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几天来，我一直在尝试实现 Bresenham 的线路算法，但一直面临着奇怪的问题。我能够画出一些线条，但其他线条似乎不起作用并导致无限循环。我花了几个小时试图让 Qemu 工作并尝试了一些模拟器，但似乎无法解决问题。下面是我应该画线的代码。

drawline:
; r0 = screen_address
; r1 = x0
; r2 = y0
; r3 = x1
; r4 = y1
; r5 = colour

push {r6-r10}   ; store the registers we will alter, thoughtful, right?
cmp r1,r3       ; compare x0 and x1
subgt r6,r1,r3  ; deltax = x0 - x1
movgt r7,#-1    ; stepx = -1
suble r6,r3,r1  ; deltax = x1 - x0
movle r7,#1     ; stepx = 1

cmp r2, r4      ; compare y1 and y0
subgt r8,r4,r2  ; deltay = y1 - y0
movgt r9,#-1    ; stepy = -1
suble r8,r2,r4  ; deltay = y0 - y1
movle r9,#1     ; stepy = 1

add r10,r6,r8   ; error = deltax - deltay

line_pixel_loop:
    teq r1,r3
    teqeq r2,r4
    popeq {r6-r10}
    bxeq lr

    push {r3, lr}   ; store registers which we will alter
    ; the following are inferred:
    ; r0 = screen address
    ; r1 = x, current
    ; r2 = y, current
    mov r3,r5       ; colour
    bl drawpixel    ; draw the pixel
    pop {r3, lr}    ; restore the altered registers

    cmp r8,r10, lsl#1   ; compare deltay (now negative) with 2 * error
    addle r10, r8       ; if less than: error += deltay
    addle r1, r7        ; if less than: x0 += stepx

    cmp r6,r10, lsl #1  ; compare deltax with 2 * error
    addge r10, r6       ; if greater than: error += stepx
    addge r2, r9        ; if greater than: y0 += stepy

    ; cmp r1,r3         ; compare x0 and x1
    ; cmpeq r2,r4   ; if equal: compare y0 and y1
    ; bne line_pixel_loop   ; if not equal, continue the loop
    b line_pixel_loop


pop {r6-r10} ; clean up

bx lr ; return

这两条线毫无问题地绘制：

push {r0-r5}
mov r0,r7     ;screen
mov r1,#300   ;x0
mov r2,#100   ;y0
mov r3,#200   ;x1
mov r4,#200   ;y1
mov r5,r6     ;colour
bl drawline
pop {r0-r5}

push {r0-r5}
mov r0,r7     ;screen
mov r1,#300   ;x0
mov r2,#100   ;y0
mov r3,#400   ;x1
mov r4,#200   ;y1
mov r5,r6     ;colour
bl drawline
pop {r0-r5}

这条线不会绘制并阻止程序进一步执行：

push {r0-r5}
mov r0,r7     ;screen
mov r1,#350   ;x0
mov r2,#265   ;y0
mov r3,#353   ;x1
mov r4,#266   ;y1
mov r5,r6     ;colour
bl drawline
pop {r0-r5}
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