Can this be done in fewer instructions?
JonnyMac
Posts: 9,378
Reviewing old P1 laser tag code I decided to freshen it up a bit am move some code from Spin to PASM -- more as an exercise than anything. In laser tag we can have coincident signals arriving which get logically OR'd by the sensor. For this reason we measure each pulse width to ensure it's +/- 10% of what it should be.
With SIRCS-style bits there is a leader pulse (2.4ms), a "1" bit (1.2ms), and a "0" bit (0.6ms). That the 1 and 0 bits are even divisions of the leader pulse make things easy. This is what I've done. For you super PASM wonks, would you have arrived at the 10% value differently?
mov t2, sbittix ' make copy of start bit ticks
shr t2, #5 ' t2 = 1/32
mov t3, t2 ' t3 = 1/32
shl t3, #1 ' t3 = 2/32
add t2, t3 ' t2 = 3/32 (~1/10)
Comments
Funny, just the act of posting and looking at the code in the forums made me think this might be [very marginally] better.
mov t2, sbittix ' make copy mov t3, sbtittx ' make copy shr t2, #4 ' t2 = 2/32 shr t3, #5 ' t3 = 1/32 add t2, t3 ' t2 = 3/32 (~1/10)Deleted P1
You could make the assumption your gear is well within the spec and go with a tighter 1/16 instead. That would get you down to a move and shift.
Is the 10% figure more or less arbitrary? It sounds like it is. in which case why not use 12.5% (1/8) as the threshold?
10% seems to be what's agreed upon for SIRCS bits. That said, this is my laser tag code and I can do what I want. This was just a bit of a Saturday exercise
For the numbers you're using you should have quite a bit of headroom, so you could do something like:
mov t2, sbittix ' make copy add t2, sbittix ' t2 = 2 * sbittix add t2, sbittix ' t2 = 3 * sbittix shr t2, #5 ' t2 = (3/32) * sbittixwhich would save an instruction and a scratch register.
The repeated adds seem a bit inelegant, but they're just as fast as shift+add combinations for doing multiply by 3.
There we go. An optimisation of what was already fast. Saturday accomplished.
Absolutely. Thank you, Eric. Sometimes I just want to see a different way of accomplishing something, and I appreciate the feedback.