Using a micro with no hardware multiply? You can optimize square multiples with lookup tables!
The 8088 has a hardware multiplier, but it’s quite slow:
MUL with byte arguments: 69 cycles plus 1 for each set bit in AL plus 1 if the high byte of the result is 0
MUL with word arguments: 123 cycles plus 1 for each set bit in AX plus 1 if the high word of the result is 0
Signed multiplies are even slower (taking at least 80 cycles for a byte, 134 cycles for a word), and depend on the number of set bits in the absolute value of the accumulator, the signs of the operands and whether or not the explicit operand is -0x80 (-0x8000 for word multiplies). I also measured some word IMULs apparently taking a half-integer number of cycles to run, suggesting that there’s either some very weird behavior going on with the 8088’s multiplier or that there’s a bug in my timing program (possibly both).
Can we beat the 8088’s hardware multiplier with a software routine?
To learn how to do this, read further!
Have an amazing project to share? Join the SHOW-AND-TELL every Wednesday night at 7:30pm ET on Google+ Hangouts.
Join us every Wednesday night at 8pm ET for Ask an Engineer!
Learn resistor values with Mho’s Resistance or get the best electronics calculator for engineers “Circuit Playground” – Adafruit’s Apps!
Maker Business — Airbnb’s Internal University to Teach Data Science
Wearables — Faking wood
Electronics — Trouble with LM741
Biohacking — Nike’s Unlimited Stadium Will Put Your Best Foot Forward
Sorry, the comment form is closed at this time.