汇编语言程序——用A逆时针画矩形

要求：从屏幕左上方开始逆时针用‘A’画一个矩形。

首先知道屏幕长80字符、宽25字符，用x,y记录当前位置，然后循环控制方向，到达边界转变方向即可。这里做了点扩展，由外向内填满整个屏幕。更多细节的完善和扩展请自行实现。

;des 表示方向 1下 2右 3上 四左
    delay equ 500
    org 100h
start:
    mov ax,cs
    mov ds,ax
    mov ss,ax
    mov ax,0B800h
    mov es,ax
    mov byte[char],'A'
loop:   ;大循环，里面有四个小循环
    mov ax,word[i]
    mov bx,13
    sub ax,bx
    jz  end
    mov ax,word[i]
    mov word[y],ax
    dec ax
    mov word[x],ax
    mov byte[des],1
    inc word[i]
down:
    inc word[x]
    mov bx,word[x]
    mov ax,26
    sub ax,word[i]
    sub ax,bx
    jz  d2r
    jmp show
d2r:
    mov ax,25
    sub ax,word[i]
    mov word[x],ax
    mov byte[des],2
right:
    inc word[y]
    mov bx,word[y]
    mov ax,81
    sub ax,word[i]
    sub ax,bx
    jz  r2u
    jmp show
r2u:
    mov ax,80
    sub ax,word[i]
    mov word[y],ax
    mov byte[des],3
up:
    dec word[x]
    mov bx,word[x]
    mov ax,-2
    add ax,word[i]
    sub ax,bx
    jz  u2l
    jmp show
u2l:
    mov ax,-1
    add ax,word[i]
    mov word[x],ax
    mov byte[des],4
left:
    dec word[y]
    mov bx,word[y]
    mov ax,-1
    add ax,word[i]
    sub ax,bx
    jz  loop
    jmp show
end:
    jmp $
show:                ;打印
    xor ax,ax
    mov ax,word[x]
    mov bx,80
    mul bx
    add ax,word[y]
    mov bx,2
    mul bx
    mov bp,ax
    mov ah,0Fh
    mov al,byte[char]
    mov word[es:bp],ax
    jmp loop_wait
loop_wait:            ;延迟打印
    dec word[count]
    jnz loop_wait
    mov word[count],delay
    dec word[dcount]
    jnz loop_wait
    mov word[count],delay
    mov word[dcount],delay
    mov al,1
    cmp al,byte[des]
    jz  down
    mov al,2
    cmp al,byte[des]
    jz  right
    mov al,3
    cmp al,byte[des]
    jz  up
    mov al,4
    cmp al,byte[des]
    jz  left
datadef:
    count dw delay
    dcount dw delay
    i dw 0        ;当前层数
    x dw 0        ;坐标
    y dw 0
    des db 1      ;方向初始化为向下
    char db 'A'

效果图如下：