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base:multiplication_with_a_constant

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 base:multiplication_with_a_constant [2015-04-17 04:33]127.0.0.1 external edit base:multiplication_with_a_constant [2021-02-10 01:24] (current)strobe fix a typo 2021-02-10 01:24 strobe fix a typo2020-04-10 17:25 tww 2020-04-09 21:30 ftc Added credits2020-03-30 22:27 tww 2015-04-17 04:33 external edit Next revision Previous revision 2021-02-10 01:24 strobe fix a typo2020-04-10 17:25 tww 2020-04-09 21:30 ftc Added credits2020-03-30 22:27 tww 2015-04-17 04:33 external edit Line 1: Line 1: ====== Multiplication with a constant ====== ====== Multiplication with a constant ====== + Multiplication by a specific constant can be a lot simpler and faster than a general-purpose routine to multiply two given numbers together. Multiplication by a specific constant can be a lot simpler and faster than a general-purpose routine to multiply two given numbers together. Line 14: Line 15: adc \$2 ; Add the two results together adc \$2 ; Add the two results together + + ---- + + By TWW: + + Optimization of the above posted by unknown: The above is based on 8*A + 2*A. This routine would not allow any higher number than A = 25 as it would overflow (8 * 26) + (2 * 26) = 208 + 52 = 262 (However 25 fits perfectly f.ex. when you'd want to calculate a character line based on Y). + + With this in mind, you can do 2*A + 8*A which saves you the asl \$2. Also as long as A cannot be higher than 25, the upper 4 bits (high nybble) will always be #%0000. This means that the asl instructions will never set carry and the clc can be dropped. + + This gives the following code to multiply A with 10 (assuming A is less than 26): + + + asl      // Multiply A with 2 + sta \$2   // Store A * 2 for later + asl      // Multiply A with 4 + asl      // Multiply A with 8 + adc \$2   // Add A * 8 + A * 2 + + (12 cycles / 7 bytes) vs (17 cycles / 10 bytes) + + ---- You may also use a table, which is even faster, but would take up more space. You may also use a table, which is even faster, but would take up more space.
base/multiplication_with_a_constant.1429237983.txt.gz · Last modified: 2015-04-17 04:33 by 127.0.0.1

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