Log inRegister an accountBrowse CSDbHelp & documentationFacts & StatisticsThe forumsAvailable RSS-feeds on CSDbSupport CSDb Commodore 64 Scene Database
You are not logged in 
CSDb User Forums


Forums > C64 Coding > (Ab)use of dummy accesses
2020-03-09 17:04
Groepaz

Registered: Dec 2001
Posts: 9539
(Ab)use of dummy accesses

For the next release of my "No more Secrets" doc i am preparing a chapter related to the dummy access which happen when the CPU performs an internal operation. Once again i am looking for some examples on how to (ab)use it :) I guess everyone knows "inc $d019" - but i am sure there is more than this. And not only with RMW instructions. So if you have anything in your mind - just drop it here!

here are some related notes which i pasted together. feel free to proofread and point out mistakes :)
 
... 55 posts hidden. Click here to view all posts....
 
2020-04-09 00:07
Zyron

Registered: Jan 2002
Posts: 2241
:o
2020-04-09 07:25
MagerValp

Registered: Dec 2001
Posts: 985
Neat!
2020-04-09 10:21
Groepaz

Registered: Dec 2001
Posts: 9539
that calls for a test program that breaks emulation :)
2020-04-09 12:10
oziphantom

Registered: Oct 2014
Posts: 409
Quoting lft
The engineers must have assumed (wrongly) that the CPU will always trigger a transfer on the last cycle of an instruction, so that the next cycle is guaranteed to be a read (to fetch the next instruction).

Or explicitly put in the programming guide/datasheet.
'Thou shall not write to thing with anything else other than ABS.'
2020-04-09 14:23
Oswald

Registered: Apr 2002
Posts: 4609
I dont get it, if REU starts to use bus as soon as VIC signals on BA to processor to stop, then it will lead to a bus conflict ?
2020-04-09 21:46
JackAsser

Registered: Jun 2002
Posts: 1720
Quote: I dont get it, if REU starts to use bus as soon as VIC signals on BA to processor to stop, then it will lead to a bus conflict ?

Nobody said that... :) It monitors BA... I don't know, but I would assume the REU will not transfer when BA is low, even though technically could use cycles 0..2 there. It might do, I dunno.
2020-04-09 23:47
Groepaz

Registered: Dec 2001
Posts: 9539
but where is the test program? :)
2020-04-10 11:54
Walt

Registered: May 2004
Posts: 17
I am working on a REU demo and I had to do some test code because I experienced different behavior when using REU to magic byte on top of 8 sprites in a row.

Timing was different on VICE 2.4, VICE 3.x, real REU (Thanks Hedning :)), 64 Ultimate and 1541 Ultimate 2+...

The C64 Ultimate and VICE 2.4 behaved the same and real REU and VICE 3.x behaved the same (Nice work on VICE :))

So yeah, a test program for BA and REU would be nice...
2020-04-10 12:20
Groepaz

Registered: Dec 2001
Posts: 9539
there are related test programs, but not for the ff00 trigger with inc as lft described :) see https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/REU/
2020-05-17 16:43
Groepaz

Registered: Dec 2001
Posts: 9539
.... so i wrote one -> https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/REU..

works as LFT described on real hardware. fails in VICE :)
Previous - 1 | 2 | 3 | 4 | 5 | 6 | 7 - Next
RefreshSubscribe to this thread:

You need to be logged in to post in the forum.

Search the forum:
Search   for   in  
All times are CET.
Search CSDb
Advanced
Users Online
Hydrogen/Glance
hedning/G★P
Mibri/ATL^MSL^PRX
zscs
Jammer/TooMany
Scorpion/Contex / Ar..
Menace/Spaceballs
Sander
Brush/Elysium
Guests online: 80
Top Demos
1 Uncensored  (9.7)
2 Edge of Disgrace  (9.6)
3 Coma Light 13  (9.6)
4 Memento Mori  (9.6)
5 Comaland 100%  (9.6)
6 The Shores of Reflec..  (9.6)
7 Unboxed  (9.6)
8 Lunatico  (9.6)
9 Remains  (9.5)
10 C=Bit 18  (9.5)
Top onefile Demos
1 Dawnfall V1.1  (9.5)
2 Gumbo Revised  (9.5)
3 Smile to the Sky  (9.5)
4 Daah, Those Acid Pil..  (9.5)
5 Bad Boy  (9.5)
6 Crystal Gazer  (9.5)
7 Cuarentenauta  (9.5)
8 Instinct  (9.5)
9 The Night It Snowed  (9.5)
10 Rewind  (9.4)
Top Groups
1 PriorArt  (9.4)
2 Booze Design  (9.4)
3 Censor Design  (9.4)
4 Fossil  (9.4)
5 Performers  (9.3)
Top Webmasters
1 Perff  (9.6)
2 Slaygon  (9.6)
3 Morpheus  (9.5)
4 Sabbi  (9.2)
5 CreaMD  (9.2)

Home - Disclaimer
Copyright © No Name 2001-2020
Page generated in: 0.05 sec.