6

我一直在阅读“从头开始编程”这本书来学习 Linux 中的汇编编程。我在解决第 3 章末尾的一个练习时遇到了麻烦。练习说修改以下程序以使用数据集末尾的地址来终止循环。这只是一个在一组数据中查找最大数的简单程序，它目前只是使用数字零来标记数据的结尾。

#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    .data

data_items:
    .long 3, 67, 34, 14, 45, 75, 54, 187, 44, 87, 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                      # The biggest

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 the beginning if new value
                                        # Isn't larger
movl    %eax, %ebx                      # Move the value as the largest
jmp     start_loop                      # Jump to the beginning of loop

loop_exit:

# %ebx is the status code for the exit system call
# and it contains the maximum number
movl    $1, %eax                         # 1 is the exit() system call
int     $0x80

我知道我可以硬编码数据列表的长度，或者我可以将它存储在数据的第一个字节中，但练习要求使用最后一个元素的地址来终止循环。书中提到使用符号来标记结束。我认为我的问题是我只是不明白如何获取地址。如果我知道如何获得它，我可以将它存储在寄存器中。任何帮助表示赞赏。

4

2 回答 2

4

带有一些适用于 Mac OSX 的小模组...

.data

data_items:
    .long 3, 67, 34, 14, 45, 75, 54, 187, 44, 87, 22, 11, 66, 0
data_end:

.text

.globl    start
start:
        movl    $0, %edi                       # Move 0 into the index register
        movl    data_items (, %edi, 4), %eax   # Load the first data value
        movl    %eax, %ebx                     # The biggest
        leal    data_end, %ecx                 # Load and save ending address

start_loop:
        leal    data_items(, %edi, 4), %eax    # Get address of next value
        cmpl    %eax, %ecx                     # Check to see if we've hit the end
        je      loop_exit
        incl    %edi                           # Increment index
        movl    (%eax), %eax                   # Load value
        cmpl    %ebx, %eax                     # Compare values
        jle     start_loop                     # Jump to the beginning if new value
                                               # Isn't larger
        movl    %eax, %ebx                     # Move the value as the largest
        jmp     start_loop                     # Jump to the beginning of loop

loop_exit:
        movl    $1, %eax                       # 1 is the exit() system call
        pushl   %ebx
        pushl   $0

        int     $0x80
于 2012-10-13T18:16:55.923 回答
0

问问自己：我知道数据的起始地址吗？我怎么知道？我知道数据的大小吗？我怎么知道这个？我可以从这些信息中得知结束地址吗？

于 2012-10-13T15:56:20.417 回答