CODEWITHSUNDEEP
assemblyx86-16
Yair B.
Yair B.

Reputation: 332

Branch table in assembly 8086

I try to do a branch table in assembly 8086.

I wrote it:

    org 100h                       

    ; START THE PROGRAM

    mov si, 1
    jmp cs:table[si]

    table dw C1, C2, C3

C1:
    add bx, 1
    jmp EXIT

C2:
    add bx, 2
    jmp EXIT

C3:
    add bx, 3
    jmp EXIT
C4:  
    add bx, 4
    jmp EXIT

; EXIT PRORGRAM   
EXIT:
    mov ah, 0
    int 16h
    ret

The code not working, It's jump to the incorrect label.

Someone can give me example or fix my code ?

Upvotes: 3

Views: 2516

Answers (1)

Margaret Bloom
Margaret Bloom

Reputation: 44046

Be careful that

jmp cs:table[si]

actually is a mangled version of

jmp cs:[table + si]

It is not an array indexing.
The assembly language has no types, to avoid confusion don't use such an unorthodox syntax.

Suppose table starts at 1000h. Then

                               Table in memory at 1000h

With SI = 1 the offset to jump to is taken at 1000h + 1 = 1001h.
The WORD at that location occupies 1001h and 1002h, thus it crosses C1 and C2.

Using the index directly to access an array is a comfort not available in assembly, we need to scale the index based on the size of the items.
In this case each item is a WORD, 2 bytes, so the index must be scaled by 2.

Index    Scaled index    Address accessed
 0            0             1000h
 1            2             1002h
 2            4             1004h

Thus

 mov si, 1
 shl si, 1         ;Scale index, SI = SI*2^1 = SI*2
 jmp cs:table[si]

or better, if the index is known at "compile time"

 mov si, 2         ;Use scaled index directly
 jmp cs:table[si]

Upvotes: 10

Related Questions