Unsigned integer math in BlackBox

From: [at]} <Wojtek>
Date: Sun, 16 Nov 2003 12:04:32 -0500 (EST)

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Hello:

 I just reached a conclusion, that lack of unsigned integer math
in BlackBox is a royal pain in the bottom. If not for my respect
to our OuS friends, I would have used more explicit language
to describe their language design decision.

Here is the story. For last three days I has chasing missing bits
in the memory pointer which I read from an field-programmable
gate array. As usual in such circumstances, I was suspecting
bad FPGA timing, or bugs in the embedded microcontroller C-code,
or perhaps bad solder joints on my PCB. It did not occur to me
that our rock-solid BlackBox can introduce subtle bugs. Right?
Wrong. It can and it did. Here is how.

The FPGA memory pointer is read in two 8-bit chunks, an MSB and LSB.
The LSB contains lower 8 bits, while the MSB contains the remaining
bits. The MSB and LSB arrive in a byte buffer, and then they are
combined in the PC into one word. Written in the (vastly inferior
and known for its obscurity) C-language this would have been simple:

  unsigned char lsb, msb;
  unsigned int memPtr;
  msb = buffer [2];
  lsb = buffer [3];
  memPtr = (msb << 8) + lsb;

In the (vastly superior and known for its clarity) Component Pascal
I have written code, which I thought was doing the same thing.

   VAR buffer: ARRAY 256 of BYTE;
       memPtr: INTEGER;
   memPtr := ASH (buffer [2], 8) + buffer [3];

This is nice and looks innocent enough. Then the hell broke loose.
Sometimes the memory pointer was correct. But sometimes it was not.
Sometimes, but not always, bits were missing in the upper part of
the pointer. After three days of debugging it turned out that
the correct code should have been:

  VAR lsb, msb: INTEGER;
       buffer: ARRAY 256 of BYTE;
       memPtr: INTEGER;
  msb := ORD (BITS (ASH (buffer [2], 8)) * {8..15});
  lsb := ORD (BITS (buffer [3]) * {0..7});
  memPtr := msb + lsb;

I leave it to you to figure out why this obfuscated code is the
correct CP version of "(msb << 8) + lsb". It is also important
to note that the bug was showing only for some bit patterns,
but not for the others, what made it exceptionally hard to debug
the original CP code.

I want to point out that the CP language is not complete. The unsigned
math should have been there. The users have been arguing with OuS for
years that the unsigned math is essential, all in vain. I hope that
I have made it more clear why the unsigned should have been in the
language.

Thank you,

Wojtek Skulski skulski{([at]})nowhere.xy

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Received on Sun Nov 16 2003 - 18:04:32 UTC