Re: 6502 CPU's Read/Write Question

[Eric Smith <eric@brouhaha.com>]

I wrote:
> 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. 

Bryan Parkoff wrote:
> Okay, how do you know for sure?  I am curious.

Well, there are various degress of "sure".

To a first approximation, you can be "sure" because I've been designing
and using 6502-based hardware for 18 years, and have a lot of experience
with it.

But if you don't want to trust me on it, you can look it up in the
published documentation.  For instance, in the Rockwell R6500
Microcomputer System Hardware Manual, Rockwell Document Number
29650-N31, Rev. 1, dated August 1978, the bus cycles performed by an STA
absolute are explicitly shown in section A.3.2 on page A-7.  So you can
be reasonably "sure" unless the documentation is wrong.

If you want to be even more "sure", you can hook up a logic analyzer.

Eric