PDP-8 Code Optimization (was Re: HP-35/45 Simulator for PDP-8)
v.slyngstad at frontier.com
Wed Sep 14 07:17:20 CDT 2016
From: Kyle Owen: Tuesday, September 13, 2016 7:12 PM
> TAD OVFA /A XOR B
> AND OVFB
> CMA IAC
> TAD OVFA
> TAD OVFB
There seems to be an issue with my implementation of XOR.
Before the CMA IAC there needs to be CLL RAL, to reposition
the carries before subtracting them. Both XORs are affected.
Since all we are interested in is the sign of the XOR results,
only some of the sums end up wrong. Mostly where both
operands are negative; there will be a carry out of the sign
bit when we add them together.
Kyle: I wrote to code to exhaustively enumerate the operand
space, and some Perl to generate "correct" answers. Wrote
the results to simulated paper tape. With the modification
described, the 250Mb result files match. Good thing that
paper tape was a simulated one :-).
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