practice
we’re doing 16 bit real mode assembly in class this semester so i’m making all my class work inside an emulator. this time, i’m doing something in 32 bit protected mode linux (as a refresher).
it’s supposed to be a program that waits for user input and tells if it’s a palindrome or not.
%define _NR_read 0x3 %define _NR_write 0x4 global _start section .text _start mov edx, ebuf ; size_t count mov ecx, buf ; void *buf xor ebx, ebx ; STDIN mov eax, _NR_read ; sys_read() int 0x80 ; call kernel ; -- strlen() -- cld ; clear direction flag mov edi, buf ; start of buffer mov eax, 0 ; null terminator mov ecx, -1 ; counter = 255 repne scasb ; repeat till NULL terminator not ecx ; invert ecx lea ecx, [ecx-2] ; ecx = strlen(buf) ; -- setup pointers -- mov esi, buf ; esi to first letter mov edi, buf ; same dec ecx add edi, ecx ; edi to last letter mov ebx, 1 ; STDOUT mov eax, _NR_write ; sys_write() do_check mov cl, [esi] mov ch, [edi] cmp cl, ch ; are they equal? jne wrong ; no. word is not a palindrome ; -- work towards the middle inc esi dec edi cmp edi, esi ; are we at the middle? jns do_check ; no. repeat check ; -- palindrome! -- mov edx, (badmsg-gudmsg) mov ecx, gudmsg int 0x80 ; call kernel jmp exit wrong ; -- not palindrome! -- mov edx, (endmsg - badmsg) mov ecx, badmsg int 0x80 exit xor eax, eax xor ebx, ebx inc eax int 0x80 section .data gudmsg db 'string is a palindrome', 0xa, 0x0 badmsg db 'string is not a palindrome', 0xa, 0x0 endmsg section .bss buf resb 0x32 ; 50 byte buffer ebuf equ $-buf
note to self:
try using the return value of read() next time?
Comment by sleepy jenkins — July 26, 2006 @ 7:40 am