double buffer video in assembler -


i'm trying double buffer video in assembler, in case have problem, don't know how solve , can't close after key pressed. apparently problem in inc di:

(i'm trying paint 320*200 pixel white color buffer)

.model small .386 .stack 400h .data  modovideo db ? vram dw 0  xval dw ? yval dw ?  .code main proc    mov ax,@data    mov ds,ax    mov ah,0fh   int 10h   mov modovideo,al    mov ah,0   mov al,13h   int 10h    ; segmento de memoria =====================================    mov ah,48h   mov bx,4000 ; 64000/16   int 21h   mov vram,ax    ; escribir en el segmento de memoria =======================================  mov es,vram  ;offset = 320*y + x     ;mov xval,160     ;mov yval,100     ;mov ax,320     ;mul yval     ;add ax,xval   mov di,0 mov al,7  mov cx,640 paso1:     mov es:[di],al     inc di ; <-----------     loop paso1  ; volcar sobre pantalla ======================================================  mov ds,vram xor si,si mov dx,0a000h mov es,dx xor di,di mov cx,64000 rep movsb     mov ah,1 int 21h  salir: mov al,modovideo mov ah,0 int 10h  mov ah,4ch int 21h      main endp

a few things:

  • check return value when allocate memory (carry set on error, ax contains error code, 8 meaning out of memory) [note if you're using com file of available memory allocated program , allocation fail]
  • you need clear/set direction flag before using string instructions (cld in case, see e.g. here)
  • after trashing ds in copying need restore before accessing modovideo (it uses ds implicitly).

with changes (and make work in com file) should work. used nasm (compile nasm -f bin -o gfx.com gfx.asm) , dosbox test it. note method employ allocate memory buggy it's been while since programmed dos.

     org 0x100  start:     ; allocate buffer %if 0     ; use 21h call allocate memory (for exe files)     mov ah, 0x48     mov bx, 64000/16      int 0x21     jc error ; carry set on error     mov [vscr_seg], ax %else      ; com files (most/all) available memory, grab of     mov bx, word [0x0002] ; last paragraph psp     sub bx, 64000/16 ; make room buffer     mov [vscr_seg], bx      mov ax, ds     add ax, 0x1000 ; program start paragraph + 64k (max of com file)     cmp ax, bx ; did fit?     jae error %endif     ; clear buffer     mov es, [vscr_seg]     xor di, di     xor ax, ax     mov cx, 32000     cld     rep stosw       ; previous video mode     mov ah, 0x0f     int 0x10     mov [previous_video_mode], al      ; set mode 13h (320x200 256 colors)     mov ax, 0x0013     int 0x10      ; fill half buffer color 15     mov es, [vscr_seg]     xor di, di     mov ax, 0x0f0f     mov cx, 16000     cld     rep stosw      ; , fill in pixel colors @ bottom row     mov di, 199 * 320     mov cx, 255     xor al, al .fill:     ; below 2 instructions equal stosb when direction flag cleared     mov [es:di], al      inc di      inc al     loop .fill      ; copy buffer screen     push ds ; remember save ds!     mov ds, [vscr_seg]     mov ax, 0xa000     mov es, ax     xor di, di     xor si, si     mov cx, 32000     cld     rep movsw     pop ds ; ... , restore again      ; wait keypress     mov ah, 0x01     int 0x21      ; restore video mode     mov ah, 0x00     mov al, [previous_video_mode]     int 0x10      ; skip error block     jmp exit  error:     ; print bx , ax facilitate debugging     push ax     call print_hex_word     pop ax     mov bx, ax     call print_hex_word      mov ah, 0x09     mov dx, error_string     int 0x21  exit:     ; exit     mov ax, 0x4c00     int 0x21  ; print 16-bit word in bx, trashes ax , dx (at least) print_hex_word:     mov dx, bx     shr dx, 12     call print_hex_digit     mov dl, bh     call print_hex_digit     mov dl, bl     shr dl, 4     call print_hex_digit     mov dl, bl     call print_hex_digit     ; new line     mov ah, 0x02     mov dl, 13     int 0x21     mov dl, 10     int 0x21     ret  print_hex_digit:     push bx     , dl, 0x0f     add dl, '0'     cmp dl, '9'     jle .print     add dl, 'a' - '0' - 10 .print:     mov ah, 0x02     int 0x21     pop bx     ret   previous_video_mode db 0 vscr_seg dw 0 error_string db 'error occurred!', 13, 10, 7, '$'

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