Question

代码如下：

section .data
    first_num dd 0
    second_num dd 0
    print_fmt db '%.2lf ', 0
section .bss
    result resb 8
section .text
extern printf, strtol
global main
%macro string_to_number 2
    PUSH rdi
    PUSH rsi
    PUSH rdx
    PUSH rax

    mov rdi, %2
    mov rsi, 0
    mov rdx, 10
    mov rax, 0
    call strtol
    mov [%1], eax

    POP rax
    POP rdx
    POP rsi
    POP rdi
%endmacro
%macro print 0
    PUSH rdi
    PUSH rsi
    PUSH rdx
    PUSH rax

    mov rdi, print_fmt
    mov rsi, [result]
    mov rax, 0
    call printf

    POP rax
    POP rdx
    POP rsi
    POP rdi
%endmacro
main:
    PUSH rbp
    mov rbp, rsp

    cmp rdi, 3
    jl .end

    add rsi, 8
    string_to_number first_num, [rsi]
    add rsi, 8
    string_to_number second_num, [rsi]

    fild dword [first_num]
    fsqrt
    fstp qword [result] ; <--THIS
    print

.end:
    POP rbp
    mov rax, 0
    ret

程序尚未完成。我只想打印第一个命令行参数的平方根并进行测试。我使用gdb调试了所有内容，直到FSTP命令为止，一切都正常工作。此时，正确的根位于st0（FPU堆栈的顶部）中，并且在FSTP命令之后，结果位置中没有写入任何内容（它为零），并且st0为零。我不明白会发生什么。我尝试使用result而不是FST来更改FSTP的大小，但没什么不同。

这是makefile：

test.o: test.asm
    nasm -f elf64 -g -F dwarf test.asm -o test.o
test: test.o
    gcc -o test test.o -no-pie

我正在用64位Linux编译它。

这是来自终端的复制粘贴：

linux@DESKTOP-QF0T5NR:/mnt/c/Users/Path$ gdb -q --arg build/zad5 12 13
Reading symbols from build/zad5...done.
(gdb) break 60
Breakpoint 1 at 0x4005ab: file zad5.asm, line 60.
(gdb) start
Temporary breakpoint 2 at 0x400540: file zad5.asm, line 47.
Starting program: /mnt/c/Users/Path/build/zad5 12 13

Temporary breakpoint 2, main () at zad5.asm:47
47              PUSH rbp
(gdb) continue
Continuing.

Breakpoint 1, main () at zad5.asm:60
60              fstp qword [result]
(gdb) i r f
st0            3.46410161513775438635   (raw 0x4000ddb3d742c2655000)
st1            0        (raw 0x00000000000000000000)
st2            0        (raw 0x00000000000000000000)
st3            0        (raw 0x00000000000000000000)
st4            0        (raw 0x00000000000000000000)
st5            0        (raw 0x00000000000000000000)
st6            0        (raw 0x00000000000000000000)
st7            0        (raw 0x00000000000000000000)
fctrl          0x27f    639
fstat          0x3820   14368
ftag           0x3fff   16383
fiseg          0x33     51
fioff          0x4005a9 4195753
foseg          0x2b     43
fooff          0x601038 6295608
fop            0x0      0
(gdb) p *(double*)result
$1 = 0
(gdb) next
61              print
(gdb) p *(double*)result
$2 = 0
(gdb)

NASM中的FSTP指令不会写入内存

0 个答案: