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base:multiplication_with_a_constant [2020-03-30 22:27]
tww
base:multiplication_with_a_constant [2021-02-10 01:24] (current)
strobe fix a typo
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 ====== 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.
  
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 ---- ----
  
-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).+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. 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):+This gives the following code to multiply A with 10 (assuming A is less than 26):
  
 <code> <code>
base/multiplication_with_a_constant.1585600032.txt.gz · Last modified: 2020-03-30 22:27 by tww