[Coco] Re: 64K Mode

[John Collyer]

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>
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> 
> > 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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