bB Modulus?

[pacgreg]

I want to make a constant timer that controls a bunch of stuff, including activating a block of code every 5 cycles, but it needs to also go to 45, is there a way i could activate something every multiple of 5 like if timer % 5 == 0 then goto..., but in basic?

[ZackAttack]

Off the top of my head I can think of 3 options:

 

1) Use BCD for the variable that holds the cycle count. Then AND it with %00001111 and check if it is equal to 0 or 5. This option will use a few more CPU cycles than the other two, but is good if you absolutely must use mod 5 and don't have a lot of ROM space available.

 

2) Create a 45 byte lookup table with a 1 stored in every 5th byte and 0 in the rest. Then simply index into the lookup table with the cycle count variable and compare that value with 1. This option is good if you must use mod 5 and have more ROM space than CPU cycles.

 

3) Make it mod 4 or 8 (or any power of 2). Then you can simply AND the count variable with n-1 to obtain mod n quickly. This is the best option to use if you can tolerate using a value that is a power of 2. Ex: cycleCount AND %00000011 is equivalent to cycleCount % 4

[pacgreg]

The second option seems like its useful for this and possibly more things in the future, is it called a data array in the batari page?

[ZackAttack]

Yes, I'm referring to this. Just use the cycle count variable to index into the array.

  if modFiveLookup[cycleCount]!=0 then goto FifthCycle

[bogax]

what else does the timer have to do?
do you really want 45 or do you want
0..44?

 count = count + 1
 if count = 68 then count = 0
 if count & 7 = 5 then count = count + 3

will go through a cycle with a length of
45

but it doesn't count 0..44

bits 0..2 count 0..4 and
bits 3..6 (ie count/8) count 0..8