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Difference between revisions of "MMU RAMROM Mode"

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Color Computer 3 Memory Management Unit in ROM/RAM
 
Color Computer 3 Memory Management Unit in ROM/RAM
  
The MMU in the CoCo 3 has access to 63 blocks of RAM each 8 kilobytes in size. In ROM/RAM mode the last four blocks don't access RAM. They access either internal or external ROM.
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The MMU in the CoCo 3 has access to 64 blocks of RAM each 8 kilobytes in size. In ROM/RAM mode the last four blocks don't access RAM. They access either internal or external ROM. The RAM/ROM mode is entered by writing any value to address $FFDE. RAM mode is selected by writing any value to address $FFDF.
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In ROM/RAM mode there are three different ROM configurations available. They are selected by setting bits 1 and 0 of address $FF90 to:
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{| border="1" cellspacing="0" cellpadding="5" align="center"
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! $3C
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! $3D
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! $3E
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! $3F
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|-
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| Extnd Color BASIC
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| Color BASIC
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| 8K Cart ROM
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| 8K Cart ROM
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|}
  
 
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*

Revision as of 20:54, 1 May 2007

Color Computer 3 Memory Management Unit in ROM/RAM

The MMU in the CoCo 3 has access to 64 blocks of RAM each 8 kilobytes in size. In ROM/RAM mode the last four blocks don't access RAM. They access either internal or external ROM. The RAM/ROM mode is entered by writing any value to address $FFDE. RAM mode is selected by writing any value to address $FFDF.

In ROM/RAM mode there are three different ROM configurations available. They are selected by setting bits 1 and 0 of address $FF90 to:

$3C $3D $3E $3F
Extnd Color BASIC Color BASIC 8K Cart ROM 8K Cart ROM

The ROM pages (3C-3F) can be written to any of the eight avaiable MMU slots. In all cases the last two bits the page page are ignored by the MMU and substituted by the last two bits of the slot number.

For example, writing $3D to slot 2 of the MMU does not map the second 8k page of ROM to addresses $4000-$5FFF. It maps page $3E (the third ROM page), because:

This essemtially means that Page $3C will only appear in slots 0 and 4; $3D in 1 and 5; $3E in 1 and 6; $3F in 1 and 7.