LC3字符输入陷阱自定义提示：数字输入子程序实现咨询

Got it, let's tackle this LC-3 subroutine problem where we need to replace the default IN trap's prompt with our custom message. Here's how to fix and complete your code properly:

First, let's refine your existing code and build out the full solution, then break down how each part works.

Full Working Code #

.ORIG x3000

; Main program: Set up custom input handler, then trigger IN trap
JSR SETUP_CUSTOM_IN
TRAP x23          ; Calls our modified IN routine
HALT              ; Exit after input is received

; Subroutine to save registers (fixed address case and added R0)
SAVE
    ST R7, SAVE_R7  ; Save return address
    ST R3, SAVE_R3
    ST R2, SAVE_R2
    ST R1, SAVE_R1
    ST R0, SAVE_R0  ; R0 is used for output, so we need to save it too
    RET

; Subroutine to restore saved registers
RESTORE
    LD R0, SAVE_R0
    LD R1, SAVE_R1
    LD R2, SAVE_R2
    LD R3, SAVE_R3
    LD R7, SAVE_R7
    RET

; Subroutine to replace the default IN trap vector with our handler
SETUP_CUSTOM_IN
    ST R7, SETUP_R7 ; Save return address for this setup routine
    
    ; Save the original IN trap routine address (stored at x0023)
    LD R0, TRAP_IN_ADDR
    LDR R1, R0, #0
    ST R1, ORIG_IN_ROUTINE
    
    ; Overwrite the trap vector with our custom handler's address
    LD R1, CUSTOM_IN_ADDR
    STR R1, R0, #0
    
    LD R7, SETUP_R7 ; Restore setup routine's return address
    RET

; Our custom IN trap handler: prints prompt, then uses original IN logic
CUSTOM_IN_HANDLER
    JSR SAVE        ; Save registers before modifying them
    
    ; Print our custom prompt using PUTS trap
    LD R0, PROMPT_ADDR
    TRAP x22        ; PUTS outputs the null-terminated string
    
    ; Call the original IN routine to get user input (stores char in R0)
    LD R0, ORIG_IN_ROUTINE
    JSRR R0         ; Jump to the saved original IN routine
    
    JSR RESTORE     ; Restore all saved registers
    RTI             ; Return from trap mode (required for trap handlers)

; Data storage section
PROMPT_ADDR .FILL x3100       ; Address of our prompt string
TRAP_IN_ADDR .FILL x0023      ; Address of the IN trap vector
CUSTOM_IN_ADDR .FILL x3020    ; Address of our custom handler
ORIG_IN_ROUTINE .BLKW 1       ; Holds the original IN routine address

; Saved registers storage
SAVE_R0 .BLKW 1
SAVE_R1 .BLKW 1
SAVE_R2 .BLKW 1
SAVE_R3 .BLKW 1
SAVE_R7 .BLKW 1
SETUP_R7 .BLKW 1

; Null-terminated prompt string (required for PUTS trap)
.ORIG x3100
    .STRINGZ "Please enter a number between 0 and 9:"

.END

Key Breakdowns #

• Register Safety: The SAVE and RESTORE subroutines ensure we don't overwrite any register values the main program relies on. We added R0 because we use it with the PUTS trap to print the prompt.
• Trap Vector Modification: The SETUP_CUSTOM_IN routine first saves the original IN trap address (stored at x0023), then replaces that vector with our custom handler. This makes TRAP x23 run our code instead of the default.
• Custom Prompt: We use the PUTS trap (x22) to print our string, which is marked with .STRINGZ to automatically add the null terminator PUTS needs to know when to stop printing.
• Reusing Original IN: After printing the prompt, we jump to the original IN routine we saved—this lets us keep the standard input functionality (reading a character into R0) without rewriting it from scratch.
• RTI Instruction: Trap handlers must end with RTI instead of RET to properly exit trap mode and return to the main program.

Quick Notes #

• LC-3 assemblers are case-sensitive for address prefixes—always use lowercase x for hex addresses (fixed your original X302F to x302F equivalents).
• If you need to restore the default IN trap later, add a cleanup subroutine that writes ORIG_IN_ROUTINE back to x0023.

内容的提问来源于stack exchange，提问作者Kacper Miklus