Has Anyone Written PDP-8 .XOR. Code Using the MQ Register (Without the EAE)?
kylevowen at gmail.com
Wed Dec 23 22:43:54 CST 2015
Steve Gibson talks about XOR on the PDP-8 here:
In summary, Steve uses this condensed code using "carryless addition" for
CLA / clear accumulator (AC) since all we have is add!
TAD ArgOne / add (TAD) ArgOne to the just-zeroed AC
AND ArgTwo / AND ArgTwo to determine where the carrys will be
CLL RAL / clear the LINK (CLL) and rotate the accumulator left (RAL)
CMA IAC / compliment (CMA) & increment (IAC) the accumulator (negate)
TAD ArgOne / add the first argument to the negated accumulator
TAD ArgTwo / and add the second argument as well
/ the accumulator now contains the XOR of ArgOne & ArgTwo
Best I can tell, Charles proposed the same thing, but for whatever reason
changed the last instruction to DCA instead of TAD.
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