Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!uunet!mcvax!hp4nl!eutrc3!rcbaem
From: rcbaem@eutrc3.urc.tue.nl (Ernst Mulder)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Can anyone anser these Questions?
Message-ID: <864@eutrc3.urc.tue.nl>
Date: 22 Aug 89 19:10:38 GMT
Reply-To: rcbaem@eutrc3.uucp
Organization: Eindhoven University of Technology, The Netherlands
Lines: 72


 Well, anyway. Thank you for trying to answer my previous questions. At this
time all problems are solved. I even started a new project. I finished this
program, and well, getting the hang of it, I thought why not start a new
one? But I'm faced with a new problem:

 I have a very nice calculator, called a PC-2 (Radio Shack) known as a
PC-1500 (Sharp) as well (it's originally a Sharp machine). Since the only
calculator resembling an existing one, on the Mac, I know of is a DA
version of HP's HP12, I desided to write my own one.
 The program is finished (took one evening :) except for one (essential) thing:
I have to write an interpreter for the PC-2's machine code. The PC-2 is
both BASIC and Machine-code programmable, so I'll HAVE to write an
interpreter implementing the PC-2's microprocessor.

The problems:

1: The interpreter is probably going to be very slow.
2: The PC-2 can access 64K bytes of memory. Therefore I allocated a 64K
   locked object, with a pointer to it. Let's call this pointer memStart.

   Then I got a terrible fight with Pascal...

   The PC-2's processor has 16bit registers. So I have to access bytes
   in the 64K memory block. An instruction like LDA (P) could
   be implemented as follows (in pascal): (A is a Byte, P is an integer
   resembling a register of the PC-2's processor)

         A := Ptr(LongInt(memStart) + P)^;

   PROBLEM!! Assume P has the (integer) value -1. (=$FFFF) and say
   memStart = 0; (LongInt(memStart) + P) then gives -1 instead of $FFFF..
   Whatever typecasting I tried, I end up with $FFFF beging extended
   to the LongInt $FFFFFFFF... I finally found a solution as follows:

   type register = record case boolean of
                     true: (h:integer;
                            l:integer);
                     false:(hl:LongInt)
                   end; 

   var P:register;

   When at initialisation time P.hl is set to 0 the following DOES work
   as expected:
        P.hl := 0;

        p.l := 0;
        p.l := p.l - 1;    (p.l is now $FFFF (or -1))

        A := Ptr(LongInt(memStart) + p.hl)^;

   (When memStart had the value 0, then this addition would indeed give
    the wanted address $FFFF).

Some suggestions of mine are:

1: a) Don't try to write this program, it's bound to be slow.
   b) Write it in assembly instead of in Pascal. (But LSP's code is
      rather efficient, I found extremely nice trics wandering through
      disassembled LSP compiled programs..)
   c) Claim the program runs on 68030 Macs only :)

2: Pascal is a terribly rotten language thanks to it's excellent type
   restrictions. Write it in Assembly or in C..

Okay, but I don't have an Assembler, nor a C compiler..

Any ideas?

 Ernst.
   >