CODEWITHSUNDEEP
assemblyx86dosx86-16
JackRobinson
JackRobinson

Reputation: 225

Assembly jmp not jumping

The problem is that it does not jump at doi even if dl and bl are equal. Anyone knows why?

assume cs:code,ds:data
data segment
sir1 db "abc"
l1 equ $-sir1
sir2 db "a"
l2 equ $-sir2
bunbun db "Este!$"
nu db "NU este!$"
iesire db "Apasa Enter!$"

data ends

code segment
start:
    mov ax,data
    mov ds,ax

    mov bp,offset sir1
    mov si,offset sir2
    dec bp
    push bp
    push si
    mov ah,l1
    mov bh,l2

unu:
    pop si
    pop bp
    inc bp
    dec ah
    mov dl,sir1[bp]
    mov bl,sir2[si]
    push bp
    push si
    cmp dl,bl
    je doi
    cmp ah,0
    je nu_bun
    jmp unu

doi:
    inc si
    inc bp
    dec ah
    mov dl,sir1[bp]
    mov bl,sir2[si]
    cmp dl,bl
    jne unu
    cmp bh,0
    je bun
    jmp doi

bun:
    mov dx,offset bunbun
    mov ah,09h
    int 21h
    mov ah, 09h
    mov dx,offset iesire
    int 21h
    mov ah, 0ah
    int 21h
    mov ax,4c00h
    int 21h

nu_bun:
    mov dx,offset nu
    mov ah,09h
    int 21h
    mov ah, 09h
    mov dx,offset iesire
    int 21h
    mov ah, 0ah
    int 21h
    mov ax,4c00h
    int 21h

code ends
end start

Upvotes: 0

Views: 1116

Answers (1)

Ken Kin
Ken Kin

Reputation: 4693

This compares implicit-length 0-terminated strings. (Unlike the strings in the question that are either explicit length (with l1 equ $-sir1 but no special byte marking the end) or have a $ terminator.)

It also assumes ES = DS, or that the 2nd string is pointed to by ES:BP. (The question is using [BP] for one of them, which is probably a mistake unless you're using a "tiny" code model where DS = SS.)

doi:
   ; cld             ; assume DF=0
    xchg di, bp
    mov cx, ax
    xor ax, ax

cmp_next:            ; do{
    lodsb              ; AL = [si]        ; si++
    scasb              ; cmp al, [es:di]  ; di++
    jnz not_equal
    test  al, al
    jnz   cmp_next   ; }while(al!=0);
   ; else fall through if we reached the end without finding a difference
do_equal:
    xchg di, bp
    mov ax, cx
    jmp somewhere_togo_when_equal

not_equal:
    xchg di, bp
    mov ax, cx
    jmp somewhere_togo_when_not_equal

Upvotes: 3

Related Questions