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I want to know another way to count character of a string not use 0Ah/int21h. Thank for your help :D

Hoàng Minh Toàn
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1 Answers1

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Start with a register pointing to the first character of the string and another register set to zero.

Loop until the contents of the first register are the NUL character (or whatever terminator you want), incrementing both those registers.

At the end, the second register will have the length.

In pseudo-asm-code (since it's very likely this is classwork):

    push  r1, r3                 ; preserve registers
    load  r1, address-of-string
    xor   r2, r2                 ; xor a number with itself gives 0
startloop:
    load  r3, [r1]               ; get memory contents
    beq   endloop                ; if NUL, we're at string end
    inc   r1                     ; otherwise incr both and loop back
    inc   r2
    bra   startloop
endloop:
    pop   r3, r1                ; restore registers
                                ; r2 now has the length.

Your job is to now turn that into real assembly code (most likely x86 given your mention of int 21h).

paxdiablo
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  • yeah it's my homework :D i try to find out another way but my code is too long. tks you again – Hoàng Minh Toàn Oct 24 '16 at 12:24
  • BTW, `xor same,same` is only a good zeroing idiom on x86. On many other architectures, including ARM, [it's architecturally required to carry a dependency on the input register (to preserve memory_order_consume semantics)](http://stackoverflow.com/questions/37222999/convert-c-to-assembly-with-predicated-instruction/37224546#comment61983585_37224546), so `mov r2, 0` is the same size (or smaller on ARM Thumb2) and significantly better. But clearly you were just trying to write x86 asm in disguise, so +1 :) – Peter Cordes Oct 24 '16 at 13:15