问题描述
该程序为什么不打印到屏幕上,我是否在INT 80命令上缺少某些内容?
How come this program is not printing out to the screen, am I missing something on the INT 80 command?
section .bss
section .data
hello: db "Hello World",0xa ;10 is EOL
section .text
global _start
_start:
mov ecx, 0; ; int i = 0;
loop:
mov dl, byte [hello + ecx] ; while(data[i] != EOF) {
cmp dl, 0xa ;
je exit ;
mov ebx, ecx ; store conetents of i (ecx)
; Print single character
mov eax, 4 ; set sys_write syscall
mov ecx, byte [hello + ebx] ; ...
mov edx, 1 ; move one byte at a time
int 0x80 ;
inc ebx ; i++
mov ecx, ebx ; move ebx back to ecx
jmp loop ;
exit:
mov eax, 0x01 ; 0x01 = syscall for exit
int 0x80 ;
添加
我的Makefile:
sandbox: sandbox.o
ld -o sandbox sandbox.o
sandbox.o: sandbox.asm
nasm -f elf -g -F stabs sandbox.asm -l sandbox.lst
修改后的代码:
section .bss
section .data
hello: db "Hello World",0xa ;10 is EOL
section .text
global _start
_start:
mov ecx, 0; ; int i = 0;
while:
mov dl, byte [hello + ecx] ; while(data[i] != EOF) {
cmp dl, 0xa ;
je exit ;
mov ebx, ecx ; store conetents of i (ecx)
; Print single character
mov eax, 4 ; set sys_write syscall
mov cl, byte [hello + ebx] ; ...
mov edx, 1 ; move one byte at a time
int 0x80 ;
inc ebx ; i++
mov ecx, ebx ; move ebx back to ecx
jmp while ;
exit:
mov eax, 0x01 ; 0x01 = syscall for exit
int 0x80 ;
推荐答案
不打印的原因之一是因为ebx
应该保留值1以指定stdin
,而另一个原因是sys_write
占用了指针(字符串的地址)作为参数,而不是实际的字符值.
One of the reasons it's not printing is because ebx
is supposed to hold the value 1 to specify stdin
, and another is because sys_write
takes a pointer (the address of your string) as an argument, not an actual character value.
无论如何,让我向您展示一种简化程序结构的方法:
Anyway, let me show you a simpler way of structuring your program:
section .data
SYS_EXIT equ 1
SYS_WRITE equ 4
STDOUT equ 1
TRAP equ 0x80
NUL equ 0
hello: db "Hello World",0xA,NUL ; 0xA is linefeed, terminate with NUL
section .text
global _start
_start:
nop ; for good old gdb
mov ecx, hello ; ecx is the char* to be passed to sys_write
read:
cmp byte[ecx], NUL ; NUL indicates the end of the string
je exit ; if reached the NUL terminator, exit
; setup the registers for a sys_write call
mov eax, SYS_WRITE ; syscall number for sys_write
mov ebx, STDOUT ; print to stdout
mov edx, 1 ; write 1 char at a time
int TRAP; ; execute the syscall
inc ecx ; increment the pointer to the next char
jmp read ; loop back to read
exit:
mov eax, SYS_EXIT ; load the syscall number for sys_exit
mov ebx, 0 ; return a code of 0
int TRAP ; execute the syscall
像我一样,用NUL终止字符串可能更简单,或者您也可以执行$-hello
以在编译时获取其长度.我还会在循环中的每次迭代中为sys_write
设置寄存器(如您所愿),因为sys_write
不会保留所有寄存器.
It can be simpler to NUL terminate your string as I did, or you could also do $-hello
to get it's length at compile time. I also set the registers up for sys_write
at each iteration in the loop (as you do), since sys_write
doesn't preserve all the registers.
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