MIPS汇编代码计算姓名ASCII最值时最小值错误的修复咨询

我写了一段MIPS汇编代码，用来读取输入的姓名（忽略姓名中间的空格），计算所有字符的ASCII最大值和最小值。运行时最大值结果正确，但最小值出错：输入Isaac Newton后，输出最小值为10，预期应该是73。相关代码、实际输出和预期输出如下，怎么修复？

.data
prompt: .asciiz "Enter your first and last name: "
max_msg: .asciiz "\nMaximum ASCII value: "
min_msg: .asciiz "\nMinimum ASCII value: "
newline: .asciiz "\n"

.text
globl main

main:
    # Prompt for input
    li $v0, 4       # system call for print string
    la $a0, prompt  # load address of prompt
    syscall

    # Read input
    li $v0, 8       # system call for read string
    la $a0, buffer  # load address of buffer
    li $a1, 255     # set maximum length of input
    syscall

    # Find maximum and minimum ASCII values
    la $t0, buffer   # load address of buffer into $t0

    li $t1, 0x20     # initialize $t1 to the ASCII value of space
    li $t2, 0x7F     # initialize $t2 to the largest possible ASCII value
    li $t3, 0x00     # initialize $t3 to the smallest possible ASCII value
    li $t4, 0         # initialize $t4 to 0

loop:
    lb $t5, ($t0)    # load byte from current position
    beq $t5, 0, done # if byte is null terminator, we're done

    # if byte is a space, skip it
    beq $t5, $t1, next

    # if this is the first non-space character, initialize $t4
    beq $t4, 0, init

    # find maximum ASCII value
    bgt $t5, $t3, is_max
    j is_min

is_max:
    move $t3, $t5    # set $t3 to current byte
    j next

    # find minimum ASCII value
is_min:
    blt $t5, $t2, set_min
    j next

set_min:
    move $t2, $t5    # set $t2 to current byte
    j next

init:
    # initialize $t4 to the first non-space character
    move $t4, $t5
    j next

next:
    addi $t0, $t0, 1 # move to next byte
    j loop           # jump to loop

done:
    # Print results
    li $v0, 4         # system call for print string
    la $a0, max_msg   # load address of max message
    syscall

    li $v0, 1         # system call for print integer
    move $a0, $t3     # load maximum ASCII value
    syscall

    li $v0, 4         # system call for print string
    la $a0, min_msg   # load address of min message
    syscall

    li $v0, 1         # system call for print integer
    move $a0, $t2     # load minimum ASCII value
    syscall

    li $v0, 4         # system call for print string
    la $a0, newline   # load address of newline
    syscall

    # Exit program
    li $v0, 10        # set system call number to 10 for exit
    syscall           # perform system call to exit
.data
buffer: .space 256   # allocate space for input buffer

实际输出：
Enter your first and last name: Isaac Newton
Maximum ASCII value: 119
Minimum ASCII value: 10

预期输出：
Enter your first and last name: Isaac Newton
Maximum ASCII value: 119
Minimum ASCII value: 73

问题原因 #

1. 初始值未正确初始化：第一个非空格字符仅存入$t4，未同步设置为$t2（最小值）和$t3（最大值），导致最小值的初始基准还是0x7F（127），后续会被输入末尾的换行符（ASCII 10）覆盖。
2. 未过滤换行符：MIPS的字符串读取调用会把用户输入的换行符（ASCII 10）存入缓冲区，代码仅跳过空格，未处理换行符，导致其参与最小值比较。

修复步骤 #

1. 用第一个有效字符初始化最大/最小值：修改init标签，将第一个非空格、非换行字符同时赋值给$t2和$t3，确立正确的比较基准。
2. 过滤换行符：在跳过空格的判断后，新增对换行符（0x0A）的检测，跳过该字符。

修复后的代码 #

.data
prompt: .asciiz "Enter your first and last name: "
max_msg: .asciiz "\nMaximum ASCII value: "
min_msg: .asciiz "\nMinimum ASCII value: "
newline: .asciiz "\n"

.text
globl main

main:
    # Prompt for input
    li $v0, 4       # system call for print string
    la $a0, prompt  # load address of prompt
    syscall

    # Read input
    li $v0, 8       # system call for read string
    la $a0, buffer  # load address of buffer
    li $a1, 255     # set maximum length of input
    syscall

    # Find maximum and minimum ASCII values
    la $t0, buffer   # load address of buffer into $t0

    li $t1, 0x20     # initialize $t1 to the ASCII value of space
    li $t2, 0x7F     # initialize $t2 to the largest possible ASCII value
    li $t3, 0x00     # initialize $t3 to the smallest possible ASCII value
    li $t4, 0         # initialize $t4 to 0 (flag for first valid char)

loop:
    lb $t5, ($t0)    # load byte from current position
    beq $t5, 0, done # if byte is null terminator, we're done

    # Skip space and newline characters
    beq $t5, $t1, next
    li $t6, 0x0A     # ASCII value of newline
    beq $t5, $t6, next

    # Initialize max and min with first valid character
    beq $t4, 0, init

    # Update maximum ASCII value
    bgt $t5, $t3, update_max
    # Update minimum ASCII value
    blt $t5, $t2, update_min
    j next

update_max:
    move $t3, $t5
    j next

update_min:
    move $t2, $t5
    j next

init:
    # Set first valid char as both initial max and min
    move $t3, $t5
    move $t2, $t5
    li $t4, 1         # Mark first char as processed
    j next

next:
    addi $t0, $t0, 1 # move to next byte
    j loop           # jump to loop

done:
    # Print results
    li $v0, 4         # system call for print string
    la $a0, max_msg   # load address of max message
    syscall

    li $v0, 1         # system call for print integer
    move $a0, $t3     # load maximum ASCII value
    syscall

    li $v0, 4         # system call for print string
    la $a0, min_msg   # load address of min message
    syscall

    li $v0, 1         # system call for print integer
    move $a0, $t2     # load minimum ASCII value
    syscall

    li $v0, 4         # system call for print string
    la $a0, newline   # load address of newline
    syscall

    # Exit program
    li $v0, 10        # set system call number to 10 for exit
    syscall           # perform system call to exit
.data
buffer: .space 256   # allocate space for input buffer

修复说明 #

• 新增换行符过滤逻辑，避免无效字符干扰计算；
• 第一个有效字符同时作为最大值和最小值的初始值，确保后续比较基准正确；
• 优化分支逻辑，让最大/最小值的更新流程更清晰。

内容的提问来源于stack exchange，提问作者Sayma Sultana