[Coco] Re: 64K Mode

[jdaggett at gate.net]

John

That is what I have been saying. This is for the SAM CHIP ONLY 
COCO 1 and 2 Only

In Memeory Map type 0 (ZERO)

Ram is in the lower 32K and ROM and vectors in the upper 32K. 
With 64K of ram on board,  the Ram is configured as two (2) banks 
of 32K mapped in the lower 32K of the memory map. The "P" 
register controls which page is mapped in.

In Memeory Map Type 1 (ONE)

Ram is configured from address $0000 to $FEFF. The upper 256 
bytes are not accessable as they are I/O and vector sapce. 

james



On 14 Jan 2004 at 10:49, John Collyer wrote:

From:           	"John Collyer" 
<johncollyer at zoominternet.net>
To:             	"CoCoList for Color Computer Enthusiasts" 
<coco at maltedmedia.com>
Subject:        	Re: [Coco] Re: 64K Mode
Date sent:      	Wed, 14 Jan 2004 10:49:25 -0500
Send reply to:  	CoCoList for Color Computer Enthusiasts 
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> OK, Everyone lets get this correct.  Here are the details:
> 
> The page number bit, P1 is cleared by BASIC when the computer
> is turned on.  The effect of P1 set can only be used by a 64K CoCo. A
> 64K RAM is considered as two 32K pages.  When the page number bit is
> clear page 0 is the first 32K and page 1 is the upper 32K.  With TY
> clear (ROM/RAM mode) and the page number bit clear the first 32K RAM
> occupies addresses $0000 through $7FFF and ROM extends upward from
> there.  Setting the page number bit results in the upper 32K of RAM,
> which was not accessible with TY clear, being accessed by addresses
> $0000 through $7FFF.  This allows you to have two BASIC programs in
> RAM at the same time. One in Page 0 and the other in page 1.
> 
> TY | Memory map mode | Set         | Clear
>                                    $FFDF  | $FFDE
> 
> P1 | Page Number           | Set         | Clear
>                                    $FFD5     $FFD4
> 
> This is how it really works!
> 
> John Collyer
> 
> 
> 
> ----- Original Message ----- 
> From: <jdaggett at gate.net>
> To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
> Sent: Tuesday, January 13, 2004 11:35 PM Subject: Re: [Coco] Re: 64K
> Mode
> 
> 
> > 
> > What ever.
> > 
> > Correct I shorted the upper range 16 bytes. $FFF0 is the start of
> > I/O. The Ram is switched in two banks and are located between $0000
> > and $7FFF when memory map type 0 is selected. 
> > 
> > The HD63x09 has vector for illegal opcode trap at $FFF0 and 
> > $FFF1. The MC68x09 does not. 
> > 
> > james
> > 
> > On 13 Jan 2004 at 18:04, Alex wrote:
> > 
> > From:           Theodore (Alex) Evans 
> > <alxevans at concentric.net>
> > Subject:        Re: [Coco] Re: 64K Mode
> > Date sent:      Tue, 13 Jan 2004 18:04:41 -1000
> > To:             CoCoList for Color Computer Enthusiasts 
> > <coco at maltedmedia.com>
> > Send reply to:  CoCoList for Color Computer Enthusiasts 
> > <coco at maltedmedia.com>
> > <mailto:coco-
> > request at maltedmedia.com?subject=unsubscribe>
> > <mailto:coco-
> > request at maltedmedia.com?subject=subscribe>
> > 
> > > On Jan 13, 2004, at 3:39 PM, jdaggett at gate.net wrote:
> > > 
> > > > The Sam Chip in the Sam Chip of the CoCo 1/2 hasa two memeory
> > > > map types
> > > >
> > > > Type 0 is ram from $0000 to $7FFF and Rom from $8000 to $FDFF
> > > > and from $FFF2 to $FFFF.  $FF00 to $FFDF is I/O and $FFE0 to
> > > > $FFF1 unused.
> > > >
> > > > Type 1 is all ram $0000 to $FDFF. $FF00 to $FFDF is I/O and
> > > > $FFE0 to $FFF1 unused. $FFF2 to $FFFF is ROM and vector
> > > > location.
> > > 
> > > I am almost positive that the the area switched extends up to
> > > $FEFF with only 256 bytes for I/O and vectors.  In fact according
> > > to some notes that I made a long time ago the destinations for the
> > > all but one of the 6809 vectors (I don't have a note for the 6309
> > > unimplemented instruction at $FFF0) lie in in the FE00-$FEFF
> > > range.  SWI3 pointing to $FEEE, SWI2 to $FEF1, FIRQ to $FEF4, IRQ
> > > to $FEF7, SWI1 to $FEFA, and NMI to $FEFD with reset being the
> > > exception which points to $8C1B
> > > 
> > > 
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> > 
> > 
> > 
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