base:multiplication_with_a_constant
Differences
This shows you the differences between two versions of the page.
Next revision | Previous revisionNext revisionBoth sides next revision | ||
base:multiplication_with_a_constant [2015-04-17 04:33] – external edit 127.0.0.1 | base:multiplication_with_a_constant [2020-04-09 21:30] – Added credits ftc | ||
---|---|---|---|
Line 1: | Line 1: | ||
====== Multiplication with a constant ====== | ====== Multiplication with a constant ====== | ||
+ | |||
+ | By TWW | ||
+ | |||
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 17: | ||
adc $2 ; Add the two results together | adc $2 ; Add the two results together | ||
</ | </ | ||
+ | |||
+ | ---- | ||
+ | |||
+ | EDIT: 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 25): | ||
+ | |||
+ | < | ||
+ | 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.txt · Last modified: 2021-02-10 01:24 by strobe