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 > TIL: The instruction after SEI can be executed before a pending IRQ is handled
2017-11-07 16:56
Krill

Registered: Apr 2002
Posts: 894
TIL: The instruction after SEI can be executed before a pending IRQ is handled

As described here: http://visual6502.org/wiki/index.php?title=6502_Timing_of_Inter..

I never knew this, after all those years, and thought i'd share this as a heads-up.

Thanks to bubis for pointing it out to me!
 
... 88 posts hidden. Click here to view all posts....
 
2017-11-11 08:58
Oswald

Registered: Apr 2002
Posts: 4123
Quote: The only interrupt source that is enabled at program start is CIA1. And we disable all CIA1 interrupts. So where would the interference come from again?

assumption is the root of all bugs. if you change enviroment you're facing hrs of debugging until you find the faulty non sei/cli irq setup, that relies on having the system in a known state.
2017-11-11 09:38
Oswald

Registered: Apr 2002
Posts: 4123
anyhow I realise this is a religious argument, everyone should just do it as he likes. doesnt really matter.
2017-11-11 10:48
ChristopherJam

Registered: Aug 2004
Posts: 680
Oswald, if there's a source you don't know about to disable, you're going to have bugs regardless of whether you SEI/CLI or not, as it's going to hit while your code isn't expecting it.

And if you do know about it, you can disable it, in which case an SEI before you disable it will create extra work for you, or an SEI after you disable it will do nothing.

Either way all you are doing is adding extra bytes to your init code with zero improvements to reliability.

It may even cause your raster interrupt to have it's first occurrence on the wrong line altogether, which can occasionally produce a frame of garbage at the start of your routine.
2017-11-11 11:28
Groepaz

Registered: Dec 2001
Posts: 8267
Quote:
Yes, you'd have to be running code stored in read-sensitive IO registers, at the precise cycle an IRQ is expected. I find it hard to imagine any kind of scenario where that would save you any cycles or memory.

it could be a fun riddle for obfuscation though :)
2017-11-11 11:40
ChristopherJam

Registered: Aug 2004
Posts: 680
(actually scratch that "it may even" para from the end of my last comment, I didn't quite think that one through - if a CIA interrupt goes off during setup unacknowledged, the consequences are going be considerably worse than a single frame...)
2017-11-11 11:45
Krill

Registered: Apr 2002
Posts: 894
Quoting Oswald
this is a religious argument
This may be an academic discussion (unless you need the size, 4K and all), but it's certainly nothing to do with religion. Well, unless you are a cargo cultist. :)
2017-11-11 11:56
Oswald

Registered: Apr 2002
Posts: 4123
"Either way all you are doing is adding extra bytes to your init code with zero improvements to reliability."

An IRQ may destroy A while you try to lda #$7f sta $dc0d. another irq from the other CIA may re-enable $dc0d. You may have interrupt sources from VICII too, fucking you up in all sorts of ways. (rewriting fffe/f after you've set them up for example).

"if there's a source you don't know about to disable, you're going to have bugs regardless of whether you SEI/CLI or not, as it's going to hit while your code isn't expecting it."

you may just forget about your super size effective irq setup and it will backfire when you least expect it.
2017-11-11 12:34
Krill

Registered: Apr 2002
Posts: 894
Now you're grasping at straws.
2017-11-11 13:06
Oswald

Registered: Apr 2002
Posts: 4123
Quote: Now you're grasping at straws.

Now you're trolling.
2017-11-11 13:26
Oswald

Registered: Apr 2002
Posts: 4123
anyhow I have real machine experience when I've been setting up raster irq without acking timer interrupt for ages, and then at a case it refused working, and I wondered how could I forget to set it up correctly. Since then I religiously ack it.
Previous - 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 - 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
iAN CooG/HVSC
Squad333
sta64
Deev/Onslaught
celticdesign/G★P/MDG
insane/altair^rab^tscc
Carrion/SHAPE
Guests online: 46
Top Demos
1 Uncensored  (9.7)
2 Edge of Disgrace  (9.7)
3 Coma Light 13  (9.6)
4 The Shores of Reflec..  (9.6)
5 Lunatico  (9.6)
6 Comaland 100%  (9.5)
7 Incoherent Nightmare  (9.5)
8 Wonderland XII  (9.5)
9 Comaland  (9.5)
10 Wonderland XIII  (9.5)
Top onefile Demos
1 FMX Music Demo  (9.6)
2 Pandemoniac Part 2 o..  (9.6)
3 Daah, Those Acid Pil..  (9.5)
4 Synthesis  (9.5)
5 Dawnfall V1.1  (9.5)
6 Dawnfall  (9.4)
7 Treu Love [reu]  (9.4)
8 Field Sort  (9.4)
9 Pro Memoria 4  (9.3)
10 KAOS 64  (9.3)
Top Groups
1 Oxyron  (9.4)
2 Booze Design  (9.4)
3 Censor Design  (9.3)
4 Crest  (9.3)
5 The Judges  (9.3)
Top Logo Graphicians
1 Pal  (9.5)
2 Jailbird  (9.3)
3 Yazoo  (9.2)
4 Mermaid  (9.2)
5 Elko  (9.1)

Home - Disclaimer
Copyright © No Name 2001-2017
Page generated in: 1.061 sec.