assembly - 在模式 13 中读取像素时出现错误数据(386 程序集 - DOS)

Question

对于一个学校项目，我正在为 DOSBOX 编写一个汇编绘图程序。在我的程序中，用户使用鼠标左键以某种颜色绘制像素。为此，我在模式 13 中使用直接写入。用户可以通过右键单击来更改该颜色，这会读取鼠标指向的像素的颜色。

这工作得很好，直到用户启动某个旨在删除屏幕的子例程，并显示可供选择的调色板(具有相同的右键单击功能)。然后，用户右键单击调色板中的一种颜色，使用 int 10h,0dh 读取该颜色。无论用户在调色板屏幕的何处右击，用户随后写入屏幕的颜色始终为白色。

tl;dr 右键单击​​任意位置会正确更改画笔颜色，但如果在调色板模式下完成，后续颜色始终为白色。这是我的代码:

GRAPHICS equ 13h
H_HOTSPOT equ 7
V_HOTSPOT equ 7
H_RES equ 320
V_RES equ 200
PIXELCOUNT equ H_RES*V_RES
DISPLAY_SEG equ 0A000h

EXIT_KEY equ 1071h          ; q key
COLORPICKER_KEY equ 1177h   ; w key

PALETTE_SIZE equ 16
SQUARE_SIZE equ 10
SQUARE_PADDING equ 2
SQUARE_ROW_JUMP equ H_RES - SQUARE_SIZE
PALETTE_ROW_JUMP equ H_RES - (SQUARE_PADDING + SQUARE_SIZE)*PALETTE_SIZE

sseg SEGMENT
    db 256 dup(?)
sseg ENDS

dseg SEGMENT

stdBrushMask    dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 0000000100000000b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b
                dw 1111111011111111b

                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 1111111011111111b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b
                dw 0000000100000000b

stdBrushHotSpots dw 7
                 dw 7

pickerToolMask  dw 1111100001000001b
                dw 1111100000000000b
                dw 1111100000000000b
                dw 1111100000000000b
                dw 1111100000000000b
                dw 1111000000000000b
                dw 1110000000000000b
                dw 1100000000000000b
                dw 1000000000000000b
                dw 1000000000000000b
                dw 1000000000000000b
                dw 1000000000011111b
                dw 0000000000111111b
                dw 0000000001111111b
                dw 0000000011111111b
                dw 0000111111111111b

                dw 0000000000000000b
                dw 0000001100011100b
                dw 0000001111111110b
                dw 0000000111111110b
                dw 0000000111111110b
                dw 0000001111111100b
                dw 0000011111111100b
                dw 0000111111111100b
                dw 0001111111111110b
                dw 0011111111100110b
                dw 0001111111000000b
                dw 0001111110000000b
                dw 0011111100000000b
                dw 0111001000000000b
                dw 0110000000000000b
                dw 0000000000000000b

pickerToolHotSpots dw 1
                   dw 14

foreground_color dw 000fh
background_color dw 0000h
colorpicker_flag db 00

prev_position    dw H_RES   ;   Cache of the last drawn pixel's coordinates, to avoid duplicate writes.
                 dw V_RES

pos_backup       dw H_RES/2
                 dw V_RES/2 ;   Cache of the mouse position before switching to palette mode or back.

video_mode_not_available db "Mode 13h is not supported on this computer. To use this program, get a VGA or MCGA graphics card / monitor.$"

display_backup db PIXELCOUNT dup(?)
dseg ENDS

cseg SEGMENT
assume cs:cseg, ds:dseg, ss:sseg

;---------------Mouse Procs-------------
stdBrush PROC
    push bx cx ax dx
    mov ax, dseg
    mov es, ax
    mov bx, stdBrushHotSpots
    mov cx, stdBrushHotSpots + 2
    mov ax, 9
    mov dx, offset stdBrushMask
    int 33h
    mov ax, DISPLAY_SEG
    mov es, ax
    pop dx ax cx bx
    ret
stdBrush ENDP

pickerTool PROC
    push bx cx ax dx
    mov ax, dseg
    mov es, ax
    mov bx, pickerToolHotSpots
    mov cx, pickerToolHotSpots + 2
    mov ax, 9
    mov dx, offset pickerToolMask
    int 33h
    mov ax, DISPLAY_SEG
    mov es, ax
    pop dx ax cx bx
    ret
pickerTool ENDP

mouseReset PROC
    push ax
    mov ax, 0
    int 33h
    pop ax
    ret
mouseReset ENDP

showCursor PROC
    push ax
    mov ax, 1
    int 33h
    pop ax
    ret
showCursor ENDP

hideCursor PROC
    push ax
    mov ax, 2
    int 33h
    pop ax
    ret
hideCursor ENDP

getCursorStat PROC
    push ax
    mov ax, 3
    int 33h
    pop ax
    ret
getCursorStat ENDP

initCursor PROC
    mov ax, dseg
    mov es, ax
    call mouseReset
    call stdBrush
    call showCursor
    mov ax, DISPLAY_SEG
    mov es, ax
    ret
initCursor ENDP

;--------------------Graphics Procs-------------
graphicsCompat PROC
    ; Checks if mode 13h is supported on this machine.
    mov ax, 1a00h
    int 10h ; Get display combination
    cmp al, 1ah
    je mode_13_supported
    mov ah, 9
    mov dx, offset video_mode_not_available
    int 21h ; Display error message
    inc sp  ; Discard the return address
    inc sp
    push offset exit    ; Replace return address with exit procedure address
    ret                 ; Return to exit procedure.

    mode_13_supported:
        ;No error, mode 13 is supported
    ret
graphicsCompat ENDP

graphicsMode PROC
    mov ah, 0
    mov al, GRAPHICS
    int 10h
    ret
graphicsMode ENDP

pixel PROC
    push bp
    mov bp, sp
    push ax bx cx dx
    xor bx, bx
    mov cx, ss:[bp+6]   ; x coord
    mov dx, ss:[bp+4]   ; y coord
    cmp cx, H_RES
    jnc pixel_outofbounds
    cmp dx, V_RES
    jnc pixel_outofbounds
    mov ax, H_RES
    mul dx
    add ax, cx
    mov di, ax      ; Puts the pixel's offset in di
    mov ax, ss:[bp+8]   ; read color argument
    stosb
  pixel_outofbounds:
    pop dx cx bx ax bp
    ret 6
pixel ENDP

backupScreen PROC
    push ax di si
    mov ax, DISPLAY_SEG
    mov ds, ax
    mov ax, dseg
    mov es, ax
    mov cx, PIXELCOUNT
    mov si, 0
    mov di, offset display_backup
    call hideCursor
    rep movsb
    call showCursor
    mov ds, ax
    mov ax, DISPLAY_SEG
    mov es, ax
    pop si di ax
    ret
backupScreen ENDP

restoreScreen PROC
    push di si
    mov cx, PIXELCOUNT
    mov si, offset display_backup
    mov di, 0
    rep movsb
    pop si di
    ret
restoreScreen ENDP

;-------------------Colorpicker Procs----------
paletteModeToggle PROC
    push ax
    call hideCursor
    mov pos_backup, cx
    mov pos_backup+2, dx
    mov al, colorpicker_flag
    not al
    mov colorpicker_flag, al
    test al, al
    jz palette_mode_off
    palette_mode_on:
        call backupScreen
        call graphicsMode
        call paletteDraw
        call showCursor
        call pickerTool
        pop ax
        jmp input_loop
    palette_mode_off:
        call graphicsMode
        call restoreScreen
        call showCursor
        call stdBrush
    pop ax
    jmp input_loop
paletteModeToggle ENDP

paletteSquareDraw PROC
    push bp
    mov bp, sp
    push ax cx dx

    mov dx, ss:[bp+4]   ; retrieves ypos
    mov cx, ss:[bp+6]   ; retrieves xpos
    mov ax, H_RES
    mul dx
    add ax, cx
    mov di, ax  ; Puts the real pixel address in di.
    mov ax, ss:[bp+8]   ;retrieves color

    xor ch, ch
    square_loop:
        xor cl, cl
        square_row_loop:
            stosb
            inc cl  ; increment horizontal counter
            cmp cl, SQUARE_SIZE
            jb square_row_loop
        inc ch                      ; increment the vertical counter
        add di, SQUARE_ROW_JUMP     ; move di to the start of the next row
        cmp ch, SQUARE_SIZE
        jb square_loop

    pop dx cx ax bp
    ret 6
paletteSquareDraw ENDP

paletteDraw PROC

    xor cx, cx  ; xpos = 0
    xor dx, dx  ; ypos = 0
    xor ax, ax  ; color = 0
    xor bx, bx  ; squarecounter = 0

    palette_loop:   ; for i in paletterows:
        xor bl, bl
        add dx, SQUARE_PADDING
        palette_row_loop:   ;   for j in palettecols:
            add cx, SQUARE_PADDING  ; xpos += padding
            push ax
            push cx
            push dx
            call paletteSquareDraw  ; DrawSquare()

            add cx, SQUARE_SIZE
            inc ax  ; next color
            inc bl  ; squarecounter++
            cmp bl, PALETTE_SIZE
            jb palette_row_loop
        ; prep for new row
        xor bl, bl
        xor cx, cx
        add dx, SQUARE_SIZE
        inc bh
        cmp bh, PALETTE_SIZE
        jb palette_loop
        ret

paletteDraw ENDP

pickColor PROC
    push bp
    mov bp, sp
    push ax dx si
    mov dx, ss:[bp+4]
    mov cx, ss:[bp+6]
    xor bh, bh
    mov ah, 0dh
    int 10h
    mov foreground_color, ax
    pop si dx ax bp
    ret 4
pickColor ENDP

;--------------------------Program Entry-------------------------
main:
    mov ax, dseg
    mov ds, ax
    mov ax, sseg
    mov ss, ax
    mov ax, DISPLAY_SEG
    mov es, ax

    call graphicsCompat
    call graphicsMode
    call mouseReset
    call stdBrush
    call showCursor
    mov ax, DISPLAY_SEG
    mov es, ax

input_loop:

    ;------------Mouse Input Detection------------------------------------
    call getCursorStat
    shr bx, 1
    jnc no_left_click
    ; Left click detected
    mov al, colorpicker_flag
    test al, al
    jnz no_left_click   ; no left clicks in palette mode.
    shr cx, 1
    cmp cx, prev_position
    jnz draw_pixel
    cmp dx, prev_position + 2
    jz no_left_click
  draw_pixel:
    call hideCursor
    mov ax, foreground_color
    mov prev_position, cx
    mov prev_position + 2, dx
    push ax cx dx
    call pixel
    call showCursor


  no_left_click:
    mov bx, 1
    mov ax, 6
    int 33h
    test bx, bx
    jz keyboard_detection
    ; Right click detected
    shr cx, 1
    push cx dx
    call pickColor
  keyboard_detection:
    ;------------Keyboard Input Detection----------------------------------
    mov ah, 1
    int 16h
    jz no_keystroke
    xor ah, ah
    int 16h
    cmp ax, EXIT_KEY
    je exit
    cmp ax, COLORPICKER_KEY
    jne no_palette_toggle
    jmp paletteModeToggle
  no_palette_toggle:
  no_keystroke:
    jmp input_loop

exit:
    mov al, 3
    mov ah, 0
    int 10h
    mov ax, 4c00h
    int 21h

cseg ENDS
end main

为清楚起见，截图: 在绘图模式下使用蓝色笔刷。 切换到调色板模式后，我右键单击亮绿色方 block 。 切换回绘图模式后，画笔正在绘制白色像素。 在绘图模式下右键单击蓝色像素。 画笔颜色正确切换为蓝色。

在这个版本中我使用 int 10h,0dh 来读取像素，但是当我尝试直接从段 A000 读取时出现了同样的问题(在调试期间，当屏幕明显不是时，它似乎完全被零填充空的)。我转而使用 int 10h 来查看直接读取是否是问题所在。

虽然我确实使用了直接写入。

底线问题:我在读取这些像素时做错了什么？为什么我的程序最初能正确读取像素颜色，但在清空屏幕、更改光标并绘制一堆彩色方 block 后，它只读取白色？为什么在清除这些方 block 并将屏幕恢复到之前的状态后它又起作用了？

Answer 1

您的问题是鼠标驱动程序使用软件光标。 VGA 卡不支持硬件光标，这意味着为了显示光标，鼠标驱动程序需要将其绘制到 VGA 帧缓冲区。这意味着当显示选择器光标并且您读取当前鼠标位置的像素时，您实际上正在读取选择器光标热点的颜色，即白色。使用画笔光标时不会发生此问题，因为热点处的像素是透明的。