Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.cbm
Subject: Re: C-128 MMU techniques and memory maps
Message-ID: <5504@cbmvax.UUCP>
Date: 14 Dec 88 18:54:44 GMT
References: <2140@nunki.usc.edu>
Organization: Commodore Technology, West Chester, PA
Lines: 89

in article <2140@nunki.usc.edu>, aliu@nunki.usc.edu (Alejandro Liu) says:
> Keywords: C-128, MMU, virtual memory, memory map

> Hi UseNet Users:
>       I have been reading some of the specs about the C-128, because a
> program  I am writing, and I need some information.  

Sounds to me like you need a C128 Programmer's Reference Manual.

> First I heard that the 128's MMU can handle 1 Meg of memory, 

Nope.  The current MMU is limited to 128K of RAM, plus the ROM.  In theory,
the MMU design would permit an MMU to be built that would support 1 meg of
RAM, though in practice, only 256K of RAM would be possible without some
software problems (the banking method _changes_ when you go over 256k).

> Second, How do you use the MMU to do Bank Switching?  I know there is a 
> register at location $FF00, but what does each bit means?  

The MMU register map looks something like this:


Location     Register

 $D500       Configuration Register
 $D501       Preconfiguration Register A
 $D502       Preconfiguration Register B
 $D503       Preconfiguration Register C
 $D504       Preconfiguration Register D
 $D505       Mode Configuration Register
 $D506       RAM Configuration Register
 $D507       Page 0 Pointer LOW
 $D508       Page 0 Pointer HIGH
 $D509       Page 1 Pointer LOW
 $D50A       Page 1 Pointer HIGH
 $D50B       MMU Version Register

 $FF00       Configuration Register (same as $D500)
 $FF01       Load Configuration Register A
 $FF02       Load Configuration Register B
 $FF03       Load Configuration Register C
 $FF04       Load Configuration Register D

The configuration and preconfiguration registers have the same format:
  Bit 7:     Unused (for 256K banking in a new MMU)
  Bit 6:     Select RAM bank 0 or 1
  Bit 5,4:   ROM HI  Space (00 = System ROM, 11 = RAM)
  Bit 3,2    ROM MID Space (00 = System ROM, 11 = RAM)
  Bit 1:     ROM LOW Space ( 0 = System ROM,  1 = RAM)
  Bit 0:     I/O Space (0 = System I/O, 1 = HIGH ROM Space)

If you write to the CR, the banking takes place immediately.  If you write to
a PCR, nothing immediately happens.  If you write to an LCR (doesn't matter 
what you write), the contents of the corresponding PCR get transferred to the
CR and the banking gets set.  This allows several common setups to be stored
and used without having to have I/O space turned on or having to disturb a 
register.
  
The RAM Configuration register looks like this:
  Bit 7:     Unused (for 256K banking in a new MMU)
  Bit 6:     Select bank 0 or 1 for the VIC chip
  Bit 5,4:   For expansion banking beyond 256K in a new MMU
  Bit 3:     Sharing banks at top    of RAM (0 = Not Shared, 1 = Shared)
  Bit 2:     Sharing banks at bottom of RAM (0 = Not Shared, 1 = Shared)
  Bit 1,0:   Amount shared (00 = 1K, 01 = 4K, 10 = 8K, 11 = 16K)

The page registers specify the actual location of the 8502's page 0 and 1.
Bit 0 of each HIGH register specifys the bank, while the LOW register
specifys the page in that bank.  You should write first to the high register,
then to the low, for each pair.  

> I know that there is a Kernal subroutine that calculates the
> value, but I don't like the 15 Bank system they have a Commodore.
> What are the exact additions to the Kernal Jump table?  I only know
> the old jumping points, but I know that they have added quite a few
> subroutines, I would like to know what are them and where are they.

I don't know how the software works, maybe Fred will say something about
it.  You should really get the C128 Programmer's Reference Manual, which
tell all about this stuff.  And I'm not just saying that 'cause I helped
write it :-)


>                     aliu@nunki.usc.edu (Alejandro Liu) 
> (Simple .signature, $CHEAP$) 
-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
              Amiga -- It's not just a job, it's an obsession