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From: ecarroll@csvax1.cs.tcd.ie (Eddy Carroll)
Newsgroups: comp.sys.cbm
Subject: Re: memory management
Message-ID: <52833@csvax1.cs.tcd.ie>
Date: 21 Aug 89 13:07:18 GMT
References: <9313@cs.Buffalo.EDU>
Organization: Computer Science Department, Trinity College Dublin
Lines: 32

In article <9313@cs.Buffalo.EDU>, bowen@cs.Buffalo.EDU (Devon E Bowen) writes:
> I'm working under C Power which seems to shut down the BASIC ROM for
> extra program space, but it leaves the I/O section and the kernal ROM.
> I need some extra buffer space for my code and I don't need any of the
> kernal functions, so I want to shut down that ROM area. I grab byte
> $01, and it with $fd (to clear the kernel ROM bit) and put it back.

[ Goes on to describe how accessing the 80-column chip locks up the system ]

> Maybe having both the BASIC ROM and kernal ROMs
> shut off automatically shuts off the I/O area?

Hi there, your last comment above is exactly what is happening. When you
switch out the Kernal ROM, this ALWAYS switches out the BASIC ROM as well.
In this situation, Bit 0 of loc $01 now controls whether RAM or I/O is mapped
into the $D000 area. With the bit set, the I/O area is mapped in, with the
bit clear, RAM is mapped in. Since C-Power is clearing that bit, and you then
clear the Kernal bit, you end up with a full 64K of continuous RAM. The
solution is very simple: After ANDing the contents of $01 with $FD, just
OR it with $01 before storing it back.

BTW, as an aside: be very careful if you are accessing the 80 column chip
from within interrupts as well as from your main code. (This caused me some
trouble in a comms package I did 18 months go, which had an onscreen clock).
You need to setup a semaphore to guarantee exclusive access to the registers,
otherwise weird things will happen.

Eddy
-- 
Eddy Carroll               ----* Genuine MUD Wizard  | "You haven't lived until
INTER: ecarroll@cs.tcd.ie                            |    you've died in MUD!"
 UUCP: {..uunet}!mcvax!ukc!cs.tcd.ie!csvax1!ecarroll |  -- Richard Bartle