NASM jmp wonkiness

Question

I'm writing a Forth inner interpreter and getting stuck at what should be the simplest bit. Using NASM on Mac (macho)

msg  db "k thx bye",0xA     ; string with carriage return
len  equ $ - msg            ; string length in bytes

xt_test:
    dw xt_bye     ; <- SI Starts Here

    dw 0
    db 3,'bye'
xt_bye dw $+2       ; <- Should point to...
    push dword len  ; <-- code here
    push dword msg  ; <--- but it never gets here
    push dword 1
    mov  eax, 0x4   ; print the msg
    int 80h
    add  esp, 12
    push dword 0
    mov eax, 0x1    ; exit(0)
    int 80h

_main:
    mov si,xt_test ; si points to the first xt
    lodsw          ; ax now points to the CFA of the first word, si to the next word
    mov di,ax
    jmp [di]       ; jmp to address in CFA (Here's the segfault)

I get Segmentation Fault: 11 when it runs. As a test, I can change _main to

_main:
    mov di,xt_bye+2
    jmp di

and it works

EDIT - Here's the simplest possible form of what I'm trying to do, since I think there are a few red herrings up there :)

a dw b
b dw c
c jmp _my_actual_code

_main:
    mov si,a
    lodsw
    mov di,ax
    jmp [di]

EDIT - After hexdumping the binary, I can see that the value in b above is actually 0x1000 higher than the address where label c is compiled. c is at 0x00000f43, but b contains 0x1f40

Answer 1

First, it looks extremely dangerous to use the 'si' and 'di' 16-bit registers on a modern x86 machine which is at least 32-bit.

Try using 'esi' and 'edi'. You might be lucky to avoid some of the crashes when 'xt_bye' is not larger than 2^16.

The other thing: there is no 'RET' at the end of xt_bye.

One more: see this linked question Help with Assembly. Segmentation fault when compiling samples on Mac OS X

Looks like you're changing the ESP register to much and it becomes unaligned by 16 bytes. Thus the crash.

One more: the

jmp [di]

may not load the correct address because the DS/ES regs are not used thus the 0x1000 offset.