打印汇编浮点数

问题描述

我试图从assemler调用printf函数打印浮点值。它正常工作与字符串和整数值，但无法打印浮动。这里是工作code的例子：

 全球主要
EXTERN的printf段.data
  消息：DB字符串是：％D％X％的，10，0
  end_message：DB..字符串结尾，0.text段
  主要：
    MOV EAX，为0xFF
    MOV EDI，消息
    movsxd RSI，EAX
    MOV RDX，为0xFF
    MOV RCX，end_message
    XOR RAX，RAX
    调用printf
    RET
 

So, the parameters are passed through registers: edi contains address of a formatting string, rsi and rdx contain the same number to print in decimal and hex styles, rcx contains end of a string, rax contains 0 as we do not have a float to print.This code works fine but something changes while trying to print float:

global main

extern printf

section .data
    val: dq 123.456
    msg: db "Result is: %fl",10, 0

section .text
    main:
    mov rdi,msg
    movsd xmm0,[val]
    mov eax,1
    call printf

    mov rax, 0
    ret

This code snipped can be compiled but returns segmentation fault being executed. It seems that the problem is in wrong value of xmm0 but trying to change movsd xmm0,[val] to movsd xmm0,val gives an

message.The compiler is NASM running on openSuSe 12.3

Update. I tried to make a c program and produce a .S assembly. It gives a very weird solution:

main:
.LFB2:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    subq    $32, %rsp
    movl    %edi, -4(%rbp)
    movq    %rsi, -16(%rbp)
    movq    val(%rip), %rax
    movq    %rax, -24(%rbp)
    movsd   -24(%rbp), %xmm0
    movl    $.LC0, %edi
    movl    $1, %eax
    call    printf
    movl    $0, %eax
    leave
    .cfi_def_cfa 7, 8
    ret

Is it possible to write a simple printf example?

解决方案

for your assembler problem:you need to align the stack before your main program starts.

insert

sub rsp, 8

right after main:

then add it again before ret:

add rsp, 8

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