linux - Why do I have a segfault when using cmp instruction? -


i using yasm on linux x86_64.

i following introduction assembly language on internet. i've covered basic data types. i'm diving loops contruction. good, first attempt failure. code segfault around use of cmd instruction:

    segment .data       dd  0x01,0x02,0x03,0x04 b       dd  0x03,0x03,0x03,0x03 product dd  0x00      segment .text     global _start _start:      mov     rax,0  begin_while:     cmp     rax,4     jnl     end_while      inc     rax     jmp     begin_while end_while:      mov     rax,1     xor     rbx,rbx     int     0x80

but when add few lines after _start label, behaves expected. i.e. no segfault.

    push    rbp     mov     rbp,rsp     sub     rsp,16

the book reading uses construct time time. not everytime. understand has procedure follow when calling function. guess might related libc runtime. anyway, not understand why it's needed. far few simple programs wrote (not much) never had use it. today, using jmp.

does have explanation it?

well, using x86 interrupt call 

int     0x80

which should changed 

syscall

for x86_64. relying on x86 compatibility while using 64-bit registers. pointed out jester, code compile , run without error on x86_64. (i have confirmed both ways without error on amd64 linux) however, extent true platforms unclear.

in writing x86_64 code, should change:

mov     rax,1

to

mov     rax, 0x3c  ; 60 decimal

to setup proper x86_64 exit syscall instead of relying on x86 compatibility. (in x86_64, syscall number 1 __nr_write 1, see: /usr/include/asm/unistd_64.h compared /usr/include/asm/unistd_32.h)


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