

Just going to reply to myself here. I think I am completely wrong on my original reply. You should use ADC. If the byte overflows the carry bit will be set, and then you can test with BCC or BCS.
http://www.6502.org/users/obelisk/6502/reference.html#ADC
"ADC - Add with Carry
This instruction adds the contents of a memory location to the accumulator together with the carry bit. If overflow occurs the carry bit is set, this enables multiple byte addition to be performed."




I was thinking about it, and since you want to count to 05FF, it might be better to load 05FF into the two bytes and use SBC and count down until you hit 0000. Then the 05 isn’t causing more code to test for.