Physical frame ownership

This figure documents the historical i386 source baseline. Current native x86-64/AArch64 qualification is recorded separately; this is not an ELF64 or native SMP implementation diagram.

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Physical frame ownership PMM determines available frames from Multiboot information, marks usable regions free and reserves occupied kernel, bitmap and heap frames. Allocation finds a free bit, marks it used and returns a 4 KiB frame; freeing clears ownership and makes the frame reusable. Exhaustion returns failure. map->bitmap bitmap->reserve reserve->cursor cursor->found found->alloc yes found->fail no alloc->free owner releases free->cursor frame reusable map Multiboot available-memory map bitmap PMM bitmap 1 bit per 4 KiB frame reserve Reserve kernel + bitmap + heap frames cursor Allocation: scan from next-free cursor found Free bit found? alloc Set bit → increment used count return physical frame fail Return allocation failure free Free frame: clear bit adjust used count + scan cursor
Physical frame ownership PMM determines available frames from Multiboot information, marks usable regions free and reserves occupied kernel, bitmap and heap frames. Allocation finds a free bit, marks it used and returns a 4 KiB frame; freeing clears ownership and makes the frame reusable. Exhaustion returns failure. map->bitmap bitmap->reserve reserve->cursor cursor->found found->alloc yes found:e->fail:e no alloc->free free:e->cursor:e frame reusable map Multiboot available-memory map bitmap Bitmap ownership 1 bit per 4 KiB frame reserve Reserve kernel, bitmap and heap frames cursor Scan from next-free cursor found Free bit found? alloc Set bit; increment used count return physical frame free Owner frees frame: clear bit; adjust count + cursor fail Return allocation failure

The bitmap tracks ownership; the next-free cursor is only a search hint.

Text equivalent and source references
  • PMM determines available frames from Multiboot information, marks usable regions free and reserves occupied kernel, bitmap and heap frames.
  • Allocation finds a free bit, marks it used and returns a 4 KiB frame; freeing clears ownership and makes the frame reusable. Exhaustion returns failure.

Allocation flow. Source paths refer to the atlas baseline; the kernel checkout is private.

  • kernel/core/memory.c · pmm_init, pmm_alloc_frame, pmm_free_frame, heap_init
  • include/memory.h · PAGE_SIZE

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