Programmer 7500/MAX_PROGRAMMERS.iso/INFO/PROGJRN/PJ_7_6.ARC/SPLITSCN.ASM

Assembly Source File | 1989-08-13 | 10.3 KB | 376 lines

;
; *** Listing 1 ***
;
; 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_LINE equ    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
Start    endp
;*********************************************************************
; Waits for the leading edge of the vertical sync pulse.
;
; Input: none
;
; Output: none
;
; Registers altered: AL, DX
;
WaitForVerticalSyncStart    proc    near
    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
;
WaitForVerticalSyncEnd    proc    near
    mov    dx,INPUT_STATUS_0
WaitVerticalSync2:
    in    al,dx
    test    al,08h
    jz    WaitVerticalSync2
WaitNotVerticalSync2:
    in    al,dx
    test    al,08h
    jnz    WaitNotVerticalSync2
    ret
WaitForVerticalSyncEnd    endp
;*********************************************************************
; 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