Branch table in assembly 8086

Question

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 ?

Answer 1

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

                               

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]