【发布时间】：2017-10-28 15:36:26
【问题描述】：
下面的代码假设显示哪个数字更大或者它们是否相等和总和，但我得到的只是总和，如果我删除第一部分显示的总和部分，显示是否不是数字相等
section .rodata
prompt1 db "Enter an integer: ",0 ; 0 is null character
prompt2 db "Enter another integer: ",0
num_format db "%ld",0
format_str1 db "The greater of %ld and %ld is %ld.",10,0 ; 10 is LF
format_str2 db "The sum of %ld and %ld is %ld.",10,0 ; 10 is LF
equalprompt db "%ld and %ld are equal.",10,0 ; 10 is LF
section .text
global main ; main and _start are both valid entry points
extern printf, scanf ; these will be linked in from glibc i
main:
; prologue
push rbp ; save base pointer to the stack
mov rbp, rsp ; base pointer = stack pointer
sub rsp, 16 ; make room for two long integers on the stack
push rbx ; push callee saved registers onto the stack
push r12 ; push automatically decrements stack pointer
push r13
push r14
push r15
pushfq ; push register flags onto the stack
; prompt for first integer
mov rdi, dword prompt1 ; double word is 4 bytes; a word is 2 bytes
; rdi = 32-bit address of prompt1
xor rax, rax ; rax is return value register - zero it out
call printf ; call the C function from glibc
; read first integer
lea rsi, [rbp-8] ; load effective address - this instruction
; computes the address 8 bytes above the
; base pointer - the integer read by scanf
mov rdi, dword num_format ;
xor rax, rax ; zero out return value register
call scanf ; call C function
; scanf reads the input as an integer
; prompt for second integer
mov rdi, dword prompt2
xor rax, rax
call printf
; read second integer
lea rsi, [rbp-16]
mov rdi, dword num_format
xor rax, rax
call scanf
; determine if num2 (second integer) is greater than num1 (first integer)
xor rbx, rbx ; RBX = 0x0
mov rax, [rbp-16] ; RAX = num2 ; load rax with value at rdb-16
cmp rax, [rbp-8] ; compute (num1) - (num2) and set condition codes
; in machine status word register based on result
jl lessthan ; jump if num1 <= num2
je equal
; num1 > num2
mov rdi, dword format_str1
mov rsi, [rbp-8] ; num1
mov rdx, [rbp-16] ; num2
mov rcx, [rbp-16] ; greater of the two
;mov rcx, 0
; add rcx, rsi
; add rcx, rdx
; xor rax, rax
jmp sum
lessthan:
; num1 < num2
mov rdi, dword format_str1
mov rsi, [rbp-8] ; num1
mov rdx, [rbp-16] ; num2
mov rcx, [rbp-8] ; greater of the two
;mov rcx, 0
;add rcx, rsi
; add rcx, rdx
; xor rax, rax
jmp sum
equal:
mov rdi, dword equalprompt
mov rsi, [rbp-8] ; num1
mov rdx, [rbp-16] ; num2
jmp sum
sum:
mov rdi, dword num_format ;
mov rdi, dword format_str2
mov rsi, rsi ; num1
mov rdx, rdx ; num2
mov rcx, 0
add rcx, rsi
add rcx, rdx
jmp exit
exit:
call printf
; epilogue
popfq
pop r15
pop r14
pop r13
pop r12
pop rbx
add rsp, 16 ; set back the stack level
leave
ret
示例输出如下：
Enter an integer: 7
Enter another integer: 3
The sum of 7 and 3 is 10
【问题讨论】：
-
请删除行号，以便其他用户可以组装您的代码并将其插入调试器。
-
call printf在第 120 行只能做一个或另一个。我认为您应该在第 109 行添加另一个call printf。 -
您不需要在函数中保留 FLAGS，它们会像
rcx、rdx等一样被调用破坏。此外，您不需要推送/弹出您从未接触过的寄存器（如r12）。这只是一个浪费的指令，就像jmp exit/exit:。您无需跳过空行；无论如何，执行总是转到下一条指令。