The '2600 has 128 bytes of internal ram stored in the PIA chip, with it being used as both Zero-Page and stack memory. It's also the only place it's possible to use read-write instructions.
The RAM can be read and written to at $80-$FF and $180-$1FF.
The following instructions must use this internal ram to operate correctly, except when the Accumulator is being operated on.
Legal instructions:
- DEC (Decrement)
- INC (Increment)
- ASL (Arithmetic Shift Left)
- LSR (Logical Shift Right)
- ROR (Rotate Right)
- ROL (Rotate Left)
Illegal instructions:
- DCP (Decrement & Compare)
- ISC (Increment & Subtract With Carry)
- SLO (Arithmetic Shift Left & OR)
- SRE (Logical Shift Right & EOR)
- RRA (Rotate Right & Add With Carry)
- RLA (Rotate Left & AND)