Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!willett!ForthNet
From: ForthNet@willett.UUCP (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: PYGMY Forth
Message-ID: <460.UUL1.3#5129@willett.UUCP>
Date: 13 Feb 90 23:36:34 GMT
Organization: Latest link in the ForthNet chain.  (Pgh, PA)
Lines: 58

Category 1,  Topic 45
Message 24        Tue Feb 13, 1990
F.SERGEANT [Frank]           at 01:01 CST
 
 To Doug Philips re changing registers in Pygmy CODE words

 Here's a quote from screen 2102 (in ASM.DOC):

         If your routine disturbs CS, DS, BP, SP, SI, or BX
      it must restore it.  The direction flag must be left clear.

   Yes, it is perfectly proper to change the other registers without 
restoring them or to change the above registers if you restore them.   Forget
about CS, you aren't going to change it.  Suppose you want to do a DOS Int $21
call that needs other registers set up than the built  in words DOS or DOS2
provide.  For example, you might want to use  function $40 "Write to a File or
Device" and you want to use a buffer  outside of Pygmy's segment.

   CODE WRITE-BUF
    ( buf-offset  buf-seg  handle  count -  #bytes-written f)
    BX CX MOV,  ( move count from TOS to CX)
    BX POP,     ( put handle in BX)
    AX POP,     ( put seg temporarily in AX)
    DX POP,     ( put offset in DX)
           (  now we have all our parameters off the data stack)
           (  and it is free for us to use to save DS          )
    DS PUSH,    ( save DS)
    AX DS MOV,  ( reload it with segment)
    $4000 #, AX MOV,   ( tell DOS we want function $40 )
    $21 INT,           ( do it )
           ( now AX holds bytes actually written if carry is not set)
           ( or an error code if carry is set )
    DS POP,    ( restore DS )
    AX PUSH,   ( push either count or error code )
    BX BX SBB, ( convert the carry flag to a forth flag of -1 or 0 )
               ( this might have been clearer if written this way  )
               (    CS, IF,  -1 #, BX MOV, ELSE, BX BX SUB,  THEN, )
    NXT,
  END-CODE

  Note that we don't bother preserving AX or CX or DX.  BX is altered,  but
carefully, as we must see that it holds Top of Stack.  DS was saved before the
$21 INT, and restored afterwards.  I haven't tested the  above routine put it
should get the general idea across.  You could  change the code to accomodate
a different order for the operands.

  You know what must go in the various registers by consulting a  reference
book on DOS calls.  I'm looking at a Microsoft programmer's  reference manual
for MS-DOS 2.0.  

 And, what's the worst that can happen if you screw it up?  Probably  just
wiping out your hard disk.  Current backups might add to your  peace of mind,
but Forth programmers live dangerously and crash the  system all the time.  

  -- Frank
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