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8086 Assembly : Memcpy procedure for 80188 embedded system

Posted on 2003-10-30
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Hi

I need to write a "memcpy" procedure in assembly for an embedded system running on an 80188 microprocessor. Memcpy() copies n bytes from memory area s2 to s1 and returns s1. It is called in C code as follows:

void *memcpy(void *s1, const void *s2, size_t n)

I am very new to assembly programming and is hoping for some advice how this can be done. Btw, is there a compiler which translate C code into assembly?

Any advice is very much appreciated. Thank you.

Regards
Wilson
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Question by:Wtwh
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dimitry earned 200 total points
ID: 9650933
Any C-compiler should have an option to produce Asm-output.

void memcpy(void far *s1, const void far *s2, size_t n)

_memcpy proc      near
      push      bp
      mov      bp,sp
      push      si
      push      di
      push      ds

      mov      cx, [bp+12]
      lds      si, [bp+8]
      les      di, [bp+4]

      rep      movsb

      pop      ds
      pop      di
      pop      si
      pop      bp
      ret
_memcpy endp
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Expert Comment

by:MrT-RSI
ID: 9659291
To speed things up you might wanna replace movsb with movsw, especially for larger data blocks:

mov cx,[bp+12]
lds si,[bp+8]
les di,[bp+4]

shr cx,1   ;divide by 2
jnc cxeven
  movsb   ;move 1 byte if odd
cxeven:
rep movsw   ;move rest of bytes in words

Or was the 80188 the 16 bit processor with the 8 bit mem access ? then stick with the original.
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LVL 22

Expert Comment

by:grg99
ID: 9665740

You'd better do a "cld" beforehand!

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