Re: 6502 CPU's Read/Write Question

["Bryan Parkoff" <nospam@nospam.com>]

Eric,

    Okay, how do you know for sure?  I am curious.

Bryan Parkoff

"Eric Smith" <eric@brouhaha.com> wrote in message 
qhy8caexc6.fsf@ruckus.brouhaha.com">news:qhy8caexc6.fsf@ruckus.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.