Assembly PC汇编语言(Paul Carter)-Prime.Asm示例帮助了解JO和JE
C代码: 但是如果跳转是因为溢出检查,或者如果!(系数*系数<0)? 可以吗?正在比较哪些值?它只是检查ZF? 但它不是因为另一个原因(另一条指令)而被修改过吗Assembly PC汇编语言(Paul Carter)-Prime.Asm示例帮助了解JO和JE,assembly,x86,overflow,instructions,Assembly,X86,Overflow,Instructions,C代码: 但是如果跳转是因为溢出检查,或者如果!(系数*系数
提前感谢您的帮助。是的,当EDX寄存器变为非零时,MUL指令设置OF标志。换句话说,当乘法产生的结果不再适合32位无符号整数时。正在测试的表达式是
factor*factor
,其中guess
是32位无符号整数。因此,代码生成器生成好代码,如果设置了溢出标志,则表达式将始终为false,不管猜测的值是多少。另一种情况更糟,如果存在溢出,则表达式的结果可能为真
不幸的是,它随后摸索跳转目标,英特尔手册指定MUL未定义ZF标志。对我来说,这看起来像是一个代码生成器错误。目前还不清楚是什么编译器生成了这段代码,或者这是否只是C代码的手工输入替代品。代码生成器通常不会生成这样的代码,因此我很容易假设这只是手写代码中的一个bug。谢谢您的回答!OF的情况现在很清楚了。关于JE。。。在此之前,执行以下指令:cmp eax,[Limit]。如果eax=[Limit]未生成JMP且while循环启动,则由于cmp,ZF=1。如果现在发生溢出,程序跳转到end_while_factor,则不会打印数字,因为ZF剩余1?呃,不,比较跳转到end_while_limit。有几个地方,它跳到结束,而你的因素。这只是错误代码。原作者在这里。我只是偶然发现了这个。我是“代码生成器”,所以这只是我制造的一个bug。很好的分析!
#include <stdio.h>
int main()
{
unsigned guess; /* current guess for prime */
unsigned factor; /* possible factor of guess */
unsigned limit; /* find primes up to this value */
printf("Find primes up to: ");
scanf("%u", &limit);
printf("2\n"); /* treat first two primes as special case */
printf("3\n");
guess = 5; /* initial guess */
while ( guess <= limit ) {
/* look for a factor of guess */
factor = 3;
while ( factor*factor < guess && guess % factor != 0 )
factor += 2;
if ( guess % factor != 0 )
printf("%d\n", guess);
guess += 2; /* only look at odd numbers */
}
return 0;
}
%include "asm_io.inc"
segment .data
Message db "Find primes up to: ", 0
segment .bss
Limit resd 1 ; find primes up to this limit
Guess resd 1 ; the current guess for prime
segment .text
global _asm_main
_asm_main:
enter 0,0 ; setup routine
pusha
mov eax, Message
call print_string
call read_int ; scanf("%u", & limit );
mov [Limit], eax
mov eax, 2 ; printf("2\n");
call print_int
call print_nl
mov eax, 3 ; printf("3\n");
call print_int
call print_nl
mov dword [Guess], 5 ; Guess = 5;
while_limit: ; while ( Guess <= Limit )
mov eax,[Guess]
cmp eax, [Limit]
jnbe end_while_limit ; use jnbe since numbers are unsigned
mov ebx, 3 ; ebx is factor = 3;
while_factor:
mov eax,ebx
mul eax ; edx:eax = eax*eax
**jo end_while_factor ; if answer won't fit in eax alone**
cmp eax, [Guess]
jnb end_while_factor ; if !(factor*factor < guess)
mov eax,[Guess]
mov edx,0
div ebx ; edx = edx:eax % ebx
cmp edx, 0
je end_while_factor ; if !(guess % factor != 0)
add ebx,2 ; factor += 2;
jmp while_factor
end_while_factor:
**je end_if ; if !(guess % factor != 0)**
mov eax,[Guess] ; printf("%u\n")
call print_int
call print_nl
end_if:
mov eax,[Guess]
add eax, 2
mov [Guess], eax ; guess += 2
jmp while_limit
end_while_limit:
popa
mov eax, 0 ; return back to C
leave
ret
cmp edx,0
je end_while_factor