ARM 程序集，将换行符ASCII 10更改为空终止并重新打印后，将打印出额外的字符字母 A

如何解决ARM 程序集，将换行符ASCII 10更改为空终止并重新打印后，将打印出额外的字符字母 A

该程序的目的是采用换行终止字符串 (ASCII 10) 并使其成为空终止字符串。我正在尝试将空终止字符串打印回控制台进行确认，但这是我看到的行为：

pi@raspBerrypi:~ $ ./tester

Please enter 4 different numbers between 1-5 together without space or special characters. 
 
1234
1234A
pi@raspBerrypi:~ $ 
pi@raspBerrypi:~ $ 

这是程序，我跟踪了它，但我没有看到 A 来自哪里。这是为 ARMv7 编写的程序集

 .global _start

_start:
  LDR r1,=prompt
        BL _sPrint
    
        LDR     r1,=userInput      @ point to the space allocated for input
        MOV     r2,#4              @ set the limit of character to read in
        BL _sInput
        
        LDR r1,=userInput
        BL _sPrint
        
        Ldr r1,=newline
        BL _sPrint
        
        B _exit
    
    
    @_sPrint prints out a string based on it's variable length determined by _strlen
    @strlen,and findEnd are both needed for _sPrint.
    
    _sPrint:
        MOV r7,#4          @sets r7 to console STDOUT
        MOV r0,#1          @set WRITE destination to STDOUT (terminal)
        PUSH {r0,r1,lr}   
        BL _strLen          @gets the stringlength and the end
        POP {r0,lr}    
        SWI 0
        mov pc,lr
    
    _strLen:
      mov   r2,#0
    
    findEnd:
     ldrb   r0,[r1],#1
      add   r2,r2,#1
      cmp   r0,#0
      bne findEnd
      sub   r2,#1
      mov   pc,lr
    
    _sInput:
        PUSH {R1-R8,lr}
        MOV r7,#3          @register r7 being set to 3 to indicate message being read in (read syscall)
        MOV r0,#0          @Set READ device to the STDIN (keyboard)
        SWI 0
        POP {R1-R8,lr}
    
    @String fix takes a string value at r1's address and changes the line Feed to be null termianted.   
    strfx:
        LDRB r0,#1    @loads a single byte from r1 (r1 is dereferenced),which is the _sInput to r0
        CMP r0,#10         @is r0 our newline?
        BNE strfx
        
        MOV r0,#0          @set r0 to null
        STRB r0,[r1,#-1]  @store r0's value back into r1's current address location. The final address 
        MOV PC,LR          @location of r1 newline to be the NULL in r1.

    _exit:
    MOV r7,#1
    SWI #0
    
 .data
    
    prompt:     .asciz "\nPlease enter 4 different numbers between 1-5 together without space or special characters. \n \n"
    newline:    .asciz  "\n"
    userInput:  .space 4