xv6 with Demand Paging

This branch implements demand paging for xv6 on ARM. Physical memory is allocated only when pages are actually accessed, not when virtual memory is allocated.

How It Works

1. sbrk() calls use allocuvm_demand() - just updates process size, no physical allocation
2. When process accesses unmapped page → page fault
3. dabort_handler() catches fault and calls handle_page_fault()
4. Physical memory allocated and mapped to virtual address
5. Process continues execution

Key Changes

• vm.c: Added allocuvm_demand() and handle_page_fault()
• trap.c: Modified dabort_handler() to handle page faults
• trap_asm.S: Fixed PC adjustment for ARM data abort return
• proc.c: Changed growproc() to use demand allocation
• copyuvm(): Skip unallocated pages during fork

Benefits

• Memory only allocated when actually used
• Faster process creation (sbrk() is immediate)
• Better memory utilization

ARM-Specific Details

• Detects translation faults using FSR codes 0x5 and 0x7
• Critical PC adjustment in trap_dabort for instruction retry
• Proper TLB flushing after new page mappings