Selecting a bit?

[Trip2018]

So I was wondering if there is a simple way to read and select a bit with a variable.

 

Like a possible implementation of a{6} = b{x}

 

I know course it throws an error for that specifically, but are there any alternatives? I'm trying to setup a 4 way platformer map where the bits are read across the x axis.

[bogax]

here's one way

setbyte is the kernel table for selecting bits (for eg setting playfield pixels)

 

the exact form of setbyte depends on the kernel but you could do your own table if neccesary

  if setbyte[x] & b then a{6} = 1 else a{6} = 0

 

Edited May 17, 2022 by bogax

[Trip2018]

Oh, that was a lot simpler than I was anticipating! Does the setbyte table loop once the variable is over 7 or would I need to reset the variable before I read? Very much appreciated, thanks!

[KevKelley]

So could something like this also be used to count the bits turned on or off on the screen instead of selecting, like 

  if setbyte[x] & b then c=c+1

[ZippyRedPlumber]

Bit variables can be activated by putting a 1 ex. _Bit1{1} = 1. Deactivating them is simple, change the 1 into a 0 ex. _Bit1{1} = 0

 

See also: https://www.randomterrain.com/atari-2600-memories-batari-basic-commands.html#bit

[bogax]

6 hours ago, Trip2018 said:

Oh, that was a lot simpler than I was anticipating! Does the setbyte table loop once the variable is over 7 or would I need to reset the variable before I read? Very much appreciated, thanks!

the setbyte table is for a row of playfield pixels

some bytes in the playfield are reversed but which ones depends on kernel/kernel options

you'd have to limit x

[bogax]

2 hours ago, KevKelley said:

So could something like this also be used to count the bits turned on or off on the screen instead of selecting, like 

  if setbyte[x] & b then c=c+1

yes you could do that if eg you're going to access bits randomly

if you're doing a byte at a time a table would be better