Claude/discretize unixv6 8fqk1 - #2
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Implements the four discretization goals:
1. All logical pages stored discretely (non-contiguous physical pages)
- Replace MapNode/Allocator with BitMap/BitMapAllocator for page allocation
- Each physical page tracked individually via Page[] reference count array
- UserPageManager and KernelPageManager use BitMapAllocator
2. COW (Copy-On-Write) for fork()
- NewProc() marks parent pages read-only, shares with child via page table copy
- PageFault handler detects COW faults: if ref count=1 restore RW; else
decrement ref count, allocate new page, copy contents
3. exec() allocates pages discretely, no memory copy
- Stack pages allocated one at a time, mapped via 0# shared page table entries
- Text/data pages allocated individually, frame numbers stored directly in PTEs
- Eliminates old contiguous block copy approach
4. Text segment sharing with x_caddr[10] array
- Text.h: x_caddr changed from single address to array of 10 frame numbers
- Text.cpp: XccDec() iterates per-page, decrements ref counts, frees individually
Additional changes:
- Process.h: Add PageDirectory* pPageDirectory member + GetPageDirectoryPhyAddr()
- ProcessManager: Add AllocPageDirectory(), InitProcPageDirectory(), ModifyPageTable()
- Assembly.h: FlushPageDirectory() now accepts physical address parameter
- MemoryDescriptor.cpp: MapToPageTable() simplified to just flush CR3 with process PD
- SwtchUStruct macro updated to use per-process page directory physical address
https://claude.ai/code/session_01TZ87VQcUEKKAh8e86YYppq
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