Forcing a 4 cycle branch

[Mikes360]

Hi Guys,

 

I have a situation where forcing a 4 cycle branch without actually crossing a page boundary would really help me out with my main drawing kernel.

 

I know you can do a sta.w to force that extra cycle is there an equivalent for the branch opcodes?

 

Thanks,

Mike

[Rybags]

Don't think so. Failed branch = 2 cycles, taken = 3, taken with page crossing = 4.

There's illegal opcodes that do NOP that takes >2 cycles.

There's the trick of branching into the operand of a BIT <abs> instruction that can give variable delay.

 

You might try and squeeze the extra cycle in elsewhere but it can be tricky given it's a conditional thing.

[Mikes360]

Ok thanks in my case I will just force the kernel to cross a page boundary to get the 4 cycles. I thought it would be worth asking however.

[Nukey Shay]

If you are only branching over one or two bytes, using a superfluous opcode could work.

 

Example:

 

   LDX $80
   BNE  Non_Zero
   .byte $2C
Non_Zero:
   LDX $90

 

Assuming the destination "Non_Zero" isn't crossing a page, that example burns 9 cycles whether or not the branch is taken. Byte $2C is read as BIT $abs (using 4 cycles, misinterpreting LDX $90 as it's 2-byte argument). If you need to preserve status flags, you can use a triple-NOP opcode ($0C).