How to skip spaces at the beginning of the string in assembly x86

user_14811059 0
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What loop should I write If I want to skip all spaces at the beginning of the string and start to do something with the string when my code reaches the first symbol of the string. If I have a string like

a='        s o m e word'

the code should start when 's' is reached. It should be some kind of loop but I still don't know how to write it correctly.

My try:

    mov  si, offset buff+2 ;buffer

    mov ah, [si]
    
    loop_skip_space:
        cmp ah,20h ;chech if ah is space            
        jnz increase ;if yes increase si 
        jmp done ;if no end of loop
        increase:
                          
            inc si 

    loop loop_skip_space
    
    done:
x86-16 whitespace assembly string
2 Answers

In this 16-bit code that fetches its string offset at buff+2, I believe it's a safe bet to consider this string to belong to the input gotten from executing the DOS.BufferedInput function 0Ah.
The code snippets below are based on this assumption. Next observations about the OP's code remain valid anyway.

  • The mov ah, [si] must be part of the loop. You need to verify different characters from the string, so loading following bytes is necessary.
  • Your code should exit the loop upon finding a non-space character. Currently you exit upon the first space.
  • The loop instruction requires setting up the CX register with the length of the string. You omitted that.
  mov  si, offset buff+2
  mov  cl, [si-1]
  mov  ch, 0
  jcxz done               ; String could start off empty
loop_skip_space:
  cmp  byte ptr [si], ' '
  jne  done
  inc  si                 ; Skip the current space
  loop loop_skip_space
done:

Here SI points at the first non-space character in the string with CX characters remaining. CX could be zero!

You can write better code if you stop using the loop instruction, because it's an instruction that is said to be slow. See Why is the loop instruction slow? Couldn't Intel have implemented it efficiently?.

  1. Avoiding loop and no longer requiring the use of CX
  mov  si, offset buff+2
  mov  cl, [si-1]
  test cl, cl
  jz   done               ; String could start off empty
loop_skip_space:
  cmp  byte ptr [si], ' '
  jne  done
  inc  si                 ; Skip the current space
  dec  cl
  jnz  loop_skip_space
done:
  1. Avoiding loop and using the delimiter 13 (carriage return)
  mov  si, offset buff+1
loop_skip_space:
  inc  si
  cmp  byte ptr [si], ' '
  je   loop_skip_space
user_31447700

look at the example

    STRLEN    EQU  9
    STRING    DB   'Assembler'

              CLD                    ;clear direction flag
              MOV  AL,' '            ;symbol to find.
              MOV  ECX,STRLEN         ;length of string
              LEA  EDI,STRING         ;string pointer.
              REPE SCASB            ;search itself
              JNZ  K20               ;jump if not found
              DEC  EDI                ;
; EDI points to your first not space char
    K20:      RET
user_31151290
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