Allright folks, let's hope this is an easy one: I need to access an array to achieve double-buffering (I use mode 13h) in 16-bit TASM. BUT: No matter if I use "OFFSET", "BYTE PTR [Array]", "BYTE PTR Array", or whatever I have tried already, the program reads/writes to the incorrect memory block, which is partly behind the actual start of the array.
Heres my (for now not really optimised and very messy) code:
.MODEL MEDIUM
.STACK
.DATA
XPos DW 0
YPos DB 0
Color DB 0
BoxX1 DW 0
BoxY1 DB 0
BoxX2 DW 0
BoxY2 DB 0
VPage DB 64010 DUP(0) ;TODO: Size *might* be incorrect.
PageSeg DW 0
.CODE
SetVGA13 PROC
MOV AX, 0013h ;Screen mode 13.
INT 10h ;Set screen mode to AX.
MOV AX, 0A000h ;Screen segment.
MOV ES, AX ;You can't affect segment registers
RET
ENDP
;-------DrawPixel---------------
; WORD XPos = x
; WORD YPos = y
; BYTE Color = colour
;-------------------------------
DrawPixel PROC
XOR AH, AH
MOV AL, [YPos]
MOV DX, 320
MUL DX
ADD AX, [XPos]
MOV DI, AX
MOV AL, [Color]
MOV ES, [PageSeg]
;ADD ES, DI
MOV ES:[DI],AL
;MOV ES:[DI],AL
RET
ENDP
DrawBox PROC
MOV CL, [BoxY1]
YLoop:
MOV BL, CL
PUSH CX
MOV CX, [BoxX1]
XLoop:
MOV [XPos], CX
MOV [YPos], BL
MOV [Color],CL
CALL DrawPixel
INC CX
CMP CX, [BoxX2]
JNZ XLoop
POP CX
INC CL
CMP CL, [BoxY2]
JNZ YLoop
RET
ENDP
WaitFrame PROC
PUSH DX
; Port #03DA contains VGA status
MOV DX, 03DAh
IN AL, DX
WaitRetrace:
; Bit 3 will be on if we're in retrace
TEST AL, 08h
JNZ WaitRetrace
EndRefresh:
IN AL, DX
TEST AL, 08h
JZ EndRefresh
POP DX
RET
ENDP
RestoreVideo PROC
; Return to text mode
MOV AX, 03h
INT 10h
RET
ENDP
ClearScreen PROC
XOR CX, CX
;MOV ES, [PageSeg]
ClearLoop:
MOV DI, CX
;MOV ES, [PageSeg]
MOV BX, OFFSET VPage
ADD BX, CX
MOV AL, [BX];VPage[DI];ES:[DI]
MOV [Color],AL
MOV AX, 0A000h
MOV ES, AX
MOV AL, [Color]
MOV ES:[DI],AL
INC CX
CMP CX, 64000
JNZ ClearLoop
RET
ENDP
Main:
;INITIALISE
MOV BX, OFFSET VPage
MOV [PageSeg],BX
CALL SetVGA13
;CALL MakePalette
MOV [BoxX1],33
MOV [BoxY1],33
MOV [BoxX2],99
MOV [BoxY2],99
;LOOP
GameLoop:
;DRAW
;CALL DrawBox
CALL ClearScreen
;CALL WaitFrame
;INPUT
MOV DX, 60h
IN AL, DX
CMP AL, 75
JNZ NotLeft
SUB [BoxX1],1
SUB [BoxX2],1
NotLeft:
IN AL, DX
CMP AL, 77
JNZ NotRight
ADD [BoxX1],1
ADD [BoxX2],1
NotRight:
CMP AL, 1
JNZ GameLoop
;END PROGRAM
Error:
;CALL ClearScreen
CALL RestoreVideo
MOV AH, 4Ch
INT 21h
END Main
This code shows a rainbow coloured box that you can move around with the left and right arrow keys,
;INITIALISE
MOV BX, OFFSET VPage
MOV [PageSeg],BX
That is my sad attempt to getbthe pointer to my buffer, but doesnt return the correct one
Sorry that my question was not done, i realised that when i got out of bed immediately for some reason.
Although this is not how I would code things, I'll offer up a couple of suggestions that may get you closer to resolving your problems.
When your executable loads the DS and ES registers originally point to the DOS PSP for your program. In your case you need to at least point DS to your DATA segment. The DATA segment that is filled in at run time by the DOS EXE loader can be referenced in your code by prefixing the segment name with an @ (AT) symbol. So you can replace this code:
;INITIALISE
MOV BX, OFFSET VPage
MOV [PageSeg],BX
With this:
;INITIALISE
MOV BX, @DATA ; Set up DS with our program's DATA segment
MOV DS, BX
MOV [PageSeg],BX ; VPage is in DATA segment, move segment to PageSeg
In your DrawPixel code you'll need to add the VPage offset to DI. Replace this code:
XOR AH, AH
MOV AL, [YPos]
MOV DX, 320
MUL DX
ADD AX, [XPos]
MOV DI, AX
MOV AL, [Color]
MOV ES, [PageSeg]
MOV ES:[DI],AL
with:
XOR AH, AH
MOV AL, [YPos]
MOV DX, 320
MUL DX
ADD AX, [XPos]
MOV DI, AX
MOV AL, [Color]
MOV ES, [PageSeg]
ADD DI, OFFSET VPage ; We need to add VPage's offset
; from beginning of PageSeg
MOV ES:[DI],AL
Video mode 13h is 320x200x256 colors. The amount of video ram that is needed would be 320*200*1 = 64000. This could be changed from:
VPage DB 64010 DUP(0) ;TODO: Size *might* be incorrect.
to:
VPage DB 64000 DUP(0)
With these changes the program seems to display a rainbow box and moves left and right on the screen with the arrow keys.
There may be other bugs but I don't know what you were trying to achieve. This code could be simplified greatly. My changes were the minimal ones to work with the way you coded your program and make it somewhat functional.