汇编开发贪吃蛇游戏出现多余星号（像素）的问题求助

我用汇编开发贪吃蛇游戏，当前蛇由3个星号（*）组成，遇到两个问题：

• 当蛇在垂直与水平移动间切换时，屏幕会额外生成一个星号
• 每次向左移动时，同一位置都会重复添加星号

问题截图：
操作前效果：
操作后效果：
多次左移后的效果：
左移3次后：
左移4次后：

我的代码如下：

IDEAL
MODEL small
STACK 100h
DATASEG
; --------------------------
; Your variables here
; --------------------------
saveal db, ' '  ;used in line 223-233

app dw 0       ;place of the apple
st_am dw 3
stars dw 0, 0, 0  ;places of the *

CODESEG
proc black
body:
    mov [es:si], ax
    add si, 2
    cmp si, 25*80*2
    jnz body
  ret
endp black

proc up
    mov di, 80*2
    cmp si, di
    jb not_move_up
    
    cmp si, [app]
    jnz move_up
    call apple
    
move_up:
    call delete
    
    mov di, [stars+2]
    mov [stars+4], di
    mov di, [stars]
    mov [stars+2], di
    sub di, 80*2
    mov ah, 156
    mov al, '*'
    mov [es:di], ax
    mov [stars], si
    
    sub si, 80*2
    mov ah, 156
    mov al, '*'
    mov [es:si], ax
    
not_move_up:
  ret
endp up

proc down

    mov di, (24*80*2)-1
    cmp si, di
    jg not_move_down
    
    cmp si, [offset app]
    jnz move_down
    call apple
    
move_down:
    call delete
    
    mov di, [stars+2]
    mov [stars+4], di
    mov di, [stars]
    mov [stars+2], di
    add di, 80*2
    mov ah, 156
    mov al, '*'
    mov [es:di], ax
    mov [stars], si
    
    add si, 80*2
    mov ah, 156
    mov al, '*'
    mov [es:si], ax
not_move_down:
  ret
endp down
proc left

    mov dx, 0
    mov bx, si
    mov ax, si
    mov si, 80*2
    div si
    mov si, bx
    cmp dx,0
    jz not_move_left
    
    cmp si, [offset app]
    jnz move_left
    call apple
    
move_left:
    call delete
    
    mov di, [stars+2]
    mov [stars+4], di
    mov di, [stars]
    mov [stars+2], di
    mov ah, 156
    mov al, '*'
    mov [es:di], ax
    mov [stars], si
    
    sub si, 2
    mov ah, 156
    mov al, '*'
    mov [es:si], ax
not_move_left:
  ret
endp left
proc right

    mov dx, 0
    mov bx, si
    mov ax, si
    mov si, 80*2
    div si
    mov si, bx
    cmp dx,158
    jz not_move_right
    
    cmp si, [app]
    jnz move_right
    call apple
    
move_right:
    call delete
    
    mov di, [stars+2]
    mov [stars+4], di
    mov di, [stars]
    mov [stars+2], di
    mov ah, 156
    mov al, '*'
    mov [es:di], ax
    mov [stars], si
    
    add si, 2
    mov ah, 156
    mov al, '*'
    mov [es:si], ax
not_move_right:
  ret
endp right

proc apple
    mov ax, 40h
    mov es, ax
    mov ax, [es:6ch]
    and ax, 0000001111111110b
    mov di,ax
    mov [offset app], di
    mov ax, 0b800h
    mov es, ax
    
    mov al, '@'
    mov ah, 154
    mov [es:di], ax
  ret 
endp apple
    
proc delete
    mov bx, offset stars
    mov di, [st_am]
    dec di
    shl di, 1
    mov di, [bx+di]
    mov ax, 0b800h
    mov es, ax
    
    mov al, ' '
    mov ah, 0
    
    mov [es:di], ax
  ret
endp delete
start:
    mov ax, @data
    mov ds, ax
; --------------------------
; Your code here
; --------------------------
    mov ax, 0b800h
    mov es, ax
    
    mov si,0
    mov al, ' '
    mov ah, 0
    call black
    
    mov bx, offset stars
    
    mov si, ((12*80+40)*2)-2
    mov al, '*'
    mov ah, 156
    mov [es:si], ax
    mov [bx], si
    mov si, (12*80+40)*2
    mov al, '*'
    mov ah, 156
    mov [es:si], ax
    mov [bx+2], si
    mov si, ((12*80+40)*2)+2
    mov al, '*'
    mov ah, 156
    mov [es:si], ax
    mov [bx+4], si
    mov si, ((12*80+40)*2)-2    

    call apple
    
yesOrno:
    mov ah, 1h
    int 21h
    mov [byte ptr saveal], al
    
    cmp [byte ptr saveal], 'w'
    jz w
    cmp [byte ptr saveal], 'a'
    jz a
    cmp [byte ptr saveal], 's'
    jz s
    cmp [byte ptr saveal], 'd'
    jz d
    cmp [byte ptr saveal], 'q'
    jmp exit
    
w:
    call up
    jmp yesOrno
s:
    call down
    jmp yesOrno 
a:
    call left
    jmp yesOrno
d:
    call right
    jmp yesOrno
    
exit:
    mov ax, 4c00h
    int 21h
END start

自行排查后未找到问题原因，特此求助。

内容的提问来源于stack exchange，提问作者yarden