PDP-8 Code Optimization (was Re: HP-35/45 Simulator for PDP-8)

On Tue, Sep 13, 2016 at 3:38 PM, Doug Ingraham <dpi at dustyoldcomputers.com> wrote: Need some more optimization fun? :) Vince and I were working on some code to add two signed 12 bit numbers and detect overflow, returning MAX_INT or MIN_INT in AC in the case of overflow, or the sum in AC otherwise. Here's what Vince came up with so far: CHKOVF, 0 TAD OVFA /GET A TAD OVFB /ADD B DCA OVFSUM /SAVE IT TAD OVFA /A XOR B AND OVFB CMA IAC TAD OVFA TAD OVFB SPA CLA /IF SIGNS DIFFER... JMP NOPROB /WE'RE DONE TAD OVFA /MIGHT BE OVERFLOW, A XOR SUM AND OVFSUM CMA IAC TAD OVFA TAD OVFSUM SMA /DID WE OVERFLOW (DIFFERENT SIGNS)? JMP NOPROB /NO, NO PROBLEM CLA CLL CMA RAR /YES, AC=3777 (MAX INT) DCA OVFSUM /SAVE IT TAD OVFA /GET THE SIGN OF CORRECT RESULT SPA CLA /SHOULD IT BE NEGATIVE? ISZ OVFSUM /YES, 3777 -> 4000 (MIN INT) NOPROB, CLA /GET CORRECTED SUM TAD OVFSUM JMP I CHKOVF /OUTTA HERE OVFA, 0 OVFB, 0 OVFSUM, 0 This tests the signs of both numbers; if they differ, there's no chance of overflow. If they're the same, it checks the signs of the augend with the sum; if they differ, an overflow occurred, and MAX_INT or MIN_INT will be returned depending on the sign of the augend. We tried clever tricks previously, using SNL/SZL and SMA/SPA after shifting the sign of the augend into the link and keeping the addend in the AC, but found these to be longer. Kyle