Opening a file from assembly

[Ric]

Could someone please explain why th efollowing code does not work

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      PROCassembly

      CALL start

      CLOSE#0

      END

      DEF PROCassembly

      DIM code% NOTEND AND 2047, code% 2048-1

      FOR I% = 0 TO 2 STEP 2
  
        P% = code%
        [
  
        opt I%
        .text DB "success.bbc"
        DB &D
        .start
        mov ebx,text
        mov eax,0
        mov al,1
        call "osopen"
        ret
  
        ]
      NEXT

      ENDPROC

[Richard Russell]

Could someone please explain why the following code does not work

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        mov ebx,text
        mov eax,0
        mov al,1
        call "osopen"
        ret

The relevant section of the manual states:

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CALL "osopen" ; Open a file, EDX addresses filename, AL = 0 (read),
              ; 1 (create) or 2 (update), channel number returned in EAX

The main difference from your code is that the documentation says edx but your code has ebx. Also your code has a mov eax,0 which is superfluous.

[Ric]

Thanks Richard,
I was always under the impression that if you were only loading the low or high byte, the rest of the register remained unchanged, thus the need to flush it first.

[Richard Russell]

I was always under the impression that if you were only loading the low or high byte, the rest of the register remained unchanged, thus the need to flush it first.

The rest of the register remains unchanged, yes, but that's fine since the subsequent code doesn't care about the rest of the register. If it did, osopen would be documented as receiving the parameter in eax, not in al, and your code would need to be:

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        mov edx,text
        mov eax,1
        call "osopen"
        ret

This code will work, of course, but it's wasteful because it's zeroing the other 24 bits of eax unnecessarily.

In a circumstance, which isn't the case here, when you need to zero-extend or sign-extend the LS 8 bits of eax into the remaining 24 bits then use the specific instructions provided:

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        movzx eax,al	; zero-extend al into eax
        movsx eax,al	; sign-extend al into eax

Finally, remember that on a 64-bit CPU zero-extending a 32-bit register into a 64-bit register happens automatically, it doesn't need any explicit code. For example xor eax,eax zeroes all 64-bits of rax, not just the lower 32-bits as you might expect.

[Ric]

Thanks Richard