在模式 13 中读取像素时出现错误数据（386 程序集 - DOS）
Wrong data when reading pixels in mode 13 (386 Assembly - DOS)
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，直到用户启动某个子程序，该子程序旨在擦除屏幕，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 右键单击任意位置可正确更改画笔颜色，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块。
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了int 10h,0dh来读取像素，但是当我试图直接从段A000读取时出现了同样的问题（在调试过程中，屏幕显示时似乎完全被零填充了）显然不是空的）。我改用 int 10h 看看直接读取是否有问题。
虽然我确实使用了直接写入。
底线问题：读取这些像素我做错了什么？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块后，它只读取白色？为什么在清除那些方块并将屏幕恢复到之前的状态后它又起作用了？
您的问题是鼠标驱动程序使用了软件光标。 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。使用笔刷光标时不会出现此问题，因为热点处的像素是透明的。
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，直到用户启动某个子程序，该子程序旨在擦除屏幕，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 右键单击任意位置可正确更改画笔颜色，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：
在这个版本中我使用了int 10h,0dh来读取像素，但是当我试图直接从段A000读取时出现了同样的问题（在调试过程中，屏幕显示时似乎完全被零填充了）显然不是空的）。我改用 int 10h 看看直接读取是否有问题。
虽然我确实使用了直接写入。
底线问题：读取这些像素我做错了什么？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块后，它只读取白色？为什么在清除那些方块并将屏幕恢复到之前的状态后它又起作用了？
您的问题是鼠标驱动程序使用了软件光标。 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。使用笔刷光标时不会出现此问题，因为热点处的像素是透明的。