char_out:
# Allocate 8 bytes on the stack by subtracting 8 from the stack pointer.
#
# This is done for conformance to the calling convention expected by the
# 'putchar' function, which we're about to call.
subq $8, %esp
# This is the weirdo AT&T mnemonic for the MOVSX instruction, which does a
# move with sign extension. This means that it extends a small value
# (here, a BYTE-sized value in DIL) into a larger value (here, a
# DWORD-sized value in EDI), in a way that properly accounts for the
# value's sign bit.
#
# This is necessary because the 'putchar' function expects to be passed a
# 32-bit 'int' parameter.
movsbl %dil, %edi
# It's obvious what this does: it calls the 'putchar' function.
call putchar
# Clean up the stack, undoing what we previously did to the stack pointer
# at the top of the function (the 'subq' instruction).
addq $8, %rsp
由于Lashane已经评论说,这汇编代码相当于下面的C代码:
void char_out(char P1)
{
putchar(P1);
}
或者,我想你也可以说:
void char_out(char P1)
{
int temp = (int)P1; // movsbl
putchar(temp);
}
但是C编译器会为你隐式执行,所以没有必要明确地显示扩展转换。
它只是'putchar(P1);' –
询问某人做你的工作的问题不适用于StackOverflow。向我们展示您迄今为止所尝试的内容以及您遇到困难的地方,我们很乐意为您提供帮助。 –
@olivercharlesworth这是一个很好的笑话,但你为什么改变链接? –