程序集 - 简单 lock/unlock 程序错误

Question

我写的程序有一个错误，我不明白为什么。它应该在用户按下任意键后将状态设置为 lock/unlock，并提供一些反馈。但是在两个循环之后输出出错了。示例：

The door is locked Press any key to unlock The door is unlocked Press any key to lock The door is locked q The door is unlocked q The door is locked q The door is unlocked q

q 是我按下的按钮。

这是代码。如果有任何帮助，我将不胜感激。

## program for a simple door locking device
## register use:    $v0: stores syscall code
##                  $a0: stores entered number
##                  $s0: stores status number

.data
    locked:     .asciiz "The door is locked\n"
    unlocked:   .asciiz "The door is unlocked\n"
    unlock:     .asciiz "Press any key to unlock\n"
    lock:       .asciiz "Press any key to lock\n"

.text
.globl main

main:

# store default status value
ori $s0, [=11=], 1      # store the default 1

# output the status
li $v0, 4           # system call code for print_str
la $a0, locked      # print "The door is locked"
syscall

loop: 

bgtz $s0, unlocking     # if $s0 > 0, start unlocking
beq $s0, [=11=], locking    # if $s0 = 0, start locking

unlocking:  

# output the user instruction
li $v0, 4               # system call code for print_str
la $a0, unlock          # print "Press any key to unlock"
syscall

# ask for input
li $v0, 8               # system call code for read_str
syscall

# set status to unlocked (0)
ori $s0, [=11=], 0      

# output new status
li $v0, 4               # system call code for print_str
la $a0, unlocked        # print "The door is unlocked"
syscall

j loop  

locking:

# output the user instruction
li $v0, 4               # system call code for print_str
la $a0, lock            # print "Press any key to lock"
syscall

# ask for input
li $v0, 8 # system call code for read_str
syscall

# set status to locked (1)
ori $s0, [=11=], 1  

# output new status
li $v0, 4           # system call code for print_str
la $a0, locked      # print "The door is locked"
syscall     

j loop  

Answer 1

当您调用 read_str 时，您需要将 a0 设置为指向虚拟输入缓冲区，否则您将覆盖 a0 恰好指向的任何内容。您还需要将 a1 设置为要读取的字符数。

请注意，仅读取单个字符的系统调用 12 可能是更好的选择。

参见list of syscalls。