Re: 6502 CPU's Read/Write Question

[Eric Smith <eric@brouhaha.com>]

"Bryan Parkoff" <nospam@nospam.com> writes:
>     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?
> 
> Bryan Parkoff 

No, cycle 4 can only do a read OR a write, not both.

I'm not sure what you're getting at, but a STA absolute instruction
works like this:

       Address          Data
Cycle    Bus     R/W    Bus
-----  -------  -----   ----
  1     1000    read     8D   opcode fetch
  2     1001    read     01   low byte of operand's absolute address
  3     1002    read     20   high byte of operand's absolute address
  4     2001    write    41   write data from accumulator


STA absolute does not read the target location.