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Added note to consider dropping dirty block tracking (but it requires updating the Video component to use a custom controller for ALL video modes, not just EGA/VGA modes)

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This commit is contained in:
Jeff Parsons 2015-11-09 09:08:12 -08:00
commit 00116ced94
1 changed files with 17 additions and 5 deletions
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22
modules/pcjs/lib/memory.js
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@ -109,8 +109,20 @@ function Memory(addr, used, size, type, controller, cpu)
this.fReadOnly = (type == Memory.TYPE.ROM);
this.controller = null;
this.cpu = cpu; // if a CPU reference is provided, then this must be an UNPAGED Memory block allocation
this.fDirty = this.fDirtyEver = false;
this.copyBreakpoints(); // initialize the block's Debugger info (eg, breakpoint totals); the caller will reinitialize
/*
* TODO: Study the impact of dirty block tracking. As noted in the paged block handlers (eg, writeBytePLE),
* the original purposes were to allow saveMemory() to save only dirty blocks, and to enable the Video component
* to quickly detect changes to the video buffer. But the benefit to saveMemory() is minimal, and the Video
* component has other options; for example, it now uses a custom memory controller for all EGA/VGA video modes,
* which performs its own dirty block tracking, and that could easily be extended to the older MDA/CGA video modes,
* which still use conventional memory blocks. Alternatively, we could restrict the use of dirty block tracking
* to certain memory types (eg, VIDEO memory).
*
* However, a quick test with with dirty block tracking disabled didn't yield a noticeable improvement in performance,
* so I think the overhead of our block-based architecture is swamping the impact of these micro-updates.
*/
this.fDirty = this.fDirtyEver = false;
if (BACKTRACK) {
if (!size || controller) {
@ -311,11 +323,11 @@ Memory.prototype = {
/**
* save()
*
* This gets the contents of a Memory block as an array of 32-bit values;
* used by Bus.saveMemory(), which in turn is called by X86CPU.save().
* This gets the contents of a Memory block as an array of 32-bit values; used by Bus.saveMemory(),
* which in turn is called by X86CPU.save().
*
* Memory blocks with custom memory controllers do NOT save their contents;
* that's the responsibility of the controller component.
* Memory blocks with custom memory controllers do NOT save their contents; that's the responsibility
* of the controller component.
*
* @this {Memory}
* @return {Array|Int32Array|null}