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──────────────────────────────────────────────────────────────────────────── Chapter 6 Video Display The visual presentation of an application program is one of its most important elements. Users frequently base their conclusions about a program's performance and "polish" on the speed and attractiveness of its displays. Therefore, a feel for the computer system's display facilities and capabilities at all levels, from MS-DOS down to the bare hardware, is important to you as a programmer. Video Display Adapters The video display adapters found in IBM PC─compatible computers have a hybrid interface to the central processor. The overall display characteristics, such as vertical and horizontal resolution, background color, and palette, are controlled by values written to I/O ports whose addresses are hardwired on the adapter, whereas the appearance of each individual character or graphics pixel on the display is controlled by a specific location within an area of memory called the regen buffer or refresh buffer. Both the CPU and the video controller access this memory; the software updates the display by simply writing character codes or bit patterns directly into the regen buffer. (This is called memory-mapped I/O.) The following adapters are in common use as this book is being written: ■ Monochrome/Printer Display Adapter (MDA). Introduced with the original IBM PC in 1981, this adapter supports 80-by-25 text display on a green (monochrome) screen and has no graphics capabilities at all. ■ Color/Graphics Adapter (CGA). Also introduced by IBM in 1981, this adapter supports 40-by-25 and 80-by-25 text modes and 320-by-200, 4-color or 640-by-200, 2-color graphics (all-points-addressable, or APA) modes on composite or digital RGB monitors. ■ Enhanced Graphics Adapter (EGA). Introduced by IBM in 1985 and upwardly compatible from the CGA, this adapter adds support for 640-by-350, 16-color graphics modes on digital RGB monitors. It also supports an MDA-compatible text mode. ■ Multi-Color Graphics Array (MCGA). Introduced by IBM in 1987 with the Personal System/2 (PS/2) models 25 and 30, this adapter is partially compatible with the CGA and EGA and supports 640-by-480, 2-color or 320-by-200, 256-color graphics on analog RGB monitors. ■ Video Graphics Array (VGA). Introduced by IBM in 1987 with the PS/2 models 50, 60, and 80, this adapter is upwardly compatible from the EGA and supports 640-by-480, 16-color or 320-by-200, 256-color graphics on analog RGB monitors. It also supports an MDA-compatible text mode. ■ Hercules Graphics Card, Graphics CardPlus, and InColor Cards. These are upwardly compatible from the MDA for text display but offer graphics capabilities that are incompatible with all of the IBM adapters. The locations of the regen buffers for the various IBM PC─compatible adapters are shown in Figure 6-1. ┌───────────────────────────────────────────────────────┐ │ ROM BIOS │ FE000H ├───────────────────────────────────────────────────────┤ │ System ROM, Stand-alone BASIC, etc. │ F4000H ├───────────────────────────────────────────────────────┤ │ Reserved for BIOS extensions │ │ (hard-disk controller, etc.) │ C0000H ├───────────────────────────────────────────────────────┤ │ Reserved │ BC000H ├───────────────────────────────────────────────────────┤ │ 16 KB regen buffer for CGA, EGA, MCGA, and VGA │ │ in text modes and 200-line graphics modes │ B8000H ├───────────────────────────────────────────────────────┤ │ Reserved │ B1000H ├───────────────────────────────────────────────────────┤ │ 4 KB Monochrome Adapter regen buffer │ B0000H ├───────────────────────────────────────────────────────┤ │ Regen buffer area for EGA, MCGA, and VGA │ │ in 350-line or 480-line graphics modes │ A0000H ├───────────────────────────────────────────────────────┤ │ Transient part of COMMAND.COM │ ├───────────────────────────────────────────────────────┤ │ Transient program area │ varies ├───────────────────────────────────────────────────────┤ │ MS-DOS and its buffers, │ │ tables, and device drivers │ 00400H ├───────────────────────────────────────────────────────┤ │ Interrupt vectors │ 00000H └───────────────────────────────────────────────────────┘ Figure 6-1. Memory diagram of an IBM PC─compatible personal computer, showing the locations of the regen buffers for various adapters. Support Considerations MS-DOS offers several functions to transfer text to the display. Version 1 supported only Teletype-like output capabilities; version 2 added an optional ANSI console driver to allow the programmer to clear the screen, position the cursor, and select colors and attributes with standard escape sequences embedded in the output. Programs that use only the MS-DOS functions will operate properly on any computer system that runs MS-DOS, regardless of the level of IBM hardware compatibility. On IBM PC─compatible machines, the ROM BIOS contains a video driver that programs can invoke directly, bypassing MS-DOS. The ROM BIOS functions allow a program to write text or individual pixels to the screen or to select display modes, video pages, palette, and foreground and background colors. These functions are relatively efficient (compared with the MS-DOS functions, at least), although the graphics support is primitive. Unfortunately, the display functions of both MS-DOS and the ROM BIOS were designed around the model of a cursor-addressable terminal and therefore do not fully exploit the capabilities of the memory-mapped, high-bandwidth display adapters used on IBM PC─compatible machines. As a result, nearly every popular interactive application with full-screen displays or graphics capability ignores both MS-DOS and the ROM BIOS and writes directly to the video controller's registers and regen buffer. Programs that control the hardware directly are sometimes called "ill-behaved," because they are performing operations that are normally reserved for operating-system device drivers. These programs are a severe management problem in multitasking real-mode environments such as DesqView and Microsoft Windows, and they are the main reason why such environments are not used more widely. It could be argued, however, that the blame for such problematic behavior lies not with the application programs but with the failure of MS-DOS and the ROM BIOS──even six years after the first appearance of the IBM PC──to provide display functions of adequate range and power. MS-DOS Display Functions Under MS-DOS versions 2.0 and later, the preferred method for sending text to the display is to use handle-based Int 21H Function 40H (Write File or Device). When an application program receives control, MS-DOS has already assigned it handles for the standard output (1) and standard error (2) devices, and these handles can be used immediately. For example, the sequence at the top of the following page writes the message hello to the display using the standard output handle. ────────────────────────────────────────────────────────────────────────── msg db 'hello' ; message to display msg_len equ $-msg ; length of message . . . mov ah,40h ; function 40h = write file or device mov bx,1 ; BX = standard output handle mov cx,msg_len ; CX = message length mov dx,seg msg ; DS:DX = address of message mov ds,dx mov dx,offset msg int 21h ; transfer to MS-DOS jc error ; jump if error detected . . . ────────────────────────────────────────────────────────────────────────── If there is no error, the function returns the carry flag cleared and the number of characters actually transferred in register AX. Unless a Ctrl-Z is embedded in the text or the standard output is redirected to a disk file and the disk is full, this number should equal the number of characters requested. As in the case of keyboard input, the user's ability to specify command-line redirection parameters that are invisible to the application means that if you use the predefined standard output handle, you can't always be sure where your output is going. However, to ensure that your output actually goes to the display, you can use the predefined standard error handle, which is always opened to the CON (logical console) device and is not redirectable. As an alternative to the standard output and standard error handles, you can bypass any output redirection and open a separate channel to CON, using the handle obtained from that open operation for character output. For example, the following code opens the console display for output and then writes the string hello to it: ────────────────────────────────────────────────────────────────────────── fname db 'CON',0 ; name of CON device handle dw 0 ; handle for CON device msg db 'hello' ; message to display msg_len equ $-msg ; length of message . . . mov ax,3d02h ; AH = function 3dh = open ; AL = mode = read/write mov dx,seg fname ; DS:DX = device name mov ds,dx mov dx,offset fname int 21h ; transfer to MS-DOS jc error ; jump if open failed mov handle,ax ; save handle for CON . . . mov ah,40h ; function 40h = write mov cx,msg_len ; CX = message length mov dx,seg msg ; DS:DX = address of message mov ds,dx mov dx,offset msg mov bx,handle ; BX = CON device handle int 21h ; transfer to MS-DOS jc error ; jump if error detected . . . ────────────────────────────────────────────────────────────────────────── As with the keyboard input functions, MS-DOS also supports traditional display functions that are upwardly compatible from the corresponding CP/M output calls: ■ Int 21H Function 02H sends the character in the DL register to the standard output device. It is sensitive to Ctrl-C interrupts, and it handles carriage returns, linefeeds, bell codes, and backspaces appropriately. ■ Int 21H Function 06H transfers the character in the DL register to the standard output device, but it is not sensitive to Ctrl-C interrupts. You must take care when using this function, because it can also be used for input and for status requests. ■ Int 21H Function 09H sends a string to the standard output device. The string is terminated by the $ character. With MS-DOS version 2 or later, these three traditional functions are converted internally to handle-based writes to the standard output and thus are susceptible to output redirection. The sequence at the top of the following page sounds a warning beep by sending an ASCII bell code (07H) to the display driver using the traditional character-output call Int 21H Function 02H. ────────────────────────────────────────────────────────────────────────── . . . mov dl,7 ; 07h = ASCII bell code mov ah,2 ; function 02h = display character int 21h ; transfer to MS-DOS . . . ────────────────────────────────────────────────────────────────────────── The following sequence uses the traditional string-output call Int 21H Function 09H to display a string: ────────────────────────────────────────────────────────────────────────── msg db 'hello$' . . . mov dx,seg msg ; DS:DX = message address mov ds,dx mov dx,offset msg mov ah,9 ; function 09h = write string int 21h ; transfer to MS-DOS . . . ────────────────────────────────────────────────────────────────────────── Note that MS-DOS detects the $ character as a terminator and does not display it on the screen. Screen Control with MS-DOS Functions With version 2.0 or later, if MS-DOS loads the optional device driver ANSI.SYS in response to a DEVICE directive in the CONFIG.SYS file, programs can clear the screen, control the cursor position, and select foreground and background colors by embedding escape sequences in the text output. Escape sequences are so called because they begin with an escape character (1BH), which alerts the driver to intercept and interpret the subsequent characters in the sequence. When the ANSI driver is not loaded, MS-DOS simply passes the escape sequence to the display like any other text, usually resulting in a chaotic screen. The escape sequences that can be used with the ANSI driver for screen control are a subset of those defined in the ANSI 3.64─1979 Standard. These standard sequences are summarized in Figure 6-2. Note that case is significant for the last character in an escape sequence and that numbers must always be represented as ASCII digit strings, not as their binary values. (A separate set of escape sequences supported by ANSI.SYS, but not compatible with the ANSI standard, may be used for reprogramming and remapping the keyboard.) Escape sequence Meaning ────────────────────────────────────────────────────────────────────────── Esc[2J Clear screen; place cursor in upper left corner (home position). Esc[K Clear from cursor to end of line. Esc[row;colH Position cursor. (Row is the y coordinate in the range 1─25 and col is the x coordinate in the range 1─80 for 80-by-25 text display modes.) Escape sequences terminated with the letter f instead of H have the same effect. Esc[nA Move cursor up n rows. Esc[nB Move cursor down n rows. Esc[nC Move cursor right n columns. Esc[nD Move cursor left n columns. Esc[s Save current cursor position. Esc[u Restore cursor to saved position. Esc[6n Return current cursor position on the standard input handle in the format Esc[row;colR. Esc[nm Select character attributes: 0 = no special attributes 1 = high intensity 2 = low intensity 3 = italic 4 = underline 5 = blink 6 = rapid blink 7 = reverse video 8 = concealed text (no display) 30 = foreground black 31 = foreground red 32 = foreground green 33 = foreground yellow 34 = foreground blue 35 = foreground magenta 36 = foreground cyan 37 = foreground white 40 = background black 41 = background red 42 = background green 43 = background yellow 44 = background blue 45 = background magenta 46 = background cyan 47 = background white Esc[=nh Select display mode: 0 = 40-by-25, 16-color text (color burst off) 1 = 40-by-25, 16-color text 2 = 80-by-25, 16-color text (color burst off) 3 = 80-by-25, 16-color text 4 = 320-by-200, 4-color graphics 5 = 320-by-200, 4-color graphics (color burst off) 6 = 620-by-200, 2-color graphics 14 = 640-by-200, 16-color graphics (EGA and VGA, MS-DOS 4.0) 15 = 640-by-350, 2-color graphics (EGA and VGA, MS-DOS 4.0) 16 = 640-by-350, 16-color graphics (EGA and VGA, MS-DOS 4.0) 17 = 640-by-480, 2-color graphics (MCGA and VGA, MS-DOS 4.0) 18 = 640-by-480, 16-color graphics (VGA, MS-DOS 4.0) 19 = 320-by-200, 256-color graphics (MCGA and VGA, MS-DOS 4.0) Escape sequences terminated with l instead of h have the same effect. Esc[=7h Enable line wrap. Esc[=7l Disable line wrap. ────────────────────────────────────────────────────────────────────────── Figure 6-2. The ANSI escape sequences supported by the MS-DOS ANSI.SYS driver. Programs running under MS-DOS 2.0 or later may use these functions, if ANSI.SYS is loaded, to control the appearance of the display in a hardware-independent manner. The symbol Esc indicates an ASCII escape code──a character with the value 1BH. Note that cursor positions in ANSI escape sequences are one-based, unlike the cursor coordinates used by the IBM ROM BIOS, which are zero-based. Numbers embedded in an escape sequence must always be represented as a string of ASCII digits, not as their binary values. Binary Output Mode Under MS-DOS version 2 or later, you can substantially increase display speeds for well-behaved application programs without sacrificing hardware independence by selecting binary (raw) mode for the standard output. In binary mode, MS-DOS does not check between each character it transfers to the output device for a Ctrl-C waiting at the keyboard, nor does it filter the output string for certain characters such as Ctrl-Z. Bit 5 in the device information word associated with a device handle controls binary mode. Programs access the device information word by using Subfunctions 00H and 01H of the MS-DOS IOCTL function (I/O Control, Int 21H Function 44H). For example, the sequence on the following page places the standard output handle into binary mode. ────────────────────────────────────────────────────────────────────────── ; get device information... mov bx,1 ; standard output handle mov ax,4400h ; function 44h subfunction 00h int 21h ; transfer to MS-DOS mov dh,0 ; set upper byte of DX = 0 or dl,20h ; set binary mode bit in DL ; write device information... ; (BX still has handle) mov ax,4401h ; function 44h subfunction 01h int 21h ; transfer to MS-DOS ────────────────────────────────────────────────────────────────────────── Note that if a program changes the mode of any of the standard handles, it should restore those handles to ASCII (cooked) mode before it exits. Otherwise, subsequent application programs may behave in unexpected ways. For more detailed information on the IOCTL function, see Section II of this book, "MS-DOS Functions Reference." The ROM BIOS Display Functions You can somewhat improve the display performance of programs that are intended for use only on IBM PC─compatible machines by using the ROM BIOS video driver instead of the MS-DOS output functions. Accessed by means of Int 10H, the ROM BIOS driver supports the following functions for all of the currently available IBM display adapters: Function Action ────────────────────────────────────────────────────────────────────────── Display mode control 00H Set display mode. 0FH Get display mode. Cursor control 01H Set cursor size. 02H Set cursor position. 03H Get cursor position and size. Writing to the display 09H Write character and attribute at cursor. 0AH Write character-only at cursor. 0EH Write character in teletype mode. Reading from the display 08H Read character and attribute at cursor. Graphics support 0CH Write pixel. 0DH Read pixel. Scroll or clear display 06H Scroll up or initialize window. 07H Scroll down or initialize window. Miscellaneous 04H Read light pen. 05H Select display page. 0BH Select palette/set border color. ────────────────────────────────────────────────────────────────────────── Additional ROM BIOS functions are available on the EGA, MCGA, VGA, and PCjr to support the enhanced features of these adapters, such as programmable palettes and character sets (fonts). Some of the functions are valid only in certain display modes. Each display mode is characterized by the number of colors it can display, its vertical resolution, its horizontal resolution, and whether it supports text or graphics memory mapping. The ROM BIOS identifies it with a unique number. Section III of this book, "IBM ROM BIOS and Mouse Functions Reference," documents all of the ROM BIOS Int 10H functions and display modes. As you can see from the preceding list, the ROM BIOS offers several desirable capabilities that are not available from MS-DOS, including initialization or scrolling of selected screen windows, modification of the cursor shape, and reading back the character being displayed at an arbitrary screen location. These functions can be used to isolate your program from the hardware on any IBM PC─compatible adapter. However, the ROM BIOS functions do not suffice for the needs of a high-performance, interactive, full-screen program such as a word processor. They do not support the rapid display of character strings at an arbitrary screen position, and they do not implement graphics operations at the level normally required by applications (for example, bit-block transfers and rapid drawing of lines, circles, and filled polygons). And, of course, they are of no use whatsoever in non-IBM display modes such as the monochrome graphics mode of the Hercules Graphics Card. Let's look at a simple example of a call to the ROM BIOS video driver. The following sequence writes the string hello to the screen: ────────────────────────────────────────────────────────────────────────── msg db 'hello' msg_len equ $-msg . . . mov si,seg msg ; DS:SI = message address mov ds,si mov si,offset msg mov cx,msg_len ; CX = message length cld next: lodsb ; get AL = next character push si ; save message pointer mov ah,0eh ; int 10h function 0eh = write ; character in teletype mode mov bh,0 ; assume video page 0 mov bl,color ; (use in graphics modes only) int 10h ; transfer to ROM BIOS pop si ; restore message pointer loop next ; loop until message done . . . ────────────────────────────────────────────────────────────────────────── (Note that the SI and DI registers are not necessarily preserved across a call to a ROM BIOS video function.) Memory-mapped Display Techniques Display performance is best when an application program takes over complete control of the video adapter and the refresh buffer. Because the display is memory-mapped, the speed at which characters can be put on the screen is limited only by the CPU's ability to copy bytes from one location in memory to another. The trade-off for this performance is that such programs are highly sensitive to hardware compatibility and do not always function properly on "clones" or even on new models of IBM video adapters. Text Mode Direct programming of the IBM PC─compatible video adapters in their text display modes (sometimes also called alphanumeric display modes) is straightforward. The character set is the same for all, and the cursor home position──(x,y) = (0,0)──is defined to be the upper left corner of the screen (Figure 6-3). The MDA uses 4 KB of memory starting at segment B000H as a regen buffer, and the various adapters with both text and graphics capabilities (CGA, EGA, MCGA, and VGA) use 16 KB of memory starting at segment B800H. (See Figure 6-1.) In the latter case, the 16 KB is divided into "pages" that can be independently updated and displayed. (0,0)┌─────────────────────────────────┐(79,0) │ │ │ │ │ │ │ │ │ │ │ │ │ │ (0,24)└─────────────────────────────────┘(79,24) Figure 6-3. Cursor addressing for 80-by-25 text display modes (IBM ROM BIOS modes 2, 3, and 7). Each character-display position is allotted 2 bytes in the regen buffer. The first byte (even address) contains the ASCII code of the character, which is translated by a special hardware character generator into a dot-matrix pattern for the screen. The second byte (odd address) is the attribute byte. Several bit fields in this byte control such features as blinking, intensity (highlighting), and reverse video, depending on the adapter type and display mode (Figures 6-4 and 6-5). Figure 6-6 shows a hex and ASCII dump of part of the video map for the MDA. Display Background Foreground ────────────────────────────────────────────────────────────────────────── No display (black) 000 000 No display (white) VGA only 111 111 Underline 000 001 Normal video 000 111 Reverse video 111 000 ────────────────────────────────────────────────────────────────────────── Figure 6-4. Attribute byte for 80-by-25 monochrome text display mode on the MDA, Hercules cards, EGA, and VGA (IBM ROM BIOS mode 7). Value Color ────────────────────────────────────────────────────────────────────────── 0 Black 1 Blue 2 Green 3 Cyan 4 Red 5 Magenta 6 Brown 7 White 8 Gray 9 Light blue 10 Light green 11 Light cyan 12 Light red 13 Light magenta 14 Yellow 15 Intense white ────────────────────────────────────────────────────────────────────────── Figure 6-5. Attribute byte for the 40-by-25 and 80-by-25 text display modes on the CGA, EGA, MCGA, and VGA (IBM ROM BIOS modes 0─3). The table of color values assumes default palette programming and that the B or I bit controls intensity. ────────────────────────────────────────────────────────────────────────── B000:0000 3e 07 73 07 65 07 6c 07 65 07 63 07 74 07 20 07 B000:0010 74 07 65 07 6d 07 70 07 20 07 20 07 20 07 20 07 B000:0020 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0030 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0040 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0050 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0060 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0070 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0080 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 B000:0090 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07 ────────────────────────────────────────────────────────────────────────── Figure 6-6. Example dump of the first 160 bytes of the MDA's regen buffer. These bytes correspond to the first visible line on the screen. Note that ASCII character codes are stored in even bytes and their respective character attributes in odd bytes; all the characters in this example line have the attribute normal video. You can calculate the memory offset of any character on the display as the line number (y coordinate) times 80 characters per line times 2 bytes per character, plus the column number (x coordinate) times 2 bytes per character, plus (for the text/graphics adapters) the page number times the size of the page (4 KB per page in 80-by-25 modes; 2 KB per page in 40-by-25 modes). In short, the formula for the offset of the character-attribute pair for a given screen position (x,y) in 80-by-25 text modes is offset = ((y * 50H + x) * 2) + (page * 1000H) In 40-by-25 text modes, the formula is offset = ((y * 50H + x) * 2) + (page * 0800H) Of course, the segment register being used to address the video buffer must be set appropriately, depending on the type of display adapter. As a simple example, assume that the character to be displayed is in the AL register, the desired attribute byte for the character is in the AH register, the x coordinate (column) is in the BX register, and the y coordinate (row) is in the CX register. The following code stores the character and attribute byte into the MDA's video refresh buffer at the proper location: ────────────────────────────────────────────────────────────────────────── push ax ; save char and attribute mov ax,160 mul cx ; DX:AX = Y * 160 shl bx,1 ; multiply X by 2 add bx,ax ; BX = (Y*160) + (X*2) mov ax,0b000h ; ES = segment of monochrome mov es,ax ; adapter refresh buffer pop ax ; restore char and attribute mov es:[bx],ax ; write them to video buffer ────────────────────────────────────────────────────────────────────────── More frequently, we wish to move entire strings into the refresh buffer, starting at a given coordinate. In the next example, assume that the DS:SI registers point to the source string, the ES:DI registers point to the starting position in the video buffer (calculated as shown in the previous example), the AH register contains the attribute byte to be assigned to every character in the string, and the CX register contains the length of the string. The following code moves the entire string into the refresh buffer: ────────────────────────────────────────────────────────────────────────── xfer: lodsb ; fetch next character stosw ; store char + attribute loop xfer ; until all chars moved ────────────────────────────────────────────────────────────────────────── Of course, the video drivers written for actual application programs must take into account many additional factors, such as checking for special control codes (linefeeds, carriage returns, tabs), line wrap, and scrolling. Programs that write characters directly to the CGA regen buffer in text modes must deal with an additional complicating factor──they must examine the video controller's status port and access the refresh buffer only during the horizontal retrace or vertical retrace intervals. (A retrace interval is the period when the electron beam that illuminates the screen phosphors is being repositioned to the start of a new scan line.) Otherwise, the contention for memory between the CPU and the video controller is manifest as unsightly "snow" on the display. (If you are writing programs for any of the other IBM PC─compatible video adapters, such as the MDA, EGA, MCGA, or VGA, you can ignore the retrace intervals; snow is not a problem with these video controllers.) A program can detect the occurrence of a retrace interval by monitoring certain bits in the video controller's status register. For example, assume that the offset for the desired character position has been calculated as in the preceding example and placed in the BX register, the segment for the CGA's refresh buffer is in the ES register, and an ASCII character code to be displayed is in the CL register. The following code waits for the beginning of a new horizontal retrace interval and then writes the character into the buffer: ────────────────────────────────────────────────────────────────────────── mov dx,03dah ; DX = video controller's ; status port address cli ; disable interrupts ; if retrace is already ; in progress, wait for ; it to end... wait1: in al,dx ; read status port and al,1 ; check if retrace bit on jnz wait1 ; yes, wait ; wait for new retrace ; interval to start... wait2: in al,dx ; read status port and al,1 ; retrace bit on yet? jz wait2 ; jump if not yet on mov es:[bx],cl ; write character to ; the regen buffer sti ; enable interrupts again ────────────────────────────────────────────────────────────────────────── The first wait loop "synchronizes" the code to the beginning of a horizontal retrace interval. If only the second wait loop were used (that is, if a character were written when a retrace interval was already in progress), the write would occasionally begin so close to the end of a horizontal retrace "window" that it would partially miss the retrace, resulting in scattered snow at the left edge of the display. Notice that the code also disables interrupts during accesses to the video buffer, so that service of a hardware interrupt won't disrupt the synchronization process. Because of the retrace-interval constraints just outlined, the rate at which you can update the CGA in text modes is severely limited when the updating is done one character at a time. You can obtain better results by calculating all the relevant addresses and setting up the appropriate registers, disabling the video controller by writing to register 3D8H, moving the entire string to the buffer with a REP MOVSW operation, and then reenabling the video controller. If the string is of reasonable length, the user won't,even notice a flicker in the display. Of course, this procedure introduces additional hardware dependence into your code because it requires much greater knowledge of the 6845 controller. Luckily, snow is not a problem in CGA graphics modes. Graphics Mode Graphics-mode memory-mapped programming for IBM PC─compatible adapters is considerably more complicated than text-mode programming. Each bit or group of bits in the regen buffer corresponds to an addressable point, or pixel, on the screen. The mapping of bits to pixels differs for each of the available graphics modes, with their differences in resolution and number of supported colors. The newer adapters (EGA, MCGA, and VGA) also use the concept of bit planes, where bits of a pixel are segregated into multiple banks of memory mapped at the same address; you must manipulate these bit planes by a combination of memory-mapped I/O and port addressing. IBM-video-systems graphics programming is a subject large enough for a book of its own, but we can use the 640-by-200, 2-color graphics display mode of the CGA (which is also supported by all subsequent IBM text/graphics adapters) to illustrate a few of the techniques involved. This mode is simple to deal with because each pixel is represented by a single bit. The pixels are assigned (x,y) coordinates in the range (0,0) through (639,199), where x is the horizontal displacement, y is the vertical displacement, and the home position (0,0) is the upper left corner of the display. (See Figure 6-7.) (0,0)┌─────────────────────────────────┐(639,0) │ │ │ │ │ │ │ │ │ │ │ │ │ │ (0,199)└─────────────────────────────────┘(639,199) Figure 6-7. Point addressing for 640-by-200, 2-color graphics modes on the CGA, EGA, MCGA, and VGA (IBM ROM BIOS mode 6). Each successive group of 80 bytes (640 bits) represents one horizontal scan line. Within each byte, the bits map one-for-one onto pixels, with the most significant bit corresponding to the leftmost displayed pixel of a set of eight pixels and the least significant bit corresponding to the rightmost displayed pixel of the set. The memory map is set up so that all the even y coordinates are scanned as a set and all the odd y coordinates are scanned as a set; this mapping is referred to as the memory interlace. To find the regen buffer offset for a particular (x,y) coordinate, you would use the following formula: offset = ((y AND 1) * 2000H) + (y/2 * 50H) + (x/8) The assembly-language implementation of this formula is as follows: ────────────────────────────────────────────────────────────────────────── ; assume AX = Y, BX = X shr bx,1 ; divide X by 8 shr bx,1 shr bx,1 push ax ; save copy of Y shr ax,1 ; find (Y/2) * 50h mov cx,50h ; with product in DX:AX mul cx add bx,ax ; add product to X/8 pop ax ; add (Y AND 1) * 2000h and ax,1 jz label1 add bx,2000h label1: ; now BX = offset into ; video buffer ────────────────────────────────────────────────────────────────────────── After calculating the correct byte address, you can use the following formula to calculate the bit position for a given pixel coordinate: bit = 7 - (x MOD 8) where bit 7 is the most significant bit and bit 0 is the least significant bit. It is easiest to build an 8-byte table, or array of bit masks, and use the operation X AND 7 to extract the appropriate entry from the table: (X AND 7) Bit mask (X AND 7) Bit mask ────────────────────────────────────────────────────────────────────────── 0 80H 4 08H 1 40H 5 04H 2 20H 6 02H 3 10H 7 01H ────────────────────────────────────────────────────────────────────────── The assembly-language implementation of this second calculation is as follows: ────────────────────────────────────────────────────────────────────────── table db 80h ; X AND 7 = offset 0 db 40h ; X AND 7 = offset 1 db 20h ; X AND 7 = offset 2 db 10h ; X AND 7 = offset 3 db 08h ; X AND 7 = offset 4 db 04h ; X AND 7 = offset 5 db 02h ; X AND 7 = offset 6 db 01h ; X AND 7 = offset 7 . . . ; assume BX = X coordinate and bx,7 ; isolate 0─7 offset mov al,[bx+table] ; now AL = mask from table . . . ────────────────────────────────────────────────────────────────────────── The program can then use the mask, together with the byte offset previously calculated, to set or clear the appropriate bit in the video controller's regen buffer.