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Adds a new statement of intent.

Now I need to try to decide whether I like my current all-in-one mapping for shadowing + paging, or whether it's better to split the things. I'm tending towards the latter at least until the functionality works.
This commit is contained in:
Thomas Harte 2020-12-05 19:09:21 -05:00
parent 1422d43c35
commit 55de98fb46

View File

@ -61,13 +61,48 @@ class MemoryMap {
// Current beliefs about the IIgs memory map: // Current beliefs about the IIgs memory map:
// //
// * language card banking applies to banks $00, $01, $e0 and $e1; // * language card banking applies to banks $00, $01, $e0 and $e1;
// * auxiliary memory switches apply to banks $00 only; // * auxiliary memory switches apply to bank $e0 only, but thereby also affect shadowed writes from $00;
// * shadowing may be enabled only on banks $00 and $01, or on all RAM pages. // * shadowing may be enabled only on banks $00 and $01, or on all RAM pages; and
// * whether bit 16 of the address is passed to the Mega II is selectable — this affects both the destination
// of odd-bank shadows, and whether bank $e1 is actually distinct from $e0.
// //
// So banks $00 and $01 need their own divided spaces at the shadowing resolution, // So:
// all the other fast RAM banks can share a set of divided spaces, $e0 and $e1 need //
// to be able to deal with language card-level division but no further, and the pure // * banks $00 and $01 need to be divided both by shadowing zones and by the language card;
// ROM pages don't need to be subdivided at all. // * all other fast RAM banks need be divided by shadowing zone only;
// * $e0 needs to be ready for any language/auxiliary arrangement;
// * $e1 needs to apply the language card mapping only; and
// * ROM banks don't need to be divided? Or probably they shadow writes back to $e0/$e1 too?
// Shadowing zones:
//
// $0400$0800 Text Page 1
// $0800$0C00 Text Page 2 [ROM 03 machines]
// $2000$4000 High-res Page 1, and Super High-res in odd banks
// $4000$6000 High-res Page 2, and Huper High-res in odd banks
// $6000$a000 Odd banks only, rest of Super High-res
// [plus IO and language card space, subject to your definition of shadowing]
// Language card zones:
//
// $D000$E000 4kb window, into either bank 1 or bank 2
// $E000end 12kb window, always the same RAM.
// Auxiliary zones:
//
// $0000$0200 Zero page (and stack)
// $0200$0400 [space in between]
// $0400$0800 Text Page 1
// $0800$2000 [space in between]
// $2000$4000 High-res Page 1
// $4000$C000 [space in between]
// Card zones:
//
// $C100$C2FF either cards or IIe-style ROM
// $C300$C3FF IIe-supplied 80-column card replacement ROM
// $C400$C7FF either cards or IIe-style ROM
// $C800$CFFF Standard extended card area
// Reserve region 0 as that for unmapped memory. // Reserve region 0 as that for unmapped memory.
region(); region();