使用结束地址来停止循环 [英] Using ending address to stop a loop

问题描述

你好，我是汇编语言的新手.我读了一本书来提高我的知识(从头开始编程).

Hello I'm new in assembly language. I read a book to improve my knowledge (Programming from the ground up).

我理解下面的例子，有一个问题要求修改程序并使其在到达结束地址时停止.我不知道如何在汇编中打印当前地址或将其与数字进行比较.使用 cmpl $13, %edi 检测何时到达 data_items 结尾是否正确?

I understand the following example, there is a question which demand to modify the program and make it stop when reaches an ending address. I don't know how to print the current address in assembly or to compare it with a number. And is it correct to use cmpl $13, %edi to detect when end of data_items has been reached ?

.section .data
data_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 biggest
start_loop:                     # start loop
#cmpl $22, %eax                 # check to see if we’ve hit the end using Value
#cmpl $13, %edi                 # Using Length to break loop

#I have to add a condition here  to use an ending address 
#rather than the number 0 to know when to stop.

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 beginning
loop_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

推荐答案

您似乎在尝试通过将 data_items 与一个数字进行比较来确定是否已到达末尾.你可以做的是使用循环来帮助解决这个问题.这是我想出的:

It seems like you're trying to determine whether you've reached the end of data_items by comparing it to a number. What you can do is use a loop to help with that. Here's what I came up with:

.section .data
data_items:
        .long 4,73,121,133,236,251,252
data_items_size:
        .byte 7

.section .text
.globl _start
_start:
        movl $0, %edi
        movl data_items(,%edi,4), %eax
        movl %eax, %ebx
        movl $1, %ecx

        loop:
                cmpl data_items_size, %ecx
                je   exit
                incl %ecx

                incl %edi
                movl data_items(,%edi,4), %eax
                cmpl %eax, %ebx
                jge  loop
                movl %eax, %ebx
                jmp  loop

        loop_exit:
                movl $1, %eax
                int  $0x80

data_items_size 标签包含 data_items 的大小.在这个例子中，我使用了 %ecx 寄存器作为循环的计数器.我试了一下，退出状态码是252，最后加253的时候还是252，看来是可以的了.希望我能帮到你:)

The data_items_size label contains the size of data_items. In this example, I used the %ecx register as a counter for the loop. I tried it and I got 252 as the exit status code, and when I added 253 at the end, I still got 252. So it seems to be working. Hope I helped you out :).

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