问题描述
这是乔纳森·巴特利特（Jonathan Bartlett）在书“ Programming from the ground up”中给出的程序，用于查找列表中的最大数目：
#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: #These are the data items
.long 3,67,34,222,45,75,54,44,33,22,11,66,0
.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 $0,%eax # check to see if we’ve hit the end
je loop_exit
incl %edi # load next value
movl data_items(,%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
不幸的是，本书中给出的汇编程序适用于32位处理器FWIU。因此，我在https://cs.lmu.edu/~ray/notes/gasexamples/的帮助下将程序翻译为64位。这是我的64位系统代码（不起作用）：
#variables: the registers have the following uses:
# %rdx - holds the index of the data item being examined
# %rbx - largest data item found
# %rcx - current data item
# the following memory locations are used:
# data_items - contains the item data. 0 is used to terminate the data.
data_items:
.long 8,9,3,4,15,8,0
.global _start
.text
_start:
mov $0,%rdx # move 0 into the index register
mov data_items(,%rdx,%rcx # load the first byte of data
mov %rcx,%rbx # since this is the first item,%rcx is the biggest
start_loop:
cmp $0,%rcx # check to see if we've hit the end
je loop_exit
inc %rdx # load next value
mov data_items(,%rcx
cmp %rbx,%rcx
jle start_loop
mov %rcx,%rbx # move the value as the largest
jmp start_loop
loop_exit:
#%rdi is the status code for the exit system call
mov $60,%rax
mov %rbx,%rdi # moving the final value in %rbx to %rdi to return
syscall
我的代码有什么问题？
我怀疑.long在64位汇编代码中的工作方式不同。
Reading symbols from ./a.out...
(gdb) break 1
Breakpoint 1 at 0x401000: file max.s,line 11.
(gdb) run
Starting program: /home/hackasaur/Desktop/assembly/a.out
Breakpoint 1,_start () at max.s:11
11 mov $0,%rdx # move 0 into the index register
(gdb)stepi
12 mov data_items(,%rcx # load the first byte of data
(gdb) info registers
rax 0x0 0
rbx 0x0 0
rcx 0x0 0
rdx 0x0 0
rsi 0x0 0
rdi 0x0 0
rbp 0x0 0x0
rsp 0x7ffffffee160 0x7ffffffee160
r8 0x0 0
r9 0x0 0
r10 0x0 0
r11 0x0 0
r12 0x0 0
r13 0x0 0
r14 0x0 0
r15 0x0 0
rip 0x401007 0x401007 <_start+7>
eflags 0x202 [ IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
(gdb) stepi
13 mov %rcx,%rbx # since this is the first item,%rcx is the biggest
(gdb) info registers
rax 0x0 0
rbx 0x0 0
rcx 0x900000008 38654705672
rdx 0x0 0
rsi 0x0 0
rdi 0x0 0
rbp 0x0 0x0
rsp 0x7ffffffee160 0x7ffffffee160
r8 0x0 0
r9 0x0 0
r10 0x0 0
r11 0x0 0
r12 0x0 0
r13 0x0 0
r14 0x0 0
r15 0x0 0
rip 0x40100f 0x40100f <_start+15>
eflags 0x202 [ IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
我发现在第12行38654705672而不是第8行的rcx寄存器中加载了这个乱码值mov data_items(,%rcx，每次使用mov data_items(,%rcx加载内容时，都会有一个大的乱码值加载。
我正在使用以下命令进行组装和链接：
as max.s -o max.o && ld max.o && ./a.out
并且我使用echo $?得到了上面数字列表的退出代码4，如果可以正常工作，在这种情况下应该为15。
解决方法
暂无找到可以解决该程序问题的有效方法，小编努力寻找整理中！
如果你已经找到好的解决方法，欢迎将解决方案带上本链接一起发送给小编。
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