Jeff Epler's blog

The oldest bug I ever fixed?

I've been playing with Tom Pittman's Tiny BASIC for 6800 computers. It's a very simple, small, and early BASIC implementation. It is also almost as old as me, being announced by February 1976.

In this BASIC, all numbers are 16-bit signed integers with a range of -32768 to 32767 inclusive.

Anyway, while working in Tiny BASIC, I got some weird results and boiled it down to the following:

IF 32513 < 0 THEN PRINT "WAT"

This prints "WAT", even though we usually consider 32513 to be greater than 0.

After spending an evening and a morning with it, and comparing the implementation to Tom Pittman's Tiny BASIC for 6502 computers, I believe that this bug is in the original implementation, and not the emulator I'm using.

The original version performs a 16-bit subtraction from two separate 8-bit subtractions; then, from the final flags value tries to determine whether the full result of the subtraction is greater to, less than, or equal to zero.

This works for many inputs. However, for some particular input values, the CPU flags—which are only aware of the results of the most significant byte of the subtraction—don't match up with the usual notions of comparison.

The fix, which I've implemented in my version of Tiny BASIC, is to instead compare the MSBs first as signed numbers, then if they are equal, to compare the LSBs as unsigned numbers.

I also wrote a test harness which performs about 60,000 comparisons, checking them against Python's idea of integer ordering. Now, all those comparisons (as well as my original test case) pass.

From what I can determine, the listing I have is derived from "TINY BASIC 6800 V.3R (C)1976 BY TOM PITTMAN (TB683R)". The "3" indicates it's the 4th release (0-based version numbering); TB682R was mentioned in ddj in October, 1976.

I'm a bit tickled to think I have fixed a bug that someone wrote when I was less than 1 year old cough 50 years ago cough. It's a bit much to claim I might be the first person who ever noticed or fixed the bug, but I looked through a range of old dr dobb's journal issues and never found anyone who remarked on the bug.

This is quite possibly the earliest-written bug I will ever fix.

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