Posted By
Haplo
on 2022-04-03
03:17:05
|  Help with bank switching
I've been working on the extended version of my game Tenebra (https://h4plo.itch.io/tenebra) and with the new levels I need more memory. The game is written in assembly by the way.
I'd like to use one of the two areas below:
$8000 ROM BANK LOW (BASIC)
$C000 BASIC expansions
What is the correct way to switch them out and see them as RAM? All the answers I have found on the net have been more confusing than helpful. Thanks.
P.S. Is the whole region of $4000-$7fff available to assembly programs without any bank switching? I have noticed I can use from $4000 to around $6800, but after that my stuff gets corrupted.
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Posted By
Haplo
on 2022-04-03
03:24:13
| Re: Help with bank switching
Please ignore the "P.S." bit, it was my mistake. I can see that I can use the whole $4000-$7fff without any issues.
Still any help with switching out the ROM at $8000 will be appreciated.
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Posted By
gerliczer
on 2022-04-03
03:28:23
| Re: Help with bank switching
SEI
STA $FF3F ; page out ROM
LDA #ISR.low
STA $FFFE
LDA #ISR.high
STA $FFFF
CLI
You cannot page out part of the ROM area but only the whole memory range.
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Posted By
Haplo
on 2022-04-03
03:33:55
| Re: Help with bank switching
Thanks.
I just want to use $8000-$BFFF as RAM. What is the value you are putting in $FF3F? I think the LDA is missing from your snippet.
Also my code is not interrupt driven (SEI at the beginning and no CLI), so thankfully I don't need to worry about setting up the vectors.
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Posted By
Haplo
on 2022-04-03
03:42:23
| Re: Help with bank switching
I just found this:
$FF3E:
This so called "shadow register" controls RAM/ROM paging with register $FF3F. Writing an arbitrary value to $FF3E will page in the currently configured ROMs to the upper memory area ($8000..$FFFF).
$FF3F:
This so called "shadow register" controls RAM/ROM paging with register $FF3E. Writing an arbitrary value to $FF3F will page in the RAM to the upper memory area ($8000..$FFFF).
So the actual value written is not important. I'll give it a go now.
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Posted By
Haplo
on 2022-04-03
03:49:19
| Re: Help with bank switching
It works, thanks!
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Posted By
gerliczer
on 2022-04-03
03:51:28
| Re: Help with bank switching
You should study SVS' excellent Ultimate Map and the source codes available on this site to learn more about the 264 series machines.
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Posted By
Haplo
on 2022-04-03
18:08:53
| Re: Help with bank switching
Seems like a very good source, thanks.
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