The PC's video display is a very complex system. First there is not a single common device as exists for the parallel and serial ports or even a few devices (like the keyboard systems found on PCs). No there are literally dozens of different display adapter cards available for the PC. Furthermore each adapter typically supports several different display modes. Given the large number of display modes and uses for the display adapters it would be very easy to write a book as large as this one on the PC's display adapters alone However this is not that text. This book would hardly be complete without at least mentioning the PC's video display but there are not enough pages remaining in this book to do justice to the subject. Therefore this chapter will discuss the 80 x 25 text display mode that nearly all display adapters support.

Most peripheral devices on the PC use I/O mapped input/output. A program communicates with I/O mapped devices using the 80x86 in out ins and outs instructions accessing devices in the PC's I/O address space. While the video controller chips that appear on PC video display adapters also map registers to the PC's I/O space these cards also employ a second form of I/O addressing: memory mapped I/O input/output. In particular the 80 x 25 text display is nothing more than a two dimensional array of words with each word in the array corresponding a character on the screen. This array appears just above the 640K point in the PC's memory address space. If you store data into this array using standard memory addressing instruction (e.g. mov) you will affect the characters appearing on the display.

There are actually two different arrays you need to worry about. Monochrome system (remember those?) locate their text display starting at location B000:0000 in memory. Color systems locate their text displays at location B800:0000 in memory. These locations are the base addresses of a column major order array declared as follows:

	Display: array [0..79
0..24] of word;

If you prefer to work with row major ordered arrays no problem the video display is equal to the following array definition:

	Display: array [0..24
0..79] of word;

Note that location (0 0) is the upper left hand corner and location (79 24) is the lower right hand corner of the display (the values in parentheses are the x and y coordinates with the x/horizontal coordinate appearing first).

The L.O. byte of each word contains the PC/ASCII code for the character you want to display (see Appendix A for a listing of the PC/ASCII character set). The H.O. byte of each word is the attribute byte. We will return to the attribute byte in the next section.

The display page consumes slightly less than 4 Kilobytes in the memory map. The color display adapters actually provide 32K for text displays and let you select one of eight different displays. Each such display begins on a 4K boundary at address B800:0 B800:1000 B800:2000 B800:3000 ... B800:7000. Note that most modern color display adapters actually provide memory from address A000:0 through B000:FFFF (and more) but the text display only uses the 32K from B800:0..B800:7FFF. In this chapter we will only concern ourselves with the first color display page at address B800:0. However everything discussed in this chapter applies to the other display pages as well.

The monochrome adapter provides only a single display page. Indeed the earliest monochrome display adapters included only 4K on-board memory (contrast this with modern high density color display adapters that have up to four megabytes of on-board memory!).

You can address the memory on the video display like ordinary RAM. You could even store program variables or even code in this memory. However it is never a good idea to do this. First of all any time you write to the display screen you will wipe out any variables stored in the active display memory. Even if you store such code or variables in an inactive display page (e.g. pages one through seven on a color display adapter) using this memory in this manner is not a good idea because access to the display adapter is very slow. Main memory runs two to ten times faster (depending on the machine).

The video attribute associated with each character on the screen controls underlining intensity inverse video and blinking video on monochrome adapters. It controls blinking and character foreground/background colors on color displays. The following diagrams provide the possible attribute values:

To get reverse video on the color display simply swap the foreground and background colors. Note that a foreground color of zero with a background color of seven produces black characters on a white background the standard reverse video colors and the same attribute values you'd use on the monochrome display adapter.

You need to be careful when choosing foreground and background colors for text on a color display adapters. Some combinations are impossible to read (e.g. white characters on a white background). Other colors go together so poorly the text will be extremely difficult to read if not impossible (how about light green letters on a green background?). You must choose your colors with care!

Blinking characters are great for drawing attention to some important text on the screen (like a warning). However it is easy to overdo blinking text on the screen. You should never have more than one word or phrase blinking on the screen at one time. Furthermore you should never leave blinking characters on the screen for any length of time. After a few seconds replace blinking characters with normal characters to avoid annoying the user of your software.

Keep in mind you can easily change the attributes of various characters on the screen without affecting the actual text. Remember the attribute bytes appear at odd addresses in the memory space for the video display. You can easily go in and change these bytes while leaving the character code data alone.

You might ask why anyone would want to bother working directly with the memory mapped display on the PC. After all DOS BIOS and the UCR Standard Library provide much more convenient ways to display text on the screen. Handling new lines (carriage return and line feed) at the end of each line or worse yet scrolling the screen when the display is full is a lot of work. Work that is taken care of for you automatically by the aforementioned routines. Work you have to do yourself if you access screen memory directly. So why bother?

There are two reasons: performance and flexibility. The BIOS video display routines are dreadfully slow. You can easily get a 10 to 100 times performance boost by writing directly to screen memory. For a typical computer science class project this may not be important especially if you're running on a fast machine like a 150 MHz Pentium. On the other hand if you are developing a program that displays and removes several windows or pop-up menus on the screen the BIOS routines won't cut it.

Although the BIOS int 10h functions provide a large set of video I/O routines there will be lots of functions you might want to have that the BIOS just doesn't provide. In such cases going directly to screen memory is one way to solve the problem.

Another difficulty with BIOS routine is that they are not reentrant. You cannot call a BIOS display function from an interrupt service routine nor can you freely call BIOS from concurrently executing processes. However by writing your own video service routines you can easily create a window for each concurrent thread you application is executing. Then each thread can call your routines to display its output independent of the other threads executing on the system.

The AMAZE.ASM program is a good example of a program that directly access the text display by directly storing data into the video display's memory mapped display array. This program access display memory directly because it is more convenient to do so (the screen's display array maps quite nicely to the internal maze array). Simple video games like a space invaders game or a "remove the bricks" game also map nicely to a memory mapped video display.

The following program provides an excellent example of an application that needs to access video memory directly. This program is a screen capture TSR. When you press the left shift key and then the right shift key this program copies the current screen contents to an internal buffer. When you press the right shift key followed by the left shift key this program copies its internal buffer to the display screen. Originally this program was written to capture CodeView screens for the lab manual accompanying this text. There are commercial screen capture programs (e.g. HiJak) that would normally do the job but are incompatible with CodeView. This short TSR allows one to capture screens in CodeView quit CodeView put the CodeView screen back onto the display and the use a program like HiJak to capture the output.

; GRABSCRN.ASM
;
; A short TSR to capture the current display screen and display it later.
;
; Note that this code does not patch into int 2Fh (multiplex interrupt)
; nor can you remove this code from memory except by rebooting.
; If you want to be able to do these two things (as well as check for
; a previous installation)
see the chapter on resident programs. Such
; code was omitted from this program because of length constraints.
;
;
; cseg and EndResident must occur before the standard library segments!

cseg            segment para public 'code'
OldInt9         dword   ?
ScreenSave      byte    4096 dup (?)
cseg            ends

; Marker segment
to find the end of the resident section.

EndResident     segment para public 'Resident'
EndResident     ends

.xlist
include         stdlib.a
includelib      stdlib.lib
.list


RShiftScan      equ     36h
LShiftScan      equ     2ah

; Bits for the shift/modifier keys

RShfBit         equ     1
LShfBit         equ     2

KbdFlags        equ     <byte ptr ds:[17h]>

byp             equ     <byte ptr>


; Screen segment address. This value is for color displays only.
; Change to B000h if you want to use this program on a mono display.

ScreenSeg       equ     0B800h

cseg            segment para public 'code'
assume  ds:nothing



; MyInt9-       INT 9 ISR. This routine reads the keyboard port to see
;               if a shift key scan code just came along. If the right
;               shift bit is set in KbdFlags the a left shift key scan
;               code comes along
we want to copy the data from our
;               internal buffer to the screen's memory. If the left shift
;               bit is set and a right shift key scan code comes along

;               we want to copy the screen memory into our local array.
;               In any case (including none of the above)
we always transfer
;               control to the original INT 9 handler.

MyInt9          proc    far
push    ds
push    ax

mov     ax
40h
mov     ds
ax

in      al
60h                 ;Read the keyboard port.
cmp     al
RShiftScan          ;Right shift just go down?
je      DoRight
cmp     al
LShiftScan          ;How about the left shift?
jne     QuitMyInt9

; If this is the left scan code
see if the right shift key is already
; down.

test    KbdFlags
RShfBit
je      QuitMyInt9              ;Branch if no

; Okay
right shift was down and we just saw left shift
copy our local
; data back to screen memory:

pushf
push    es
push    cx
push    di
push    si
mov     cx
2048
mov     si
cs
mov     ds
si
lea     si
ScreenSave
mov     di
ScreenSeg
mov     es
di
xor     di
di
jmp     DoMove

; Okay
we just saw the right shift key scan code
see if the left shift
; key is already down. If so
save the current screen data to our local
; array.

DoRight:        test    KbdFlags
LShfBit
je      QuitMyInt9

pushf
push    es
push    cx
push    di
push    si
mov     cx
2048
mov     ax
cs
mov     es
ax
lea     di
ScreenSave
mov     si
ScreenSeg
mov     ds
si
xor     si
si

DoMove:         cld
rep     movsw
pop     si
pop     di
pop     cx
pop     es
popf
QuitMyInt9:
pop     ax
pop     ds
jmp     OldInt9
MyInt9          endp



Main            proc
assume  ds:cseg

mov     ax
cseg
mov     ds
ax

print
byte    "Screen capture TSR"
cr
lf
byte    "Pressing left shift
then right shift
captures "
byte    "the current screen."
cr
lf
byte    "Pressing right shift
then left shift
displays "
byte    "the last captured screen."
cr
lf
byte    0

; Patch into the INT 9 interrupt vector. Note that the
; statements above have made cseg the current data segment

; so we can store the old INT 9 value directly into
; the OldInt9 variable.

cli                             ;Turn off interrupts!
mov     ax
0
mov     es
ax
mov     ax
es:[9*4]
mov     word ptr OldInt9
ax
mov     ax
es:[9*4 + 2]
mov     word ptr OldInt9+2
ax
mov     es:[9*4]
offset MyInt9
mov     es:[9*4+2]
cs
sti                             ;Okay
ints back on.


; We're hooked up
the only thing that remains is to terminate and
; stay resident.

print
byte    "Installed."
cr
lf
0

mov     ah
62h                 ;Get this program's PSP
int     21h                     ; value.

mov     dx
EndResident         ;Compute size of program.
sub     dx
bx
mov     ax
3100h               ;DOS TSR command.
int     21h
Main            endp
cseg            ends

sseg            segment para stack 'stack'
stk             db      1024 dup ("stack ")
sseg            ends

zzzzzzseg       segment para public 'zzzzzz'
LastBytes       db      16 dup (?)
zzzzzzseg       ends
end     Main

Chapter Twenty Three: The PC Video Display
29 SEP 1996