CoCo 3 RAM addressing.

The GIME chip can access 512K of memory, yet the 6809 can only access  64K.  The
barrier is broken by a MMU (Memory Management  Unit)  which  splits  the  access
into 8 blocks of 8K each.

These blocks are accessed through the registers 

FFA0-FFA7.  They represent the following address areas:
FFA0 =  $0000-$1FFF    this is the 32K of basic RAM
FFA1 =  $2000-$2FFF
FFA2 =  $4000-$5FFF
FFA3 =  $6000-$7FFF
FFA4 =  $8000-$9FFF   <- this page is Extended Color Basic
FFA5 =  $A000-$BFFF   <- this page is Color Basic
FFA6 =  $C000-$DFFF   <- this page is Disk Extended Color Basic
FFA7 =  $E000-$FFFF   <- this page is "Super" Basic & hardware I/O

In each page there can be a six bit value xx000000 to xx111111  or  hex  $00  to
$3F.  Each of these 64 values accesses a distinct area of memory. the pages  are
numbered as so

Hex Value                              Page Contents
---------------------------!--------------------------------------------------
$00-$2F                          $00000 -  $5FFFF  512K upgrade RAM
$30                              Hi-Res page #1   $60000-$61FFF
$31                              Hi-Res page #2   $62000-$63FFF
$32                              Hi-Res page #3   $64000-$65FFF
$33                              Hi-Res page #4   $66000-$67FFF
$34                              HGET/HPUT buffer $68000-$69FFF
$35                              Secondary Stack  $6A000-$6BFFF
$36                        Hi-Res text screen RAM $6C000-$6DFFF
$37                                  unused       $6E000-$6FFFF
$38                   Basic memory $0000-$1FFF    $70000-$71FFF
$39                   Basic memory $2000-$3FFF    $72000-$73FFF
$3A                   Basic memory $4000-$5FFF    $74000-$75FFF
$3B                   Basic memory $6000-$7FFF    $76000-$77FFF
$3C                   Extended Basic Interpreter  $78000-$79FFF
$3D                   Color Basic Interpreter     $7A000-$7BFFF
$3E                   Disk Basic Interpreter      $7C000-$7DFFF
$3F     Super Basic, GIME regs, I/O, Interrupts   $7E000-$7FFFF

So, if you wanted to set memory location $60000 to $00, you could:
    ORCC   #$50     SEE NOTE 1
    LDA    $FFA1    GET THE PAGE FOR THE RESTORE
    PSHS   A        SAVE THE PAGE FOR LATER
    LDA    #$30     ACCESS TO PAGE $30
    STA    $FFA1    PAGE  $60000-$61FFF IN LOACTIONS $2000-$3FFF
    LDA    #$00     THE BYTE IS 0
    STA    $2000    SET THE PROPER BYTE
    PULS   A        RESTORE THE PAGE
    STA    $FFA1    AND ACCESS TO IT
    RTS             RETURN FROM CALLER

Note 1- ALWAYS shut off interrupts during extended RAM access! If you  change  a
page that has an interrupt handler in it, and  an  interrupt  occurs,  you  will
crash the computer!

Also note that if you do a load off of one of the  registerrs  the  current  page
will be read.  This technique can also be used  in  clearing  the  hi  res  text
screen.

     ORCC   #$50
     LDA    $FFA1
     PSHS   A
     LDA    #$76
     STA    $FFA1
     LDX    #$2000    START OF THE PAGE
     LDD    #$2000   A CLEAR SPACE (32=BLANK SPACE WITH 00 ATTRIBUTE BYTE)
CR   STD    ,X++
     CMPX   #$2000+(2*80*24)   2BYTES*80CHARS*24ROWS!
     BNE    CR
     PULS   A 
     STA    $FFA1     RESTORE
     RTS

this will clear out the text screen.

Note 2-    If you are using the stack, BE SURE NEVER to  page  over  the  stack!
therefore, KNOW WHERE THE STACK IS! In basic, it  will  be  in  the  $6000-$7FFF
page. If there's a CLEAR xxx,mmmm the  stack  is  at  location  mmmm  and  those
lower.

Hi-Res screen notes- the screen is arranged in  this  format  for  each  16  bit

character:

CACACACACACACACA
CACACACACACACACA
CACACACACACACACA

where C is the ASCII code byte, and A is the attribute byte.
the attribute byte looks like this:

bit
 7    flash      (1=flash)
 6    underline  (1=underline)
 5 \
 4 -  three foreground color bits
 3 /        (palettes 8-15)
 2 \
 1 -  three background color bits
 0 /        (palettes 0-7)
-------------------------------------------------------------------------------
End Part 1