nasm中的数据库用法,尝试存储和打印字符串 [英] db usage in nasm, try to store and print string
问题描述
我试图在程序集中存储一些字符串,并使用WriteString在屏幕上打印出来.但是,我只调用其中一个字符串,它在一行中显示了所有字符串.为什么会这样?我的代码有什么问题吗?谢谢!
I was trying to store a few strings in my assembly, and use WriteString to print out on the screen. However, I only call one of the strings, it shows up all of them in one single line. Why does this happen? Is that anything wrong in my code? Thanks!
此处的代码:
%include "lib/libasm.h"
SECTION .text
global main
main:
call badrng
ret
badrng:
push eax
push edx
mov edx, badrngstr
call WriteString
pop edx
pop eax
call failure
ret
failure:
mov eax,1
int 0x80
ret
SECTION .data
nlstr: db ""
badrngstr: db "Value out of range in assignment"
baddivstr: db "Division by zero"
badcasstr: db "Value not handled in case statement"
badptrstr: db "Attempt to use a null pointer"
badsubstr: db "Subscript out of bounds"
stkovstr: db "Stack overflow"
heapovstr: db "Out of heap space"
WriteString位于libasm.s
and WriteString is in libasm.s
EXTERN main
SECTION .text
GLOBAL _start
_start:
call main
mov ebx, eax
call Exit
ret
GLOBAL AsciiDigit
AsciiDigit: ; al = digit to convert
SECTION .data
.xtable db "0123456789abcdef"
SECTION .text
push ebx
mov ebx, .xtable
xlat
pop ebx
ret
GLOBAL Crlf
Crlf:
SECTION .data
.lf db 0x0a
.lflen equ $-.lf
SECTION .text
pusha
mov ecx, .lf
mov edx, .lflen
call Write
popa
ret
GLOBAL Exit
Exit: ; ebx = error code
mov eax, 0x01
int 0x80
ret
GLOBAL Strlen
Strlen: ; edi = string
push edi
mov eax, 0
.L1:
cmp byte [edi], 0 ; end of string?
je .L2
inc edi
inc eax
jmp .L1
.L2:
pop edi
ret
GLOBAL Write
Write: ; ecx = buffer, edx = count
push ebx
mov eax, 0x04 ; sys_write = 4
mov ebx, 1 ; fd = 1 (stdout)
int 0x80
pop ebx
ret
GLOBAL WriteChar
WriteChar: ; al = the char
SECTION .data
.bb db 0
SECTION .text
pushad
mov byte [.bb], al
mov ecx, .bb
mov edx, 1
call Write
popad
ret
GLOBAL WriteHex
WriteHex: ; eax = the number
SECTION .data
.buf TIMES 16 db 0 ; buffer
.bufsz equ $-.buf ; bufsize
SECTION .text
pushad
mov edi, .buf ; point to the string
add edi, .bufsz
mov ecx, 0 ; buflen
.L1:
mov ebx, eax ; preserv eax into ebx
and al, 0x0f
call AsciiDigit
dec edi
mov byte [edi], al
inc ecx
mov eax, ebx ; restore eax
shr eax, 4
or eax, eax ; eax == zero ?
jnz .L1
;
mov edx, ecx
mov ecx, edi
call Write
popad
ret
GLOBAL WriteInt
WriteInt: ; eax = the number
SECTION .data
.isneg db 0 ; negative = 0 (false)
.buf TIMES 16 db 0 ; buffer
.bufsz equ $-.buf ; bufsize
SECTION .text
pushad
mov byte [.isneg], 0 ; negative = 0 (false)
or eax, eax ; eax is positive?
jns .L1
neg eax
mov byte [.isneg], 1 ; negative = 1 (true)
.L1:
mov edi, .buf ; point to the string
add edi, .bufsz
mov ecx, 0 ; buflen
mov ebx, 10 ; divided by 10
.L2:
mov edx, 0 ; edx:eax = the number
div ebx ; eax = Q, edx = R
or dl, 0x30 ; convert value to ASCII
dec edi
mov byte [edi], dl
inc ecx
or eax, eax ; eax == zero ?
jnz .L2
; add the sign symbol
cmp byte [.isneg], 0
je .L3
dec edi
mov byte [edi], '-'
inc ecx
.L3:
mov edx, ecx
mov ecx, edi
call Write
popad
ret
GLOBAL WriteString
WriteString:
SECTION .text
pushad
mov edi, edx
call Strlen
mov ecx, edx
mov edx, eax
call Write
popad
ret
,输出结果为
Value out of range in assignmentDivision by zeroValue not handled in case statementAttempt to use a null pointerSubscript out of boundsStack overflowOut of heap space
即使我只将edx的一个字符串移动,我也不知道为什么它将所有字符串打印出来.拜托,谁能告诉我我在哪里做错了?
I don't know why it would print all the string out, even though I only move one string the edx. Please, can anyone tell where did I do wrong?
注意:它已经过测试并在ubuntu 12.04 32bit上运行.
Note: It is tested and run on ubuntu 12.04 32bit.
推荐答案
字符串必须以null终止-您必须指出最后一个字节的零值.如果希望输出例程将字符串放在自己的行上,则可能还需要指示回车符和换行符.
The strings have to be null terminated -- you have to indicate the zero value for the last byte. You may also need to indicate carriage returns and line feeds if you want the output routine to put the string on it's own line.
您可以通过指定:
nullstr db 0
errorMessage db 'critical error', 10, 13, 0
warningMessage db 'warning', 10, 13, 0
goodbyeWorld1 db 'Goodbye World.', 0
goodbyeWorld2 db 'Goodbye World.'
endline db 10, 13, 0
endline
字符串将基于以下原理工作:由于goodbyeWorld2
并非以null终止,因此它将显示用于回车和换行的ascii代码,因此Goodbye World.
会出现在shell输出中其自己的行上.但是,由于goodByeWorld1
为空终止,因此它不会出现在自己的行上,并且输出将在最后的'.'处继续.显示的字符.
The endline
string would work on the principle that since goodbyeWorld2
is not null terminated, it would display the ascii codes for carriage feed and line return, so Goodbye World.
would appear on it's own line in the shell output. However since goodByeWorld1
is null-terminated it would not appear on it's own line and output would continue at the final '.' character displayed.
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