Linux下的一个简单汇编程序, .long声明数据的长度问题!

#PURPOSE: This program finds the maximum number of a# set of data items.##VARIABLES: The registers have the following uses:## %edi - Holds the index of the data item being examined# %ebx - Largest data item found# %eax - Current data item## The following memory locations are used:## data_items - contains the item data. A 0 is used# to terminate the data# .section .datadata_items: #These are the data items .long 3,67,34,222,45,75,54,34,44,33,22,11,66,0 .section .text .globl _start_start: movl $0, %edi # move 0 into the index register movl data_items(,%edi,4), %eax # load the first byte of data movl %eax, %ebx # since this is the first item, %eax is# the biggeststart_loop: # start loop cmpl $0, %eax # check to see if we've hit the end je loop_exit incl %edi # load next value movl data_items(,%edi,4), %eax cmpl %ebx, %eax # compare values jle start_loop # jump to loop beginning if the new # one isn't bigger movl %eax, %ebx # move the value as the largest jmp start_loop # jump to loop beginningloop_exit: # %ebx is the status code for the _exit system call # and it already has the maximum number movl $1, %eax #1 is the _exit() syscall int $0x80

 

程序中,

data_items:
.long 3,67,34,222,45,75,54,34,44,33,22,11,66,0

定义了一组数。.long指示声明一组数，每个数占32位，相当于C语言中的数组。

 

然后就逐一比较大小，最后找到一个最大的数，并把它作为程序的退出状态,调用_exit系统调用。

 

以下是具体操作结果：

 

[root@localhost Desktop]# as max.s -o max.o[root@localhost Desktop]# ld max.o -o max[root@localhost Desktop]# ./max[root@localhost Desktop]# echo $?222

输出结果是：222

很正常！结果和预期一致。

 

但是后来我把数组的第四个元素的值222改成256...

再编译运行！

[root@localhost Desktop]# as max.s -o max.o[root@localhost Desktop]# ld max.o -o max[root@localhost Desktop]# ./max[root@localhost Desktop]# echo $?0 

输出结果是：0

 

然后我又把它改成257，再编译运行！

[root@localhost Desktop]# as max.s -o max.o[root@localhost Desktop]# ld max.o -o max[root@localhost Desktop]# ./max[root@localhost Desktop]# echo $?1

输出结果是：1

 

看结果，我猜想。

1.

这是数据溢出了。

long申请的内存空间只有一个byte...8位的空间。

2.

书上说,.long声明的每个数占32位!差太远了吧！

想深一层，这会不会是Linux系统的_exit系统调用返回数据的大小只有8位！

 

个人觉得后者解释更为合理!

 

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