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update comment on memory handlin

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Jorj Bauer 2020-07-02 21:59:35 -04:00
parent 998c7cebc8
commit e8e80b9edc
1 changed files with 9 additions and 16 deletions
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vmram.h
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@ -21,24 +21,17 @@ class VMRam {
bool Test();
private:
// We need 591 pages of 256 bytes for the //e. There's not
// enough RAM in the Teensy 3.6 for both this (nearly 148k)
// and the display's DMA (320*240*2 = 150k).
// We need 591 pages of 256 bytes for the //e.
//
// We could put all of the //e RAM in an external SRAM -- but the
// external SRAM access is necessarily slower than just reading the
// built-in RAM. So this is a hybrid: we allocate some internal
// SRAM from the Teensy, and will use it for the low addresses of
// our VM space; and anything above that goes to the external SRAM.
// This previously used a split memory model where some of this was
// in internal ram and some was external - and while that's not true
// right now, it may be true again in the future.
//
// Changing this invalidates the save files, so don't just change it
// willy-nilly :)
//
// Zero-page should be in internal RAM (it's changed very often). Some
// other pages that are read or written often should probably go in
// here too. The order of the pages (in apple/applemmu.cpp) defines
// what order the pages are referenced in the VMRam object; the lowest
// wind up in internal RAM.
// If we go that way: zero-page should be in internal RAM (it's
// changed very often). Some other pages that are read or written
// often should probably go in here too. The order of the pages (in
// apple/applemmu.cpp) defines what order the pages are referenced
// in the VMRam object; the lowest should wind up in internal RAM.
// Pages 0-3 are ZP; we want those in RAM.
// Pages 4-7 are 0x200 - 0x3FF. We want those in RAM too (text pages).