6502 CPU has R/W pin. Is it possible that all instructions are always
set to read only unless R/W pin is turned on to tell some instructions to
set to write only. It uses STA instruction for read first then write.
For example:
Cycle 1: Address Bus: 1000 Data Bus: 8D -- Read only
Cycle 2: Address Bus: 1001 Data Bus: 01 -- Read only
Cycle 3: Address Bus: 1002 Data Bus: 20 -- Read only
Cycle 4: Address Bus: 2001 Data Bus: C1 -- Read only (Old Data)
Cycle 4: Turn on R/W pin -- Read $41 from Accumulator
Cycle 4: Address Bus: 2001 Data Bus: 41 (New Data)
It is like ASL using RMW. What do you think?