Mac GUI

Re: POKE and STA for speaker access

comp.sys.apple2

Author: Vladimir Ivanov
Date: 31 Jul 2014 7:05 pm
Ref: 1 2 3 4 5

On Wed, 30 Jul 2014, sicklittlemonkey wrote:

> From the Bill Mensch interview it looks like they didn't know about the  
> RWTS/disk controller problem until they tried to develop the 65C816.

I somehow doubt this is pure luck, but not losing much sleep over it,  
either. :-)

>>> STA (zp),Y has been fixed for both cases by adding a dummy read of the
>>> program counter, so the change was rW -> W and RW -> W.
>>
>> Just a small correction - as I wrote previously, it does dummy read of
>> zero page. :-)
>
> Yes, but that is an extra read of an operand (zp + 1), which can be  
> considered "expected". The operand is read. We don't care how many  
> times. ; - )It doesn't access anywhere "unexpected" like it did in the  
> NMOS 6502. The extra access used to be to the destination (or dest  
> -$100) where we do care how many times.

That feature of the NMOS 6502 was a neat trick for C64 people. Otherwise  
plenty dangerous near I/O areas in various embedded 6502 setups, so Mr.  
Mensch did the right thing and got rid of it.

>> Yes, $DE/$FE don't save a cycle on no page boundary crossing, but there is
>> still a twist - the dummy read is different depending on whether or not
>> page boundary is crossed. I guess another Apple II compatibility patch,
>> maybe.
>
> Interesting. Do you have a link for this?

My own inspection of G65SC02P.

> The 6502.org site mentions it, but there's no detailed cycle timing. I  
> would be surprised if INC/DEC abs,X did an "unexpected" read - I mean to  
> dest - $100.

No, 65SC02 does not do dangerous "semi" indexed reads at all, that's the  
beauty of it.

The no page boundary crossing case of $DE/$FE does quadruple access to the  
target address (RRRW), while the page boundary case does only triple  
access (RRW).




POKE and STA for speaker access
126 Jul 2014 5:20 pmsicklittlemonkey
226 Jul 2014 5:42 pm|- D Finnigan
327 Jul 2014 1:27 am|  \ sicklittlemonkey
427 Jul 2014 3:51 pm|     \ D Finnigan
527 Jul 2014 12:44 am|- Michael J. Mahon
627 Jul 2014 1:25 am|  |- sicklittlemonkey
727 Jul 2014 2:03 am|  |- Steve Nickolas
827 Jul 2014 3:17 am|  |  |- sicklittlemonkey
927 Jul 2014 8:52 am|  |  |  \ Vladimir Ivanov
1027 Jul 2014 5:37 am|  |  \ Michael J. Mahon
1127 Jul 2014 6:09 am|  |     \ Steve Nickolas
1228 Jul 2014 4:50 pm|  \ Scott Alfter
1328 Jul 2014 9:09 pm|     |- Bill Buckels
1428 Jul 2014 10:42 pm|     |  \ sicklittlemonkey
1529 Jul 2014 1:27 am|     |     |- Bill Buckels
1629 Jul 2014 1:30 am|     |     |- Michael J. Mahon
1729 Jul 2014 4:42 am|     |     \ Scott Alfter
1829 Jul 2014 5:08 am|     |        \ sicklittlemonkey
1929 Jul 2014 10:24 am|     |           \ Bill Buckels
2029 Jul 2014 1:30 am|     \ Michael J. Mahon
2127 Jul 2014 8:31 am\ Vladimir Ivanov
2228 Jul 2014 12:01 am   \ sicklittlemonkey
2329 Jul 2014 6:02 am      |- sicklittlemonkey
2409 May 2020 4:28 am      |  \ Nick Westgate
2530 Jul 2014 11:29 am      \ Vladimir Ivanov
2631 Jul 2014 5:27 am         \ sicklittlemonkey
2731 Jul 2014 7:05 pm            \ Vladimir Ivanov
2831 Jul 2014 10:30 pm               \ sicklittlemonkey
2901 Aug 2014 6:16 am                  \ Vladimir Ivanov