LC-3汇编程序无法处理用户输入，请求代码修正与完善指导

功能需求 #

• 提示用户输入LC-3汇编指令（按回车结束）
• 合法指令输出对应4位opcode
• 非法指令输出错误信息
• 输出后循环重启
• 输入QUIT则退出

当前问题 #

现有代码仅能打印提示信息，但输入ADD等指令无响应，输入QUIT也无法触发退出，不知道如何完成剩余功能，需要帮助。

当前代码 #

.ORIG x3000

; Constants for comparison
ADD_STR  .STRINGZ "ADD"
AND_STR  .STRINGZ "AND"
JMP_STR  .STRINGZ "JMP"
BR_STR   .STRINGZ "BR"
QUIT_STR .STRINGZ "QUIT"

; Memory locations
MESSAGE  .STRINGZ "Enter an LC-3 assembly language instruction (type QUIT to exit): "
USER_INPUT .BLKW #20

MAIN
    LEA R0, MESSAGE       ; Display the prompt message
    PUTS
    
    ; Read user input
    LEA R0, USER_INPUT    ; Store user input here
    GETC                  ; Read the first character
    OUT                   ; Echo the character

    ; Loop to read input until Enter/Return is pressed
    LOOP
        GETC              ; Read the next character
        OUT               ; Echo the character
        BRZ END_LOOP      ; Exit loop if Enter/Return is pressed
        STR R0, R1, #0    ; Store the character in memory
        ADD R1, R1, #1    ; Move to the next memory location
        BRNZP LOOP        ; Repeat the loop

    END_LOOP

    ; Compare user input to known instructions
    LEA R1, USER_INPUT    ; Load the input string pointer
    LEA R2, ADD_STR      ; Load ADD instruction pointer
    LEA R3, AND_STR      ; Load AND instruction pointer
    LEA R4, JMP_STR      ; Load JMP instruction pointer
    LEA R5, BR_STR       ; Load BR instruction pointer
    LEA R6, QUIT_STR     ; Load QUIT instruction pointer
    
    ; Compare input with ADD
    CMP_ADD
        LDR R7, R1, #0   ; Load a character from input
        LDR R0, R2, #0   ; Load a character from ADD string
        NOT R0, R0       ; Get the 2's complement of ADD character
        ADD R0, R0, #1   ; Add 1 to compare with input character
        ADD R7, R7, #1   ; Prepare to compare next character
        BRnp ADD_CHECK   ; Check condition

    ADD_CHECK
        BRz IS_ADD       ; If both characters match, it's ADD instruction
        BRnzp CMP_AND    ; Else, check for AND instruction

    IS_ADD
        ; Display the opcode for ADD
        LEA R0, ADD_STR  ; Load ADD instruction string
        PUTS             ; Display it
        BRnzp MAIN       ; Go back to the main loop

    CMP_AND
        ; Similar comparison process for AND
        ; ... (Repeat the above process for AND, JMP, BR, QUIT)

    ; Handling for other instructions and errors can be written similarly

    ; Handle QUIT instruction
    QUIT_CHECK
        ; ...

    ; Other instruction checks and error messages go here

    ; Loop back to the beginning
    BRnzp MAIN

.END

问题分析与修复方案 #

关键问题点

1. 输入初始化缺失：读取输入时未将R1初始化为USER_INPUT的起始地址，导致字符无法正确存入输入缓冲区。
2. 输入未加结束符：输入完成后没有在缓冲区末尾写入字符串结束符\0（x00），后续字符串比较无法识别输入结束。
3. 字符串比较逻辑错误：原比较逻辑未逐字符对比直到结束符，分支条件错误，无法正确匹配指令。
4. QUIT处理未实现：缺少QUIT指令的匹配逻辑和退出程序的代码。

修复后完整代码

.ORIG x3000

; Constants for comparison and output
ADD_STR  .STRINGZ "ADD"
AND_STR  .STRINGZ "AND"
JMP_STR  .STRINGZ "JMP"
BR_STR   .STRINGZ "BR"
QUIT_STR .STRINGZ "QUIT"
OPCODE_ADD .STRINGZ "Opcode: 0001\n"
OPCODE_AND .STRINGZ "Opcode: 0101\n"
OPCODE_JMP .STRINGZ "Opcode: 1100\n"
OPCODE_BR  .STRINGZ "Opcode: 0000\n"
ERROR_MSG  .STRINGZ "Error: Invalid instruction\n"
NEWLINE    .STRINGZ "\n"

; Memory locations
MESSAGE  .STRINGZ "Enter an LC-3 assembly language instruction (type QUIT to exit): "
USER_INPUT .BLKW #20

MAIN
    ; Clear input buffer (optional but good practice)
    LEA R1, USER_INPUT
    CLEAR_BUFFER
        STR R0, R1, #0    ; Store x00 to clear
        ADD R1, R1, #1
        ADD R2, R1, #-20  ; Check if we've cleared 20 bytes
        BRnp CLEAR_BUFFER

    ; Display prompt
    LEA R0, MESSAGE
    PUTS

    ; Read user input
    LEA R1, USER_INPUT    ; R1 points to input buffer start
    GETC
    OUT
    STR R0, R1, #0        ; Store first character
    ADD R1, R1, #1

    LOOP_INPUT
        GETC
        OUT
        BRz END_INPUT     ; Exit loop on Enter (ASCII x00)
        STR R0, R1, #0    ; Store character
        ADD R1, R1, #1
        BRNZP LOOP_INPUT

    END_INPUT
    ; Add string terminator to input
    AND R0, R0, #0        ; R0 = x00
    STR R0, R1, #0

    ; Check for QUIT first
    LEA R1, USER_INPUT
    LEA R2, QUIT_STR
    JSR STRING_COMPARE
    BRz EXIT_PROGRAM

    ; Check ADD
    LEA R1, USER_INPUT
    LEA R2, ADD_STR
    JSR STRING_COMPARE
    BRz PRINT_ADD_OPCODE

    ; Check AND
    LEA R1, USER_INPUT
    LEA R2, AND_STR
    JSR STRING_COMPARE
    BRz PRINT_AND_OPCODE

    ; Check JMP
    LEA R1, USER_INPUT
    LEA R2, JMP_STR
    JSR STRING_COMPARE
    BRz PRINT_JMP_OPCODE

    ; Check BR
    LEA R1, USER_INPUT
    LEA R2, BR_STR
    JSR STRING_COMPARE
    BRz PRINT_BR_OPCODE

    ; If none matched, print error
    LEA R0, ERROR_MSG
    PUTS
    BRNZP MAIN

PRINT_ADD_OPCODE
    LEA R0, OPCODE_ADD
    PUTS
    BRNZP MAIN

PRINT_AND_OPCODE
    LEA R0, OPCODE_AND
    PUTS
    BRNZP MAIN

PRINT_JMP_OPCODE
    LEA R0, OPCODE_JMP
    PUTS
    BRNZP MAIN

PRINT_BR_OPCODE
    LEA R0, OPCODE_BR
    PUTS
    BRNZP MAIN

EXIT_PROGRAM
    LEA R0, NEWLINE
    PUTS
    HALT                  ; Exit program

; String comparison subroutine: R1 = string1, R2 = string2
; Returns Z=1 if equal, Z=0 otherwise
STRING_COMPARE
    COMPARE_LOOP
        LDR R3, R1, #0    ; Get char from string1
        LDR R4, R2, #0    ; Get char from string2
        NOT R4, R4
        ADD R4, R4, #1    ; 2's complement of string2 char
        ADD R5, R3, R4    ; Compare chars
        BRnp COMPARE_END  ; Chars don't match
        BRz COMPARE_END   ; Both are null terminator
        ADD R1, R1, #1    ; Next char
        ADD R2, R2, #1
        BRNZP COMPARE_LOOP
    COMPARE_END
    RET

.END

代码说明

1. 输入处理修复：初始化R1为输入缓冲区起始地址，输入结束后添加\0结束符，确保字符串操作正常。
2. 通用字符串比较子程序：提取STRING_COMPARE子程序，避免重复代码，统一处理字符串匹配逻辑。
3. QUIT处理：优先检查QUIT指令，匹配后调用HALT退出程序。
4. Opcode输出：为每个合法指令定义对应的opcode字符串，匹配后直接输出。
5. 错误处理：所有合法指令匹配失败后，输出错误信息。

内容的提问来源于stack exchange，提问作者Felipe Cotrina