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Finally, after another keypress, Listing 30.1 halts.
LISTING 30.1 L30-1.ASM
; Demonstrates the VGA/EGA split screen in action.
;
;*********************************************************************
IS_VGA equ 1 ;set to 0 to assemble for EGA
;
VGA_SEGMENT equ 0a000h
SCREEN_WIDTH equ 640
SCREEN_HEIGHT equ 350
CRTC_INDEX equ 3d4h ;CRT Controller Index register
OVERFLOW equ 7 ;index of Overflow reg in CRTC
MAXIMUM_SCAN_LINEequ 9 ;index of Maximum Scan Line register
; in CRTC
START_ADDRESS_HIGH equ 0ch ;index of Start Address High register
; in CRTC
START_ADDRESS_LOW equ 0dh ;index of Start Address Low register
; in CRTC
LINE_COMPARE equ 18h ;index of Line Compare reg (bits 7-0
; of split screen start scan line)
; in CRTC
INPUT_STATUS_0 equ 3dah ;Input Status 0 register
WORD_OUTS_OK equ 1 ;set to 0 to assemble for
; computers that can't handle
; word outs to indexed VGA registers
;*********************************************************************
; Macro to output a word value to a port.
;
OUT_WORD macro
if WORD_OUTS_OK
out dx,ax
else
out dx,al
inc dx
xchg ah,al
out dx,al
dec dx
xchg ah,al
endif
endm
;*********************************************************************
MyStack segment para stack 'STACK'
db 512 dup (0)
MyStack ends
;*********************************************************************
Data segment
SplitScreenLine dw ? ;line the split screen currently
; starts after
StartAddress dw ? ;display memory offset at which
; scanning for video data starts
; Message displayed in split screen.
SplitScreenMsg db 'Split screen text row #'
DigitInsert dw ?
db '...$'
Data ends
;*********************************************************************
Code segment
assume cs:Code, ds:Data
;*********************************************************************
Start proc near
mov ax,Data
mov ds,ax
;
; Select mode 10h, 640x350 16-color graphics mode.
;
mov ax,0010h ;AH=0 is select mode function
;AL=10h is mode to select,
; 640x350 16-color graphics mode
int 10h
;
; Put text into display memory starting at offset 0, with each row
; labelled as to number. This is the part of memory that will be
; displayed in the split screen portion of the display.
;
mov cx,25 ;# of lines of text we'll draw into
; the split screen part of memory
FillSplitScreenLoop:
mov ah,2 ;set cursor location function #
sub bh,bh ;set cursor in page 0
mov dh,25
sub dh,cl ;calculate row to draw in
sub dl,dl ;start in column 0
int 10h ;set the cursor location
mov al,25
sub al,cl ;calculate row to draw in again
sub ah,ah ;make the value a word for division
mov dh,10
div dh ;split the row # into two digits
add ax,'00' ;convert the digits to ASCII
mov [DigitInsert],ax ;put the digits into the text
; to be displayed
mov ah,9
mov dx,offset SplitScreenMsg
int 21h ;print the text
loop FillSplitScreenLoop
;
; Fill display memory starting at 8000h with a diagonally striped
; pattern.
;
mov ax,VGA_SEGMENT
mov es,ax
mov di,8000h
mov dx,SCREEN_HEIGHT ;fill all lines
mov ax,8888h ;starting fill pattern
cld
RowLoop:
mov cx,SCREEN_WIDTH/8/2 ;fill 1 scan line a word at a time
rep stosw ;fill the scan line
ror ax,1 ;shift pattern word
dec dx
jnz RowLoop
;
; Set the start address to 8000h and display that part of memory.
;
mov [StartAddress],8000h
call SetStartAddress
;
; Slide the split screen half way up the screen and then back down
; a quarter of the screen.
;
mov [SplitScreenLine],SCREEN_HEIGHT-1
;set the initial line just off
; the bottom of the screen
mov cx,SCREEN_HEIGHT/2
call SplitScreenUp
mov cx,SCREEN_HEIGHT/4
call SplitScreenDown
;
; Now move up another half a screen and then back down a quarter.
;
mov cx,SCREEN_HEIGHT/2
call SplitScreenUp
mov cx,SCREEN_HEIGHT/4
call SplitScreenDown
;
; Finally move up to the top of the screen.
;
mov cx,SCREEN_HEIGHT/2-2
call SplitScreenUp
;
; Wait for a key press (don't echo character).
;
mov ah,8 ;DOS console input without echo function
int 21h
;
; Turn the split screen off.
;
mov [SplitScreenLine],0ffffh
call SetSplitScreenScanLine
;
; Wait for a key press (don't echo character).
;
mov ah,8 ;DOS console input without echo function
int 21h
;
; Display the memory at 0 (the same memory the split screen displays).
;
mov [StartAddress],0
call SetStartAddress
;
; Flip between the split screen and the normal screen every 10th
; frame until a key is pressed.
;
FlipLoop:
xor [SplitScreenLine],0ffffh
call SetSplitScreenScanLine
mov cx,10
CountVerticalSyncsLoop:
call WaitForVerticalSyncEnd
loop CountVerticalSyncsLoop
mov ah,0bh ;DOS character available status
int 21h
and al,al ;character available?
jz FlipLoop ;no, toggle split screen on/off status
mov ah,1
int 21h ;clear the character
;
; Return to text mode and DOS.
;
mov ax,0003h ;AH=0 is select mode function
;AL=3 is mode to select, text mode
int 10h ;return to text mode
mov ah,4ch
int 21h ;return to DOS
Startendp
;*********************************************************************
; Waits for the leading edge of the vertical sync pulse.
;
; Input: none
;
; Output: none
;
; Registers altered: AL, DX
;
WaitForVerticalSyncStartprocnear
mov dx,INPUT_STATUS_0
WaitNotVerticalSync:
in al,dx
test al,08h
jnz WaitNotVerticalSync
WaitVerticalSync:
in al,dx
test al,08h
jz WaitVerticalSync
ret
WaitForVerticalSyncStart endp
;*********************************************************************
; Waits for the trailing edge of the vertical sync pulse.
;
; Input: none
;
; Output: none
;
; Registers altered: AL, DX
;
WaitForVerticalSyncEndprocnear
mov dx,INPUT_STATUS_0
WaitVerticalSync2:
in al,dx
test al,08h
jz WaitVerticalSync2
WaitNotVerticalSync2:
in al,dx
test al,08h
jnz WaitNotVerticalSync2
ret
WaitForVerticalSyncEndendp
;*********************************************************************
; Sets the start address to the value specifed by StartAddress.
; Wait for the trailing edge of vertical sync before setting so that
; one half of the address isn't loaded before the start of the frame
; and the other half after, resulting in flicker as one frame is
; displayed with mismatched halves. The new start address won't be
; loaded until the start of the next frame; that is, one full frame
; will be displayed before the new start address takes effect.
;
; Input: none
;
; Output: none
;
; Registers altered: AX, DX
;
SetStartAddress proc near
call WaitForVerticalSyncEnd
mov dx,CRTC_INDEX
mov al,START_ADDRESS_HIGH
mov ah,byte ptr [StartAddress+1]
cli ;make sure both registers get set at once
OUT_WORD
mov al,START_ADDRESS_LOW
mov ah,byte ptr [StartAddress]
OUT_WORD
sti
ret
SetStartAddress endp
;*********************************************************************
; Sets the scan line the split screen starts after to the scan line
; specified by SplitScreenLine.
;
; Input: none
;
; Output: none
;
; All registers preserved
;
SetSplitScreenScanLine proc near
push ax
push cx
push dx
;
; Wait for the leading edge of the vertical sync pulse. This ensures
; that we don't get mismatched portions of the split screen setting
; while setting the two or three split screen registers (register 18h
; set but register 7 not yet set when a match occurs, for example),
; which could produce brief flickering.
;
call WaitForVerticalSyncStart
;
; Set the split screen scan line.
;
mov dx,CRTC_INDEX
mov ah,byte ptr [SplitScreenLine]
mov al,LINE_COMPARE
cli ;make sure all the registers get set at once
OUT_WORD ;set bits 7-0 of the split screen scan line
mov ah,byte ptr [SplitScreenLine+1]
and ah,1
mov cl,4
shl ah,cl ;move bit 8 of the split split screen scan
; line into position for the Overflow reg
mov al,OVERFLOW
if IS_VGA
;
; The Split Screen, Overflow, and Line Compare registers all contain
; part of the split screen start scan line on the VGA. We'll take
; advantage of the readable registers of the VGA to leave other bits
; in the registers we access undisturbed.
;
out dx,al ;set CRTC Index reg to point to Overflow
inc dx ;point to CRTC Data reg
in al,dx ;get the current Overflow reg setting
and al,not 10h ;turn off split screen bit 8
or al,ah ;insert the new split screen bit 8
; (works in any mode)
out dx,al ;set the new split screen bit 8
dec dx ;point to CRTC Index reg
mov ah,byte ptr [SplitScreenLine+1]
and ah,2
mov cl,3
ror ah,cl ;move bit 9 of the split split screen scan
; line into position for the Maximum Scan
; Line register
mov al,MAXIMUM_SCAN_LINE
out dx,al ;set CRTC Index reg to point to Maximum
; Scan Line
inc dx ;point to CRTC Data reg
in al,dx ;get the current Maximum Scan Line setting
and al,not 40h ;turn off split screen bit 9
or al,ah ;insert the new split screen bit 9
; (works in any mode)
out dx,al ;set the new split screen bit 9
else
;
; Only the Split Screen and Overflow registers contain part of the
; Split Screen start scan line and need to be set on the EGA.
; EGA registers are not readable, so we have to set the non-split
; screen bits of the Overflow register to a preset value, in this
; case the value for 350-scan-line modes.
;
or ah,0fh ;insert the new split screen bit 8
; (only works in 350-scan-line EGA modes)
OUT_WORD ;set the new split screen bit 8
endif
sti
pop dx
pop cx
pop ax
ret
SetSplitScreenScanLine endp
;*********************************************************************
; Moves the split screen up the specified number of scan lines.
;
; Input: CX = # of scan lines to move the split screen up by
;
; Output: none
;
; Registers altered: CX
;
SplitScreenUp proc near
SplitScreenUpLoop:
dec [SplitScreenLine]
call SetSplitScreenScanLine
loop SplitScreenUpLoop
ret
SplitScreenUp endp
;*********************************************************************
; Moves the split screen down the specified number of scan lines.
;
; Input: CX = # of scan lines to move the split screen down by
;
; Output: none
;
; Registers altered: CX
;
SplitScreenDown proc near
SplitScreenDownLoop:
inc [SplitScreenLine]
call SetSplitScreenScanLine
loop SplitScreenDownLoop
ret
SplitScreenDown endp
;*********************************************************************
Code ends
end Start
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