LC-3：处理输入值的条件语句实现及非数字输入处理求助

Got it, let's tackle this problem step by step. Below is a complete LC-3 assembly program that handles all your requirements: it uses TRAP x23 (IN) to get user input, rejects non-numeric characters, checks if the input value is between 0 and 5, and outputs "yes" or "error" accordingly.

Complete Code #

.ORIG x3000

; Define string constants with newline for clean output
YES_STR    .STRINGZ "yes\n"
ERROR_STR  .STRINGZ "error\n"

; Main program entry
MAIN
    TRAP x23          ; Read single character input, store ASCII in R0
    
    ; Check if input is NOT a digit (ASCII < '0' or > '9')
    LD R1, ASCII_0
    NOT R1, R1
    ADD R1, R1, #1    ; R1 = -x30 (two's complement of '0')
    ADD R2, R0, R1    ; R2 = input ASCII - '0'
    BRn NON_DIGIT     ; If negative, input is less than '0'
    
    LD R1, ASCII_9
    NOT R1, R1
    ADD R1, R1, #1    ; R1 = -x39 (two's complement of '9')
    ADD R2, R0, R1    ; R2 = input ASCII - '9'
    BRp NON_DIGIT     ; If positive, input is greater than '9'
    
    ; Convert ASCII digit to numeric value (0-9)
    LD R1, ASCII_0
    SUB R0, R0, R1    ; R0 now holds the numeric value (0-9)
    
    ; Check if value exceeds 5
    LD R1, MAX_VALUE
    NOT R1, R1
    ADD R1, R1, #1    ; R1 = -5
    ADD R2, R0, R1    ; R2 = value - 5
    BRp OUTPUT_ERROR  ; If positive, value > 5
    
    ; Valid input (0-5): output "yes"
    LEA R0, YES_STR
    TRAP x22
    BR END_PROGRAM

NON_DIGIT
OUTPUT_ERROR
    ; Invalid input: output "error"
    LEA R0, ERROR_STR
    TRAP x22

END_PROGRAM
    TRAP x25          ; Halt the program

; Data constants
ASCII_0    .FILL x30  ; ASCII value of '0'
ASCII_9    .FILL x39  ; ASCII value of '9'
MAX_VALUE  .FILL #5   ; Upper limit for valid input

.END

Key Explanations #

Let's walk through how each part works to make sure you understand:

1. String Setup

   • .STRINGZ creates null-terminated strings (automatically adds x00 at the end), which is required for TRAP x22 (PUTS) to know when to stop printing. The \n adds a newline for readability.

2. Input Handling

   • TRAP x23 grabs a single character from the keyboard and stores its ASCII code in R0.

3. Non-Numeric Check

   • We use two's complement subtraction to compare the input ASCII value against '0' and '9'. If the input is outside this range, we jump straight to the error output.

4. ASCII to Numeric Conversion

   • Subtracting the ASCII value of '0' (x30) from the input character converts '0' → 0, '1' → 1, ..., '9' → 9. This lets us work with the actual numeric value instead of ASCII codes.

5. Range Validation

   • We check if the numeric value is greater than 5 using two's complement subtraction. If it is, we output an error; otherwise, we output "yes".

6. Output Logic

   • LEA R0, STRING loads the memory address of our string into R0. TRAP x22 (PUTS) then prints the entire string starting at that address until it hits the null terminator.

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