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EGA and VGA Smooth Scrolling and Panning

Source Code Accompanies This Article. Download It Now.


DEC88: EGA AND VGA SMOOTH SCROLLING AND PANNING

Andrew Chalk is the president of Magna Carta Software, which offers the C Windows Toolkit, a video function library containing extensive support for the EGA and VGA. He can he reached at PO. BOX 475594, Garland, TX 75047.


Although PC applications have traditionally relied on BIOS-type rolling, smooth scrolling will figure more prominently in future applications. User expectations for WYSIWYG displays placed increased demands that programs offer oversized virtual screens and deftly handle rapid transitions of document/screen perspective. In turn, the proliferation of high-resolution monitors driven by EGA and VGA cards has made programs that draw upon the expanded features of these cards and monitors a more worthwhile use of development time.

Smooth scrolling and panning take advantage of the EGA and VGA adapters' ability to move the viewing window under the display one pixel at a time in any direction. This can be done in both text and graphics modes by manipulating various EGA/VGA video controller registers in ways that weren't possible with the older CGAs and MDAs.

This article describes a Microsoft Compatible library that allows you to integrate text-mode smooth scrolling and panning into your applications. Smooth scrolling works by updating the start address by one or more scan lines at a time inside a consistent timing loop, using the built-in hardware support of the EGA or VGA card. The result is that the entire display appears to slide up, down, left, or right, with smooth movement at a controlled rate. This process uncovers portions of the display that were previously off-screen. The effect is almost magical.

The code presented here was tested on a number of EGA and VGA cards (including the VGA on the PS/2). However, the register-level compatibility of certain EGA and VGA clones with their IBM counterparts is less than complete. With third-party VGAs in particular, you should try your code on a PS/2 if you experience problems.

A file lister (or browser) called smooth browse is also included here. It lets you load a text file and move through it, with smooth scrolling and panning, by means of the cursor keys. The source code for smooth browse is compatible with Microsoft C, Borland Turbo C, Watcom C, and Mix Power C.

To develop a library that's flexible enough to handle either an EGA or a VGA at run time, you must be sensitive to the similarities of and differences between the two adapters. Fortunately, the VGA and its predecessor, the EGA, are more compatible than the EGA and the CGA. However, since smooth scrolling and panning are not supported at the BIOS level, there are important differences that the code must account for. In this article, the term "the adapter" will be used with the information is true for both adapters.

While the following discussion applies to text-mode applications, much of it is also relevant to graphics mode. In an ironic twist of the usual state of affairs, smooth scrolling and panning are more difficult to implement in text mode than in graphics mode.

How Smooth Scrolling and Panning Work

The EGA is configured with up to 256K of read/write memory in multiples of 64K. Although IBM's original EGA came with only 64K as its standard, clones almost invariably are fully populated with RAM. IBM released the VGA with 256K as standard. The size of the display buffer is 32K on VGAs and EGAs that are fully populated with 256K of RAM. EGAs with less video memory have proportionately smaller video buffers but are comparatively rare. In recognition of this fact, we will assume that your adapter has 256K.

To ensure compatibility with earlier adapters, the address of the display buffer in the processor address space depends on the active video mode. Video modes 0-6 are CGA compatible and map into segment B800H of the processor address space. Mode 7, the monochrome mode, maps into segment B8000H of the processor address space. All other video modes are EGA- and VGA-specific; they map to segment A000H.

Although a character and attribute may have been written to display memory through DOS calls, the PC BIOS, or by direct memory mapping, it is easiest to understand smooth scrolling by assuring that our program writes directly to the display buffer. In this way we can think of the screen as an array of memory addresses, without confusing the subject with DOS or BIOS calls (which ultimately write to the screen as an array in memory addresses anyway). IBM calls the memory address that appears at the top-left comer of the screen the "start address." Whenever our program writes data to the start address and subsequent addresses within the display buffer, the data can be displayed on the CRT.

For example, if our program uses 80-column color text mode, the character written to the top-left comer of the screen is at address B800:0000H. Since the attributes of a text-mode character are determined from the byte at the next memory address (called the "attribute byte"), the attribute for this character is determined by the contents of the byte at B800:0001H.

If each screen row has 80 characters, each of which is followed by an attribute byte, then a character row requires 160 bytes of storage. In 25-line mode this implies that a single screen is 4,000 bytes of display memory. Note that this is just under 4K of memory on an adapter that contains far more video memory. If we write to the display buffer at addresses that are greater than those on the screen, our text is not immediately visible, but it can be in the display buffer nonetheless. The usual way to display it is through BIOS INT 10H, service 5 (set active display page).

On the EGA and VGA, register AL can be loaded with any value from 0 to 7 to select one of eight video pages. A video page is the 4,000 bytes of memory we have already accounted for in the example just given, plus a 96-byte margin between pages. Thus, interrupt 10H, service 5, permits us to move through the display buffer in 4096-byte chunks. As we select successively higher page numbers, the BIOS actually shows us higher display-buffer addresses, and any data written to those addresses becomes visible.

Smooth Scrolling

Smooth scrolling takes the principle behind the BIOS video paging service to its logical conclusion. The basic idea behind smooth scrolling is to update the start address by one or more scan lines at a time inside a consistent timing loop. To set the value of the start address (as a byte offset from the beginning of the display buffer), you pre controller start address low and start address high registers. For example, to set the start address to 1,000H, write 10H into the start address high register and 00H into the start address low register. In the source code shown in Listing One, page 101, setting the start address is handled by the function set_start_addr().

If we update the start address by one screen width (normally 80 columns) each time through the loop, the text on the screen moves up by a whole character row. To create increments the size of one scan line, we use the preset row scan register in the CRT controller. This register has a default value of 0. Each time that we increase it by 1, text on the screen moves up one scan line. There are 14 scan lines per character row (by default) on the EGA attached to an enhanced color display or a monochrome display, and 16 scan lines on the VGA. There are only 8 scan lines per character row on a CGA monitor (or an EGA in 43-line mode).

Smooth scrolling is handled by the function smooth_scroll(). Before this function is called for the first time, the global variable vpel, short for vertical pel value, is 0 (a pel is IBM parlance for a pixel). The algorithm used in smooth_scroll() increments the value of vpel in a loop until the screen character height is reached. Then the start address is incremented by the length of one screen width, and the pel scrolling begins anew with vpel once again 0.

Consistent timing is important when you smooth scroll, or the text will move at varying speeds. For this reason we must not try to control the timing through a loop in our program (which would be dependent on processor speed). Instead, we use built-in adapter hardware support. The image on the enhanced color display is updated 60 times a second. After each update, the CRT gun moves back to its starting position (the to-left comer of the CRT) in an action known as a "vertical retrace." Because this 60Hz refresh cycle is independent of CPU speed, if we can detect the start of the vertical retrace we can use it as a signal to our program to update the display-buffer start address and achieve our desired slow loop.

Fortunately, both the EGA and the VGA provide support to detect the vertical retrace that occurs at a processor independent speed. To poll for the start of each vertical retrace before updating the start address, we read the input status register and AND the value with 8. As many readers are aware, the EGA and PS/2 VGA can be programmed to generate an interrupt on IRQ2 at the start of each vertical retrace. We choose polling instead because the interrupt is not supported on the VGA display adapter (the card for the original IBM PC AT), and it is not certain that VGA clone developers will choose to implement it.

Smooth Panning

The principles behind smooth panning are similar to those of smooth scrolling, although the implementation is more involved. Suppose that we update the display-buffer start address by two bytes. In this case the first character on the screen disappears, and all other characters march to the left. Text moves sideways, but not smoothly. This is not panning, because the character in the first column of the second line moves to the end of the first line and so on. We want all screen lines to move to the left one character so that the leftmost character of every row disappears. This is easily achieved with the EGA and VGA due to built-in support for logical line lengths. Each CRT line can be wider than the physical screen (up to 512 bytes). If we redefine the logical line length to more than 80 characters (by addressing the CRT controller offset register at index 13H), the adapter obligingly moves all lines to the left.

Our screen update is not smooth, because we have advanced the start address by two bytes (one character and its attribute), which is too large an amount. To achieve smoothness we use two things. First, the CRT vertical retrace is used as it is in smooth scrolling to achieve consistent timing. Second, the EGA and VGA provide a register in the attribute controller at index 13H, called the horizontal pel panning register, that allows pixel-sized increments in the position of each character, permitting the appearance of smooth movement.

In the source code for smooth browse (Listing One), the program performs smooth panning by first setting a logical screen width larger than 80 characters and then executing two loops. The outer loop updates the screen start address and the inner one increments the horizontal pel panning register. The net effect is that the user sees a smoothly panned screen.

A Dissection of the Source Code Library

The code for the smooth scrolling and panning library, Listing Two, page 108, illustrates one way to implement smooth scrolling and panning in text mode on the EGA and VGA. The library requires knowledge of certain system variables, such as whether the system contains an EGA or VGA, the active video mode, the character height, and so on. Source code for functions that perform these supporting roles is included in smooth browse. The functions that our pre gram must perform prior to calling the smooth scrolling and panning library are: First, determining that an EGA or a VGA is present and active. Second, set the video mode to the one desired (if the system is not already in the correct mode) and obtain the number of screen columns. Third, detect the type of monitor installed, the amount of video RAM on the adapter (if it is an EGA), and the character height. The addresses of the input status register and the CRT controller index register depend on the presence of a monochrome or color monitor and can be derived once the type of monitor is known. Finally, the cursor size should be saved for restoration on exit.

Before it calls any of the library functions, the calling program should also perform some basic setup tasks. First, the variable left_edge must be initialized to TRUE, indicating that we are at the left edge of the file to be displayed. Second, the variable end_of_buffer must be calculated as the size of the file that is loaded divided by 2 (to allow for attribute bytes). Third, and far from obvious, the input status register must be read to initialize the attribute controller.

There are no special rules regarding the order in which you call the functions in the library, save that if you want to use a logical screen width that is different from the default (80 in video mode 2 and video mode 3), you should call set_logical_screen_width() before calling any of the other functions. A word is in order about the arguments to smooth_scroll()and smoothpan(). The first argument, count, is the number of scan lines to be scrolled or panned (negative values move back to the top and left of the video buffer). This value may be low (equal to just 1 or 2) in each call to these functions, or it could be several hundred. Although the latter implies less overhead between calls to the loops within these functions that actually do the scrolling, my experience has been that even with frequent calls to smooth_scroll() and smooth_browse() that scroll only one or two scan lines at a time, the overhead does not produce any discernible difference in speed for the user. Smooth browse illustrates this point.

The second argument in smooth_scroll() and smoothpan(), which is speed, is the desired speed of movement. This variable adjusts the speed by dictating the number of scan lines by which the screen is scrolled in each loop by smooth_scroll() and the number of pels by which it is shifted sideways by smoothpan(). If speed is set to 1, the screen slides slowly and eerily in the designated direction. If it is set to 5, the screen glides past the user at a rate that is too fast to read.

The function smooth_scroll() implements the smooth scrolling techniques described previously. The first thing that smooth_scroll() does is read the start address from the CRT controller registers. The start address is stored in two registers start_address_high(0CH) and start_address_low(0DH). These registers hold the high byte and low byte, respectively, of the screen start address. We OR the two register values together, after shifting the high byte left 8 bits, and assign the value to screen.start_addr.

The function smooth_scroll() has two loops to perform. For each character row, the start address must be incremented by the number of bytes in a logical screen line in order to move us to the next character line. Within each character line, the start address is fixed, but the starting scan line of the first character row on the screen is incremented from 0 to the character height by incrementing the value in the preset row scan register (index 8 of the CRT controller).

The function smooth_pan() is similar in operation to smooth_scroll(). Given a panning direction, there are two loops. One of these pans a full character by incrementing the start address by 1. The other writes the value of hpel (horizontal pel panning) to the horizontal pel panning register of the attribute controller chip and increments hpel by the value of speed for each iteration. smooth_pan() looks more complicated than it really is. This is due to the different pixel width of the EGA monochrome and VGA characters (9 pixels) versus EGA color characters (8 pixels). The code has to perform an extra conditional test to see if video mode 7 or a VGA is active so that the value of hpel can be iterated through 8,1,2,3,4,5,6,7,0, where 8 is the default setting at which video memory is addressed on a byte boundary. On the color EGA, the valid values for hpel are 0,1,2,3,4,5,6,7, with 0 being the default setting. Contrary to expectations, smooth panning is easier in graphics modes. With the exception of VGA mode 13H, hpel would be incremented through from 0 through 7.

The other thing worth noting about smooth_pan() is that it uses the defined logical screen width to set two values, left_edge and right_edge, so that panning respects the logical line width. After panning one logical line width to the right, panning stops. The screen can be panned back to the left, and it stops at column 1.

What do you do at the end of a smooth scrolling and panning session? Many registers on the adapter may not be in a desirable state and so must be reset. Although you could write to each one separately, you may wish to take the Chuck Norris approach to video control and just reset the video mode before exiting your program.

Limitations and Enhancements

In the earlier description of smooth scrolling and panning concepts, I glossed over two important hardware limitations. The first is the limited display buffer size. By default this is 32K, but it can be increased to 64K fairly easily (smooth browse shows how). One way around this limitation (and one that could be introduced into smooth browse) is to use dynamic display buffer allocation. This could be implemented by storing the object in RAM in the transient program area and mapping it into the display-buffer on cue.

The second limitation is the imposition that the whole screen must be scrolled or panned. The EGA and VGA can produce a split screen in which one screen is kept stationary while the other is scrolled (also used in smooth browse), but this is only a minor amelioration of a major hardware limitation. Some EGA/VGA clone makers have responded to this situation by building in hardware support for smooth-scrolling windows, but we will have to wait for the next generation of video adapters until such features are generally available to support software windowing.

Summary

The main purpose of this article was to explain the useful programming technique of smooth scrolling and panning on a machine equipped with an EGA or a VGA. It illustrated how to implement this technique with a smooth scrolling and panning library. There is also an example application, smooth browse, that uses these techniques, plus the hardware split screen and 16 video pages that are available to DDJ readers. The example applied these techniques in text mode, but they also work (with some small amendment to the sample source code) in graphics mode. In fact, the graphics mode implementation is generally easier than the text mode implementation.

Once you see smooth panning and scrolling in action, new possibilities suggest themselves, and I am sure that Dr Dobb's readers will find many new applications in which to incorporate them.

_SMOOTH SCROLLING AND PANNING_ by Andrew Chalk

[LISTING ONE]

<a name="022d_000b">

/*
Copyright (C) Magna Carta Software, 1988.  All Rights Reserved.

SMOOTH BROWSE -- A file browser to illustrate how to do smooth scrolling and
panning on the EGA and the VGA.
SYSTEM REQUIREMENTS:  EGA or VGA, Enhanced Color or Monochrome Display.
COMPILER SUPPORT: Turbo C v1.5,  MSC v5.0/5.1/Quick C,  Mix Power C v1.1
WATCOM C v6.0.
Simple compiler invocation (suggested):
    TURBO:          "TCC sb.c" (from the environment).
    MSC:            "CL /AS sb.c"
    MIX POWER C:    "PC /E sb.c"
    WATCOM C:       "WCC sb 3"
                    "WLINK FILE sb LIB ?:\watcomc\lib\clib%2, ?:\watcomc\lib\math%2
                    OPTION Map, caseexact, stack=2048"
Usage: UpArrow -- scroll up, DnArrow -- scroll down, + scroll faster,
- scroll slower, RtArrow -- smooth pan right, LtArrow -- smooth pan left
Space -- pause scrolling, PgUp, PgDn, Ctrl PgUp, Ctrl PgDn
First version: 3/5/88.  Last update 06-07-88.
*/


/* MSC SPECIFIC CODE */
#if !defined(__TURBOC__)        /* these two manifest constants are         */
#if !defined(__POWERC)          /* #defined by the respective compilers.    */
#if !defined(__WATCOMC__)           /* this one is our invention                */
                                /* Fall through... must be MSC!             */

/* MSC _dos_findfirst structure compatible with TURBOC "findfirst" function */

struct find_t {
    char ff_reserved[21];
    char ff_attrib;
    unsigned ff_ftime;
    unsigned ff_fdate;
    long ff_fsize;
    char ff_name[13];
};
#define _FIND_T_DEFINED  /* A Microsoft manifest constant to prevent redef. */

#define ffblk find_t
#define findfirst(x,y,z) _dos_findfirst(x,z,y)  /* note parm. order! */
#define bioskey(x) _bios_keybrd(x)

#define inportb(port) inp(port)
#define outportb(port,value) outp((port),(value))
#define outport(port,value) outpw((port),(value))
#define MK_FP(seg,ofs)  ((void far *)((((unsigned long)(seg)) << 16) | (ofs)))
#define peekb(a,b) (*((char far*)MK_FP((a),(b))))
#endif
#endif
#endif

/* WATCOM C SPECIFIC CODE */
#if defined(__WATCOMC__)
    #include <conio.h>
    #include <sys\types.h>
    #include <direct.h>

    #define inportb(port)           inp(port)
    #define outportb(port,value)    outp((port),(value))
    #define outport(port,value)     outpw((port),(value))
    #define peekb(a,b) (*((char far*)MK_FP((a),(b))))
    #define bioskey(x) kbhit()

    struct ffblk {
        char ff_reserved[21];
        char ff_attrib;
        unsigned ff_ftime;
        unsigned ff_fdate;
        long ff_fsize;
        char ff_name[13];
    };
    int findfirst(const char *pathname, struct ffblk *ffblk, int attrib);

#endif

/* OTHER COMPILER-SPECIFIC CODE */
#if defined(__TURBOC__)
    #include <dir.h>
#elif !defined(__TURBOC__) && !defined(__WATCOMC__)
    #include <conio.h>
    #include <direct.h>
#endif

#if !defined(__WATCOMC__)
    #include <bios.h>
#endif

#include <dos.h>
#include <process.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>

typedef unsigned char BYTE;     /* A convenient new data type */

/* MANIFEST CONSTANTS */
#define VERSION "1.0"
#define LOGICAL_WIDTH 132   /* EGA/VGA logical screen width (bytes, max. 512) */
#define MAXFILESIZE 0XFFFF  /* Maximum permisable file size to load */
#define NEWCOLOR    0X8     /* Code for new EGA color to load       */
#define BCOLOR      0       /* Screen background color              */
#define FCOLOR      15      /* Screen foreground color              */
#define TRUE        1
#define FALSE       !TRUE
#define TABSIZE     4

#define _KEYBRD_READY 1

#define KEYBOARD    0X16    /* The BIOS keyboard interrupt number */
#define VIDEO       0X10    /* The BIOS video interrupt number */

/* KEY DEFINITIONS */
#define ESC         1
#define UP_ARROW    72
#define DOWN_ARROW  80
#define LEFT_ARROW  75
#define RIGHT_ARROW 77
#define PAGE_UP     73
#define PAGE_DOWN   81
#define PLUS        78
#define MINUS       74
#define CPAGE_UP    132
#define CPAGE_DOWN  118
#define SPACEBAR    57

/* EGA AND VGA REGISTER VALUES */
#define PRESET_ROW_SCAN     8     /* Address of preset row scan reg. of CRTC */
#define START_ADDRESS_HIGH  0X0C  /* Address of start address h reg. of CRTC */
#define START_ADDRESS_LOW   0X0D  /* Address of start address l reg. of CRTC */
#define AC_INDEX            0X3C0 /* Attribute Controller Index Register     */
#define AC_HPP              0X13 | 0X20   /* Horizontal Pel Panning Register */
#define AC_MCR              0X10  /* Attribute Controller Mode Control Reg.  */
#define LINE_COMPARE        0X18  /* CRTC line compare register.             */
#define CRTC_OVERFLOW       0X07  /* CRTC overflow register.                 */
#define MAX_SCAN_LINE       0X09  /* CRTC maximum scan line register.        */

/* GLOBAL VARIABLES */
struct ffblk u_file;         /* DOS file descriptor returned by findfirst() */
FILE *p_u_file;              /* Ptr. to the user file to browse */
unsigned end_of_file, screen_size, end_of_buffer, buffer_size;
unsigned right_edge, left_edge; /* TRUE if we are at the edge of the screen */
int vpel, hpel, mode, mono;

/* The following three structures are a convenient form in which to store   */
/* video information and you can extend them with more information.         */
/* We will declare one of each type (below).                                */

struct video_descriptor {
    unsigned ev_active;             /* if TRUE, an EGA or VGA is active */
    unsigned color;                 /* if TRUE, ega drives color monitor */
    unsigned ecd;                   /* if TRUE, Enhanced Color Display */
    unsigned ram;                   /* size of EGA memory (in k) */
    BYTE char_ht, char_wi; /* character height and width (EGA/VGA) */
    BYTE far *base;        /* starting address of the video buffer */
    unsigned address;               /* starting address of the active video page */
    unsigned isr;                   /* EGA/VGA input status register address */
    unsigned crtc;                  /* CRT Controller Register address */
};

struct screen_descriptor {
    unsigned rows;
    unsigned cols;
    unsigned a_start;      /* start address of screen A in split screen mode */
    unsigned logical_width;     /* screen logical width in words. max: 0x100 */
    unsigned start_addr;        /* screen start address. max: 0X4000 */
};

struct enhanced_graphics_adapter {
    unsigned present;               /* if TRUE, EGA present */
    unsigned active;                /* if TRUE, EGA active */
};

struct video_graphics_array {
    unsigned present;               /* if TRUE, VGA present */
    unsigned active;                /* if TRUE, EGA present */

};

struct video_descriptor video;
struct screen_descriptor screen;
struct enhanced_graphics_adapter ega;
struct video_graphics_array vga;


/* FUNCTION PROTOTYPES -- In order of appearance */
int             main_menu(BYTE *p_fbuf);
void            intro_to_buffer(unsigned state);
BYTE * build_line(BYTE **p_fbuf);
int             line_to_buffer(BYTE *p_fbuf);
void            smooth_scroll(int count, unsigned speed);
int             smooth_pan(int count, unsigned speed);
unsigned        set_logical_screen_width(unsigned l_width);
void            set_start_addr(unsigned start_addr);
void            set_pel_pan(int hpel);
void            set_line_compare(unsigned scan_line);
void            load_ega_color(BYTE pregister, BYTE color);
unsigned        set_video_mode(BYTE m);
int             get_v_mode(void);
void            get_v_config(void);
unsigned        get_key(void);
int             get_cursor_size(void);
void            set_cursor_size(BYTE top_line, BYTE bottom_line);
void            error(char *fs,...);


int main(int argc, char *argv[])
{

    BYTE *p_fbuf;
    int not_found, rc;
    int csize;


    /* TEST FOR A FILENAME/PATH */
    /* No filename entered -- give syntax message and exit */
    if(argc < 2) {
        fprintf(stderr,"Smooth browser version %s by\n\tMagna Carta Software\n\tP.O. Box 475594\n\tGarland, Texas 75047\n", VERSION);
        fprintf(stderr, "Usage: sb pathname\n");
        exit(1);
    }

    /* SEE IF FILE EXISTS */
    not_found = findfirst(argv[1],&u_file,0);
    if (not_found) error("\nFile %s not found.",argv[1]);

    /* get file size. If too large (> MAXFILESIZE) then tell the user and exit */
    if (u_file.ff_fsize > MAXFILESIZE)
        error("\nMaximum file size %u bytes exceeded\n",MAXFILESIZE);

    /* TEST FOR AN ACCEPTABLE VIDEO CONFIGURATION */
    mode = get_v_mode();
    get_v_config();
    if (!vga.present && !ega.present)
        error("EGA/VGA required to run SMOOTH BROWSE");
    if (!video.ev_active)
        error("The EGA/VGA must be active to use SMOOTH BROWSE");
    if (!video.ecd && video.color)
        error("Enhanced Color or Monochrome Display required to run SMOOTH BROWSE");
    if (video.ram < 128)
        error("More video RAM required to use SMOOTH BROWSE");


    /* CREATE BUFFER FOR FILE. IF ERROR, RETURN AN ERRORLEVEL TO DOS */
    p_fbuf = (BYTE *) calloc(1, (unsigned) u_file.ff_fsize);
    if (p_fbuf == NULL) error("Not enough memory available to load file");

    /* OPEN FILE. IF THERE IS AN ERROR RETURN AN ERRORLEVEL TO DOS */
    p_u_file = fopen(argv[1], "rt");
    if (p_u_file == NULL) error("Error opening file %s",u_file.ff_name);

    /* READ IN FILE. IF THERE IS A SHORT COUNT RETURN AN ERRORLEVEL TO DOS */
    if (fread(p_fbuf,1,(int) u_file.ff_fsize,p_u_file) == NULL)
        error("Short count returned when reading file");

    /* SYSTEM CHECKS OUT AND FILE EXISTS AND IS WITHIN SIZE LIMITS */
    /* NOTE: VARIABLE MONO IS SET BY THE FUNCTION GET_V_MODE() */
    if (mode != 7) set_video_mode(3);
    else set_video_mode(7);

    /* GET THE ADDRESS OF THE INPUT STATUS REGISTER AND CRT CONTROLLER */
    if (mono) video.isr = 0x03ba;
    else video.isr = 0x03da;
    video.crtc = video.isr - 6;

    /* SAVE THE CURSOR AND TURN IT OFF */
    csize = get_cursor_size();
    set_cursor_size(0,0);

    /* Load a pleasant but unusual EGA color */
    load_ega_color(0,NEWCOLOR);


    /* DO IT */
    rc = main_menu(p_fbuf);

    /* EXIT ROUTINES -- RESTORE THE CURSOR AND RESET THE VIDEO MODE. */
    set_cursor_size((BYTE) (csize >> 8), (BYTE) csize);
    if (mono) set_video_mode(7);
    else set_video_mode(3);
    return(rc);
}



int main_menu(BYTE *p_fbuf)
{
    unsigned speed=1;
    int v_direction=0, old_v_direction=0;
    int h_direction=0, old_h_direction=0;
    unsigned scan;
    BYTE old_reg;
    int ret;

    /* SET LOGICAL SCREEN WIDTH */
    screen.logical_width = LOGICAL_WIDTH;
    set_logical_screen_width(screen.logical_width);

    /* SPLIT THE SCREEN */
    screen.a_start = screen.logical_width;
    set_line_compare((screen.rows - 1) * video.char_ht);


    /* CREATE 16 VIDEO PAGES IN VIDEO MODE 3 -- MANY CAVEATS TO THIS */
    outportb(0X3CE, 6);                 /* Point index at Misc. register */
    if (vga.active) old_reg = (BYTE) inportb(0X3CF); /* save contents */
    else old_reg = 0X0E;
    outportb(0X3CF, old_reg & 0XF7);    /* reset EGA ram to A0000-AFFFF */
    video.base = (BYTE far *) 0XA0000000L;

    /* SET SCREEN START ADDRESS FOR FILE */
    screen.start_addr = screen.a_start;
    set_start_addr(screen.start_addr);
    screen_size = screen.logical_width*(screen.rows-1);

    /* DISPLAY THE INTRODUCTORY MESSAGE */
    intro_to_buffer(TRUE);

    /* DISPLAY THE FILE */
    line_to_buffer(p_fbuf);             /* position file on screen */
    end_of_buffer = end_of_file/2 - screen_size;
    inportb(video.isr); /* a dummy read to initialize the Attribute Controller */


    left_edge = TRUE;   /* file display starts at left edge */

    /* THIS IS THE MAIN LOOP THAT MOVES LOGICAL SCREEN 'A' AROUND */
    do {
        do {
            if (v_direction) {
                smooth_scroll(v_direction,speed);
                old_v_direction = old_h_direction = 0;
            }
            else if (h_direction) {
                ret = smooth_pan(h_direction,speed);
                if (ret == -1) h_direction = v_direction = 0;
                old_h_direction = old_v_direction = 0;
            }
        } while (!bioskey(_KEYBRD_READY));

        scan = get_key();
        switch(scan) {
            case  ESC:      /* The user's key presses tell us what to do */
                 break;

            case UP_ARROW:
                v_direction = -2;
                h_direction = 0;
                break;

            case DOWN_ARROW:
                v_direction = 2;
                h_direction = 0;
                break;

            case LEFT_ARROW:
                intro_to_buffer(FALSE);
                h_direction = -1;
                v_direction = 0;
                break;

            case RIGHT_ARROW:
                intro_to_buffer(FALSE);
                h_direction = 1;
                v_direction = 0;
                break;

            case PAGE_UP:
                if (screen.start_addr > screen_size + screen.logical_width) screen.start_addr -= screen_size;
                else {
                    screen.start_addr = screen.a_start;
                    left_edge = TRUE;
                }
                vpel = 0;       /* Set the preset row scan register to 0 */
                /* address the preset row scan register */
                outport(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);
                set_start_addr(screen.start_addr);
                break;

            case PAGE_DOWN:                                 /* pg dn */
                if (screen.start_addr < end_of_buffer - screen_size)
                    screen.start_addr += screen_size;
                else screen.start_addr = end_of_buffer;
                vpel = 0;       /* Set the preset row scan register to 0 */
                outport(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);

                /* reset pel panning position */
                hpel = (mono || vga.active) ? 8 : 0;
                set_pel_pan(hpel);
                set_start_addr(screen.start_addr);
                break;

            case PLUS:                      /* '+' key -- scroll faster */
                if (speed < 5) speed++;
                break;

            case MINUS:                     /* '-' key -- scroll slower */
                if (speed > 1) speed--;
                break;

            case SPACEBAR:                  /* space bar -- toggle scrolling */
                if (v_direction) {          /* moving vertically, so stop */
                    old_v_direction = v_direction;
                    v_direction = 0;
                }
                else if (h_direction) {     /* moving horizontally, so stop */
                    old_h_direction = h_direction;
                    h_direction = 0;
                }
                else if (old_v_direction) {  /* stopped -- start vertical */
                    v_direction = old_v_direction;
                }
                else if (old_h_direction) {  /* stopped -- start horizontal */
                    h_direction = old_h_direction;
                }
                break;

            case CPAGE_UP:                   /* ctrl-pgup -- go to top */
                screen.start_addr = screen.a_start;
                left_edge = TRUE;

                /* reset pel panning position */
                vpel = 0;       /* Set the preset row scan register to 0 */
                outport(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);
                hpel = (mono || vga.active) ? 8 : 0;
                set_start_addr(screen.start_addr);
                set_pel_pan(hpel);
                break;

            case CPAGE_DOWN:                 /* ctrl-pgdn -- go to bottom */
                screen.start_addr = end_of_buffer;

                /* reset pel panning position */
                vpel = 0;   /* Set the preset row scan register to 0 */
                outport(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);
                hpel = (mono || vga.active) ? 8 : 0;
                set_pel_pan(hpel);

                set_start_addr(screen.start_addr);
                break;

            default:
                break;
        }
    } while (scan != 1);

    return (0);
}


/* INTRO_TO_BUFFER -- Writes the introductory messsage on screen 'B' */
void intro_to_buffer(unsigned state)
{
    BYTE far *ega_buf ;
    static BYTE a_sbuf[] = "SMOOTH BROWSE v1.0 -- by Magna Carta Software, 1988";
    BYTE *p_sbuf, att;

    ega_buf = video.base;
    att = ((FCOLOR & 0X7) << 4) + BCOLOR;
    if (state) {
        p_sbuf = a_sbuf;
        while (*p_sbuf) {
            *ega_buf++ = *p_sbuf++;
            *ega_buf++ = att;
        }
    }
    while (ega_buf < video.base + (screen.logical_width << 1) ) {
        *ega_buf++ = '\x20';
        *ega_buf++ = att;
    }
}


/* BUILD_LINE -- This function constructs a line of text from data read in  */
/* from the file.                                                           */
BYTE *build_line(BYTE **p_fbuf)
{
    static BYTE lbuf[LOGICAL_WIDTH + 1];/* add 1 for '\0' terminator */
    BYTE *p_lbuf;
    int i;

    p_lbuf = lbuf;  /* point to the buffer to hold the line that we build */

    /* construct a line in the buffer for display until we hit a '\n' or '\t'*/
    while (**p_fbuf != '\n') {
        if (**p_fbuf == '\t') for(i=0;i<TABSIZE;i++) *p_lbuf++ = 0x20;
        else if (**p_fbuf >= 0x20 && **p_fbuf < 0x80) *p_lbuf++ = **p_fbuf;
        (*p_fbuf)++;
        if (p_lbuf >= lbuf + screen.logical_width) break;
    }
    *p_lbuf = '\0';
    (*p_fbuf)++;        /* advance past the new line character */

    return (lbuf);
}


/* LINE_TO_BUFFER -- Moves the line from memory to the video buffer */
int line_to_buffer(BYTE *p_fbuf)
{
    BYTE far *p_vbuff, far *line, far *eob;
    BYTE *p_lbuf, att;
    unsigned i;

    p_vbuff = video.base + 2*screen.logical_width;
    att = (BCOLOR << 4) + FCOLOR;
    for (i = 0; i < 0X8000 - 2*screen.logical_width; i++) {
        p_vbuff[i++] = '\0';
        p_vbuff[i] = att;
    }
    eob = video.base + (0XFFFF - screen.logical_width);
    line = p_vbuff;
    while (*p_fbuf && p_vbuff < eob ) {
        p_lbuf = build_line(&p_fbuf);
        while (*p_lbuf) {
            *p_vbuff++ = *p_lbuf++;
            *p_vbuff++ = att;
        }
        /* go to next screen line */
        line += screen.logical_width*2;
        p_vbuff = line;
    }
    end_of_file = FP_OFF(p_vbuff);
    return (0);
}


/* SMOOTH_SCROLL scrolls the EGA video buffer the number of scan lines
indicated in "count" at a speed of "speed" scan lines per vertical retrace.
One retrace occurs each 60th of a second regardless of processor speed.
*/
void smooth_scroll(int count, unsigned speed)
{

    unsigned i;

    /* GET THE START ADDRESS OF THE SCREEN BUFFER */
    outportb(video.crtc, START_ADDRESS_HIGH);       /* High byte */
    screen.start_addr = inportb(video.crtc+1) << 8;
    outportb(video.crtc, START_ADDRESS_LOW);        /* Low byte */
    screen.start_addr |= inportb(video.crtc+1);

    /* count > 0 => scroll screen upwards. */
    /* i is the number of scan lines scrolled */
    if (count>0 && (screen.start_addr < end_of_buffer))
    for(i=0;i < count;) {
        if (vpel >= (int) video.char_ht) {
            vpel = vpel - video.char_ht;
            screen.start_addr += screen.logical_width;
        }
        if (vpel < 0) {
            if (screen.start_addr > screen.logical_width) {
                vpel += video.char_ht;
                screen.start_addr -= screen.logical_width;
            }
            else vpel = 0;
        }
        for(;vpel< (int) video.char_ht && i<count;vpel+=speed,i+=speed) {

            /* wait for a vertical retrace */
            while (!(inportb(video.isr) & 8));
            /* wait for horizontal or vertical retrace */
            while (inportb(video.isr) & 1);

            /* address the preset row scan register */
            outport(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);

            /* Reset the start address */
            set_start_addr(screen.start_addr);
        }
    }

    /* count < 0  => scroll screen characters downwards */
    /* i is the number of scan lines scrolled */
    if (count < 0) {
        if (vpel >= (int) video.char_ht) vpel -= video.char_ht;

        for(i=0;i < -count && screen.start_addr;) {
        /* This loop determines whether to update the start address */
        /* It is iterated once for each video.char_ht. */
            if (vpel < 0) {
                vpel = video.char_ht + vpel;
                if (screen.start_addr > screen.logical_width)
                    screen.start_addr -= screen.logical_width;
                else {
                    screen.start_addr = screen.a_start;
                    vpel=0;
                }
            }

            /* this loop moves the screen "speed" scan lines each time through */
            for(;vpel>=0 && i< -count;vpel-=speed,i+=speed) {

                /* wait for a vertical retrace */
                while (!(inportb(video.isr) & 8));
                /* wait for horizontal or vertical retrace */
                while (inportb(video.isr) & 1);

                /* address the preset row scan register */
                outport(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);

                /* Reset the start address */
                set_start_addr(screen.start_addr);
            }
        }
    }
}


/* SMOOTH_PAN
This function invokes smooth panning on the EGA/VGA in text mode.  The
function calculates the number of scan lines per row.  The speed variable
adjusts the speed in pixels per vertical retrace and takes values in the
range 1-8 for EGA color and 1-9 for monochrome and the VGA.
*/
int smooth_pan(int count, unsigned speed)
{
    unsigned i;

    /* count greater than zero (move viewport to the right) */
    if (count>0 && !right_edge) for(i=0;i<count;) {
        /* if we have scrolled a full character, reset start address */
        if (hpel >= 8) {
            screen.start_addr++;
            set_start_addr(screen.start_addr);
            if (!left_edge) if (!(screen.start_addr % (screen.logical_width)))
                right_edge = TRUE;
            left_edge = FALSE;
            if (!right_edge) {
                if (!(mono || vga.active)) hpel %= 8;    /* reset the pel counter */
                else {
                    hpel %= 9;
                    if (hpel == 8) {
                        set_pel_pan(hpel);
                        hpel += speed;
                        hpel %= 9;
                    }
                }
            }
            else {
                hpel = (mono || vga.active) ? 8 : 0;
                set_pel_pan(hpel);
                return (-1);
            }
        }
        for(;hpel < 8 && i < count;i+=speed, hpel+=speed) set_pel_pan(hpel);
    }

    /* count less than zero (move viewport to the left) */
    else if (count < 0) {
        if (mono || vga.active) {
            if (left_edge && hpel == 8) return (-1);
            if (hpel > 8) hpel -= 9;
        }
        else {
            if (left_edge && hpel == 0) return (-1);
            if (hpel > 7) hpel = hpel - 8;
        }
        for(i=0; i < (-count); ) {
        /* IF WE HAVE SCROLLED A FULL CHARACTER, RESET START ADDRESS */
            if (hpel<0 && !left_edge) {
                if ((mono || vga.active) && hpel == -1) {
                    hpel = 8;
                    set_pel_pan(hpel);
                    hpel = -1 - speed;
                }
                screen.start_addr--;
                right_edge = FALSE;
                hpel = (mono || vga.active) ? 9 + hpel : 8 + hpel;
                set_start_addr(screen.start_addr);
                if (!(screen.start_addr % screen.logical_width)) {
                    left_edge = TRUE;
                    screen.start_addr--;
                }
            }
            else if (hpel<0 && left_edge) i = (-count);

            for(;hpel >= 0 && i < (-count);i+=speed, hpel-=speed) set_pel_pan(hpel);
            if (left_edge && hpel < speed) {
                hpel = (mono || vga.active) ? 8 : 0;
                set_pel_pan(hpel);
                return (-1);
            }
        }
    }
    return (0);
}



/* SET_LOGICAL_SCREEN_WIDTH defines an EGA/VGA screen width for smooth panning
and sets the the global variable "screen.cols."  screen.cols must be restored
when smooth panning is finished or subsequent screen writes will address the
wrong screen locations.
l_width is the width of the logical screen in bytes. Max 512.
*/
unsigned set_logical_screen_width(unsigned l_width)
{
    l_width = (l_width > 512) ? 512 : l_width;

    /* set screen_cols */
    screen.cols = l_width;

    /* set logical screen width */
    l_width >>= 1;                                  /* convert to words */
    outport(video.crtc, (l_width << 8) | 0x013);

    return (l_width << 1);
}


/* SET_START_ADDRESS -- Sets the start address of the screen. I.e. the  */
/* address that occupies the top left of the screen.                    */
void set_start_addr(unsigned start_addr)
{
    /* address the start address high register */
    outport(video.crtc, (start_addr & 0XFF00) | START_ADDRESS_HIGH);

    /* address the start address low register */
    outport(video.crtc, (start_addr << 8) | START_ADDRESS_LOW);
}


/* SET_PEL_PAN -- Sets the horizontal pel panning register to the value     */
/* specified in "hpel".  Called by smooth_pan()                             */
void set_pel_pan(int hpel)
{
    /* first wait for end of vertical retrace */
    while (inportb(video.isr) & 8);

    /* wait for next vertical retrace */
    while (!(inportb(video.isr) & 8));

    /* address the horizontal pel paning register */
    outportb(AC_INDEX, AC_HPP);
    outportb(AC_INDEX, hpel);
}


/* SET_LINE_COMPARE -- Sets the scan line at which the screen is split.     */
void set_line_compare(unsigned scan_line)
{
    BYTE old_reg;
    if (scan_line > 256) {          /* set bit 4 of the overflow register */
        if (ega.active) {
            if ((mode >= 4 && mode <= 7) || (mode == 0X0D || mode == 0X0E))
                outport(video.crtc, (0X11 << 8) | CRTC_OVERFLOW);
            else outport(video.crtc, (0X1F << 8) | CRTC_OVERFLOW);
            if (mode < 4 && !video.ecd) outport(video.crtc, (0X11 << 8) | CRTC_OVERFLOW);
        }
        else if (vga.active) {      /* ahhh... readable registers */
            /* set bit 8 of line compare reg., which is bit 4 of the CRTC overflow reg. */
            outportb(video.crtc,CRTC_OVERFLOW);
            old_reg = (BYTE) inportb(video.crtc+1);
            outportb(video.crtc+1,old_reg | 0X10);
            /* set bit 9 of line compare reg., which is bit 6 of
            the CRTC max. scan line register */

            outportb(video.crtc,MAX_SCAN_LINE);
            old_reg = (BYTE) inportb(video.crtc+1);
            if (old_reg | 0X40) outportb(video.crtc+1,old_reg & 0XBF);
        }
        scan_line -= 256;
    }
    else {
        if (ega.active) {
            /* clear bit 8 of the line compare register (which is stored as bit */
            /* 4 of the CRTC overflow register).  This register is write only.  */
            if (!video.ecd && mode < 4) outport(video.crtc, (1 << 8) | CRTC_OVERFLOW);
            else if (video.ecd && mode < 4) outport(video.crtc, (0XF << 8) | CRTC_OVERFLOW);
            if ((mode >= 4 && mode < 7) || (mode == 0XD || mode == 0XE) ) outport(video.crtc, (1 << 8) | CRTC_OVERFLOW);
            if (mode == 7 || mode > 0X0E) outport(video.crtc, (0XF << 8) | CRTC_OVERFLOW);
        }
        if (vga.active) {
            outportb(video.crtc,CRTC_OVERFLOW); /* clear bit 8 of line compare reg. */
            old_reg = (BYTE) inportb(video.crtc+1);
            outportb(video.crtc,((old_reg & 0XEF) << 8) | CRTC_OVERFLOW);
            outportb(video.crtc,MAX_SCAN_LINE);  /* clear bit 9 of line compare reg. */
            old_reg = (BYTE) inportb(video.crtc+1);
            outportb(video.crtc,((old_reg & 0XBF) << 8) | MAX_SCAN_LINE);
        }
    }
    outport(video.crtc, (scan_line << 8) | LINE_COMPARE);
}


/************************** UTILITY FUNCTIONS *********************************/


/* LOAD_EGA_COLOR -- Load an EGA or VGA palette register with a selected color */
void load_ega_color(BYTE pregister, BYTE color)
{
    union REGS regs;

    regs.x.ax = 0x1000;     /* Service 0. Set an individual palette register */
    regs.h.bl = pregister;  /* BL holds the register to be set (0-15) */
    regs.h.bh = color;      /* BH hols the new color */
    int86(VIDEO, &regs, &regs);
}


/* SET_VIDEO_MODE -- sets the active video mode. */
unsigned set_video_mode(BYTE m)
{
    union REGS regs;

    regs.h.ah = 0;          /* service 0 - set current video mode */
    regs.h.al = m;          /* requested mode # */
    int86(VIDEO, &regs, &regs);

    return m;
}


/* GET_V_MODE -- Returns the video mode and sets the global variables       */
/* screen.cols (# of screen columns), screen.rows (# of screen rows)        */
/* video.base (screen buffer starting address), and mono.                   */
int get_v_mode(void)
{
    union REGS regs;

    regs.h.ah = 0xf;            /* get current video mode */
    int86(VIDEO, &regs, &regs);
    mode = regs.h.al;           /* current mode # */
    screen.cols = regs.h.ah;
    regs.h.dl = 0x18;           /* dummy #rows for CGA compatibility */
    regs.h.bh = 0;
    regs.x.ax = 0x1130;         /* BIOS function 0x11, sub-function 0x30 */
    int86(VIDEO,&regs,&regs);
    screen.rows = regs.h.dl + 1;/* screen rows returned in DL */
    if (mode == 7) {
        video.base = (BYTE far *) 0XB0000000L;
        mono = TRUE;
    }
    else {
        video.base = (BYTE far *) 0XB8000000L;
        mono = FALSE;
    }

    return (mode);
}


/* GET_V_CONFIG -- A portmanteau routine to determine if an EGA, VGA or     */
/* neither is present and, if an EGA or VGA is present, to set the          */
/* following video parameters:                                              */
/* vga.present = TRUE if a VGA is present in the system.                    */
/* vga.active = TRUE if the VGA is the active adapter.                      */
/* ega.present = TRUE if an EGA is present in the system.                   */
/* ega.active = TRUE if an EGA is the active adapter.                       */
/* video.ram = amount of video RAM installed on the adapter.                */
/* video.color = TRUE if the EGA/VGA drives a color display.                */
/* video.ecd = TRUE if the EGA/VGA drives an enhanced display.              */
/* video.ev_active = TRUE if the EGA or the VGA is active.                  */
/* This function should be preceeded by a call to get_v_mode().             */
void get_v_config(void)
{

    union REGS regs;
    BYTE e_byte;           /* EGA information byte in ROM data area */

    /* First, check for the presence of an VGA */
    regs.x.ax = 0x1a00;     /* Function call 0x1a -- Tech. Ref./2 p. 3-24 */
    int86(VIDEO, &regs, &regs);         /* Function supported => VGA */
    if (regs.h.al == 0x1a) {
        vga.present = TRUE;
        if (regs.h.bl < 7) vga.active = FALSE;
        else vga.active = TRUE;
    }
    else {
        vga.present = FALSE;
        vga.active = FALSE;
    }
    /* Check for the presence of an EGA and set relevant EGA/VGA parameters */
    regs.h.ah = 0x12;       /* EGA BIOS alternate select. Tech. Ref. p106. */
    regs.h.bl = 0x10;       /* return EGA information.              */
    int86(VIDEO, &regs, &regs);
    if (regs.h.bl == 0x10 || vga.present) ega.present = FALSE;
    else ega.present = TRUE;

    /* If an EGA or VGA is present then set some important parameters */
    if (ega.present || vga.present) {   /* EGA/VGA is present -- is it active? */
        video.ram = regs.h.bl*64 + 64;  /* EGA/VGA installed memory */
        if (regs.h.bh) video.color = FALSE; /* EGA/VGA drives a mono monitor */
        else video.color = TRUE;            /* EGA/VGA drives a color monitor */

        /* See if EGA/VGA drives an Enhanced Color Display */
        if (video.color) {
            if (regs.h.cl == 3 || regs.h.cl == 9) video.ecd = TRUE;
            else video.ecd = FALSE;
        }
        e_byte = peekb(0,0x487);                    /* EGA info. byte */
        if (e_byte & 8) video.ev_active = FALSE;    /* EGA not active */
        else {
            video.ev_active = TRUE;                 /* EGA is active */
            if (!vga.active) ega.active = TRUE;
        }
    }

    if (vga.active) {
        regs.x.ax = 0X1202;
        regs.h.bl = 0X30;
        int86(VIDEO, &regs, &regs);
    }
    /* check # of screen rows for font size */
    if (vga.active) video.char_ht = (BYTE) (400/screen.rows);
    else if (ega.active) if (video.ecd || mono) video.char_ht = (BYTE) (350/screen.rows);
    else video.char_ht = (BYTE) (200/screen.rows);
}


/* GET_KEY -- Returns the scan code of the next key in the keyboard buffer. */
unsigned get_key(void)
{
    union REGS regs;

    regs.h.ah = 0;     /* read next keyboard character */
    int86(KEYBOARD, &regs, &regs);
    return ((unsigned) regs.h.ah);
}


/* GET_CURSOR_ SIZE -- Get the cursor size on the active page.         */
int get_cursor_size(void)
{
    union REGS regs;
    int hi,lo;

    regs.h.ah = 3;                 /* request cursor size */
    regs.h.bh = 0;                 /* from page 0 */
    int86(VIDEO, &regs, &regs);    /* ROM BIOS video service */
    hi = regs.h.ch;                /* top scan line of cursor */
    lo = regs.h.cl;                /* bottom scan line of cursor */
    return (hi << 8) | lo;         /* combine for return */
}


/* SET CURSOR SIZE sets the cursor size. top_line=bottom_line=0 turns it off           */
void set_cursor_size(BYTE top_line, BYTE bottom_line)
{
    union REGS regs;

    regs.h.ah = 1;                      /* BIOS function 1, set cursor size */
    if (top_line == 0 && bottom_line == 0) regs.h.ch = 32; /* request cursor off */
    else {
        regs.h.ch = top_line;               /* row */
        regs.h.cl = bottom_line;            /* column */
    }
    int86(VIDEO, &regs, &regs);
}


/* ERROR -- A simple error-handler */
void error(char *fs,...)
{
    va_list argptr;
    va_start(argptr,fs);
    vfprintf(stderr,fs,argptr);
    va_end(argptr);
    exit(1);
}


#if defined(__WATCOMC__)
/*
WATCOM C does not access the DOS file system in a fashion similar to the other
compilers so we write our own "findfirst()" function.  WATCOM C uses a
function called opendir(), which could be used instead.
*/

/* NOTE: THE FOLLOWING IMPLEMENTATION IS VALID ONLY IN SMALL DATA MODELS */
int findfirst(const char *pathname, struct ffblk *ffblk, int attrib)
{
    union REGS regs;
    struct SREGS sregs;

    segread(&sregs);
    regs.h.ah = 0X1A;               /* DOS set DTA function         */
    regs.x.dx = (unsigned) ffblk;   /* address of new DTA address   */
    intdosx(&regs, &regs, &sregs);  /* this reads the registers...  */

    regs.h.ah = 0X4E;               /* DOS find first matching file */
    regs.x.cx = attrib;             /* search attribute             */
    regs.x.dx = (unsigned) pathname;/* DS:DX is address of pathname */
    intdosx(&regs, &regs, &sregs);
    return (regs.x.ax);
}
#endif


<a name="022d_000c"><a name="022d_000c">
<a name="022d_000d">
[LISTING TWO]
<a name="022d_000d">


/*
SMOOTHLIB--A library of C language routines to do smooth scrolling and panning on the EGA and the VGA.
by Andrew J. Chalk.
SYSTEM REQUIREMENTS: EGA or VGA. Enhanced Color, Monochrome or Analog Display.
COMPILER SUPPORT: MSC 5.0+/Quick C
Simple compiler invocation (suggested):
    MSC:            "CL /AS sb.c"
First version: 3/5/88.  Last update 09-01-88.
*/

#include <conio.h>
#include <stdio.h>



typedef unsigned char BYTE;     /* A convenient new data type */



/* MANIFEST CONSTANTS */
#define LOGICAL_WIDTH 132   /* EGA/VGA logical screen width (bytes, max. 512) */
#define TRUE            1
#define FALSE       !TRUE

/* Macros to make a far pointer and examine at a byte in memory */
#define MK_FP(seg,ofs)  ((void far *)((((unsigned long)(seg)) << 16) | (ofs)))
#define peekb(a,b)      (*((char far*)MK_FP((a),(b))))



/* EGA AND VGA REGISTER VALUES */
#define PRESET_ROW_SCAN     8     /* Address of preset row scan reg. of CRTC */
#define START_ADDRESS_HIGH  0X0C  /* Address of start address h reg. of CRTC */
#define START_ADDRESS_LOW   0X0D  /* Address of start address l reg. of CRTC */
#define AC_INDEX            0X3C0 /* Attribute Controller Index Register     */
#define AC_HPP              0X13 | 0X20   /* Horizontal Pel Panning Register */
#define AC_MCR              0X10  /* Attribute Controller Mode Control Reg.  */
#define LINE_COMPARE        0X18  /* CRTC line compare register.             */
#define CRTC_OVERFLOW       0X07  /* CRTC overflow register.                 */
#define MAX_SCAN_LINE       0X09  /* CRTC maximum scan line register.        */



/* GLOBAL VARIABLES */
unsigned end_of_buffer;         /* address of the end of the video buffer   */
unsigned right_edge, left_edge; /* TRUE if we are at the edge of the screen */
int vpel, hpel;                 /* vertical pel height, etc.                */
int mode, mono;                 /* video mode, monochrome (TRUE or FALSE)   */



/* STRUCTURE DEFINITIONS
The following four structures define convenients form in which to store
video information and can be extended with additional members.
We will declare one of each type (below).
*/
struct video_descriptor {
    unsigned ev_active;             /* if TRUE, an EGA or VGA is active     */
    unsigned color;                 /* if TRUE, ega drives color monitor    */
    unsigned ecd;                   /* if TRUE, Enhanced Color Display      */
    unsigned ram;                   /* size of EGA memory (in k)            */
    BYTE char_ht, char_wi;          /* character height and width (EGA/VGA) */
    BYTE far *base;                 /* start address of the video buffer    */
    unsigned address;               /* start addr. of the active video page */
    unsigned isr;                   /* EGA/VGA input status register address*/
    unsigned crtc;                  /* CRT Controller Register address      */
};


/* We place all the information related to the screen in one structure */
struct screen_descriptor {
    unsigned rows;
    unsigned cols;
    unsigned a_start;      /* start address of screen A in split screen mode */
    unsigned logical_width;     /* screen logical width in words. max: 0x100 */
    unsigned start_addr;        /* screen start address. max: 0X4000 */
};


/* This structure contains information specific to the EGA */
struct enhanced_graphics_adapter {
    unsigned present;               /* if TRUE, EGA present */
    unsigned active;                /* if TRUE, EGA active */
};


/* This structure contains information specific to the VGA */
struct video_graphics_array {
    unsigned present;               /* if TRUE, VGA present */
    unsigned active;                /* if TRUE, EGA present */

};


/* STRUCTURE DECLARATIONS */
struct video_descriptor video;
struct screen_descriptor screen;
struct enhanced_graphics_adapter ega;
struct video_graphics_array vga;



/* FUNCTION PROTOTYPES -- In order of appearance */
void        smooth_scroll(int count, unsigned speed);
int         smooth_pan(int count, unsigned speed);
unsigned    set_logical_screen_width(unsigned l_width);
void        set_start_addr(unsigned start_addr);
void        set_pel_pan(int hpel);



/*** FUNCTION DEFINITIONS BEGIN HERE ***/

/*
SMOOTH_SCROLL scrolls the EGA/VGA video buffer in text mode the number of
scan lines indicated in "count" at a speed of "speed" scan lines per
vertical retrace.
*/
void smooth_scroll(int count, unsigned speed)
{

    unsigned i;

    /* GET THE START ADDRESS OF THE SCREEN BUFFER */
    outp(video.crtc, START_ADDRESS_HIGH);       /* High byte */
    screen.start_addr = inp(video.crtc+1) << 8;
    outp(video.crtc, START_ADDRESS_LOW);        /* Low byte */
    screen.start_addr |= inp(video.crtc+1);

    /* COUNT > 0 => SCROLL SCREEN UPWARDS. */
    /* i is the number of scan lines scrolled */
    if (count>0 && (screen.start_addr < end_of_buffer))
    for(i=0;i < count;) {
        if (vpel >= (int) video.char_ht) {
            vpel = vpel - video.char_ht;
            screen.start_addr += screen.logical_width;
        }
        if (vpel < 0) {
            if (screen.start_addr > screen.logical_width) {
                vpel += video.char_ht;
                screen.start_addr -= screen.logical_width;
            }
            else vpel = 0;
        }
        for(;vpel< (int) video.char_ht && i<count;vpel+=speed,i+=speed) {

            /* wait for a vertical retrace */
            while (!(inp(video.isr) & 8));
            /* wait for horizontal or vertical retrace */
            while (inp(video.isr) & 1);

            /* address the preset row scan register */
            outpw(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);

            /* Reset the start address */
            set_start_addr(screen.start_addr);
        }
    }

    /* COUNT < 0  => SCROLL SCREEN CHARACTERS DOWNWARDS */
    /* i is the number of scan lines scrolled */
    if (count < 0) {
        if (vpel >= (int) video.char_ht) vpel -= video.char_ht;

        for(i=0;i < -count && screen.start_addr;) {
        /* This loop determines whether to update the start address */
        /* It is iterated once for each video.char_ht. */
            if (vpel < 0) {
                vpel = video.char_ht + vpel;
                if (screen.start_addr > screen.logical_width)
                    screen.start_addr -= screen.logical_width;
                else {
                    screen.start_addr = screen.a_start;
                    vpel=0;
                }
            }

            /* this loop moves the screen "speed" scan lines each time through */
            for(;vpel>=0 && i< -count;vpel-=speed,i+=speed) {

                /* wait for a vertical retrace */
                while (!(inp(video.isr) & 8));
                /* wait for horizontal or vertical retrace */
                while (inp(video.isr) & 1);

                /* address the preset row scan register */
                outpw(video.crtc, (vpel << 8) | PRESET_ROW_SCAN);

                /* Reset the start address */
                set_start_addr(screen.start_addr);
            }
        }
    }
}



/*
SMOOTH_PAN -- This function invokes smooth panning on the EGA/VGA in
text mode.  The function calculates the number of scan lines per row.
The speed variable adjusts the speed in pixels per vertical retrace and
takes values in the range 1-8 for EGA color and 1-9 for monochrome and
the VGA.
*/
int smooth_pan(int count, unsigned speed)
{
    unsigned i;

    /* count greater than zero (move viewport to the right) */
    if (count>0 && !right_edge) for(i=0;i<count;) {
        /* if we have scrolled a full character, reset start address */
        if (hpel >= 8) {
            screen.start_addr++;
            set_start_addr(screen.start_addr);
            if (!left_edge) if (!(screen.start_addr % (screen.logical_width)))
                right_edge = TRUE;
            left_edge = FALSE;
            if (!right_edge) {
                if (!(mono || vga.active)) hpel %= 8;    /* reset the pel counter */
                else {
                    hpel %= 9;
                    if (hpel == 8) {
                        set_pel_pan(hpel);
                        hpel += speed;
                        hpel %= 9;
                    }
                }
            }
            else {
                hpel = (mono || vga.active) ? 8 : 0;
                set_pel_pan(hpel);
                return (-1);
            }
        }
        for(;hpel < 8 && i < count;i+=speed, hpel+=speed) set_pel_pan(hpel);
    }

    /* count less than zero (move viewport to the left) */
    else if (count < 0) {
        if (mono || vga.active) {
            if (left_edge && hpel == 8) return (-1);
            if (hpel > 8) hpel -= 9;
        }
        else {
            if (left_edge && hpel == 0) return (-1);
            if (hpel > 7) hpel = hpel - 8;
        }
        for(i=0; i < (-count); ) {
        /* IF WE HAVE SCROLLED A FULL CHARACTER, RESET START ADDRESS */
            if (hpel<0 && !left_edge) {
                if ((mono || vga.active) && hpel == -1) {
                    hpel = 8;
                    set_pel_pan(hpel);
                    hpel = -1 - speed;
                }
                screen.start_addr--;
                right_edge = FALSE;
                hpel = (mono || vga.active) ? 9 + hpel : 8 + hpel;
                set_start_addr(screen.start_addr);
                if (!(screen.start_addr % screen.logical_width)) {
                    left_edge = TRUE;
                    screen.start_addr--;
                }
            }
            else if (hpel<0 && left_edge) i = (-count);

            for(;hpel >= 0 && i < (-count);i+=speed, hpel-=speed) set_pel_pan(hpel);
            if (left_edge && hpel < speed) {
                hpel = (mono || vga.active) ? 8 : 0;
                set_pel_pan(hpel);
                return (-1);
            }
        }
    }
    return (0);
}



/*
SET_LOGICAL_SCREEN_WIDTH defines an EGA/VGA screen width (in bytes) for
smooth panning and sets the the global variable "screen.cols."
screen.cols is the number of screen columns and must be restored when
smooth panning is finished or subsequent screen writes will address the
wrong screen locations.
l_width is the width of the logical screen in bytes. Max 512.
*/
unsigned set_logical_screen_width(unsigned l_width)
{
    l_width = (l_width > 512) ? 512 : l_width;

    /* set screen_columns */
    screen.cols = l_width;

    /* set logical screen width */
    l_width >>= 1;                                  /* convert to words */
    outpw(video.crtc, (l_width << 8) | 0x013);

    return (l_width << 1);
}



/*
SET_START_ADDRESS -- Sets the start address of the screen. I.e. the
address that occupies the top left of the screen.
*/
void set_start_addr(unsigned start_addr)
{
    /* address the start address high register */
    outpw(video.crtc, (start_addr & 0XFF00) | START_ADDRESS_HIGH);

    /* address the start address low register */
    outpw(video.crtc, (start_addr << 8) | START_ADDRESS_LOW);
}



/*
SET_PEL_PAN -- Sets the horizontal pel panning register to the value
specified in "hpel".  Called by smooth_pan()
*/
void set_pel_pan(int hpel)
{
    /* first wait for end of vertical retrace */
    while (inp(video.isr) & 8);

    /* wait for next vertical retrace */
    while (!(inp(video.isr) & 8));

    /* address the horizontal pel paning register */
    outp(AC_INDEX, AC_HPP);
    outp(AC_INDEX, hpel);
}











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