danos/docs/os-development/frame-allocator.md

The physical frame allocator

Once the kernel knows what RAM exists (memory-map.md), it needs a way to hand out that RAM: give me a free page of physical memory, and later, here's one back. That's the physical frame allocator (a "physical memory manager", hence system/kernel/pmm.zig). It deals only in fixed 4 KiB frames — the natural unit because that's the granularity the CPU's paging hardware maps — and it is the primitive everything above it stands on: page tables, the kernel heap, per-process memory all ultimately ask the frame allocator for pages.

It's generic kernel code: it operates on the neutral boot_handoff.MemoryRegion array, so there's no UEFI in it and nothing architecture-specific beyond the 4 KiB page. (Contrast architecture.md, which is where CPU-specific code lives.)

Why a bitmap

There are a few classic designs; danos starts with the simplest that still supports freeing:

• Bitmap (chosen): one bit per frame, 1 = used, 0 = free. Freeing is trivial (clear a bit), it's very compact, and you can later extend it to allocate contiguous runs by scanning for consecutive zero bits. Allocation is a linear scan, but that's cheap and easy to reason about.
• Intrusive free-list / stack: store the "next free frame" pointer inside each free frame; O(1) alloc and free. Elegant, but it can't satisfy contiguous multi-frame requests and can't answer "is this frame free?".
• Buddy allocator: great for contiguous power-of-two blocks, but more machinery than a first allocator needs.

Compactness matters less than clarity here, but it's a nice property: 128 MiB of RAM is 32768 frames — a 4 KiB bitmap, a single frame. Even 64 GiB needs only 2 MiB of bitmap.

How it works

State lives in system/kernel/pmm.zig: the bitmap slice, total_frames, used_frames, and a next_hint marking where the next allocation scan should start.

init(map) — building it from the memory map

1. Size it. Find the highest address across all RAM regions — every kind except mmio — so reserved and ACPI spans sit inside the bitmap, marked used but trackable (e.g. so the boot buffers can be freed later); total_frames = highest / page_size. Only MMIO (device address space — remember the ~12 GiB of it from memory-map.md) sits outside the bitmap and is simply never allocatable.
2. Place it (the bootstrap). The bitmap needs storage before an allocator exists — a chicken-and-egg. Solution: pick the first usable region big enough to hold the bitmap and put it there, addressing it through the physmap (boot_handoff.physicalToVirtual). The loader's bootstrap page tables already provide the physmap and the kernel's own tables keep it, so the pointer stays valid across the paging switch.
3. Mark, then free. Set the whole bitmap to used (0xff), then walk the usable regions clearing their bits. Doing it in that direction means every gap, reserved span, and hole is unallocatable by default — we only ever hand back memory the firmware explicitly called usable.
4. Take back the essentials. Re-reserve the frames the bitmap itself occupies (they're inside a usable region we just freed), plus frame 0, so an address of 0 can keep meaning "no frame".

alloc() → ?u64

Scan the bitmap from next_hint (wrapping once) for the first free bit, mark it used, advance the hint, and return frame * page_size. Returns null when no frame is free — genuine out-of-memory. The hint avoids rescanning the low, long-since-allocated frames on every call.

free(addr)

Clear the frame's bit and, if it's below next_hint, pull the hint back so the reclaimed frame gets reused soon. Bogus or double frees (a frame already marked free, or one out of range) are ignored rather than corrupting the used count.

Correctness points worth remembering

• Generic walk. Because MemoryRegion is danos's own type, the map is a plain slice — none of the variable descriptor-stride from the raw UEFI map.
• Physmap addressing. The bitmap (and the region array) is reached through the physmap via boot_handoff.physicalToVirtual, which both the loader's bootstrap tables and the kernel's own tables provide — no remapping is needed when danos switches to its own paging. The one invariant: the bitmap must sit under the bootstrap physmap's reach (4 GiB), which holds because the placement scan (step 2 above) runs from the lowest usable region up and takes the first one big enough — on the supported configurations that lands well under 4 GiB.
• Frame 0 is reserved so 0 stays a safe "none" sentinel — and the bitmap is never placed there. (An early bug did exactly that: a usable region at physical address 0 collided with a 0-means-not-found sentinel and tripped a panic. The fix was an optional plus starting the bitmap at least one page in.)
• Everything non-usable is unallocatable by construction — the "mark all used, then free usable" order gives that for free, so the kernel image, the loader's buffers, MMIO and firmware memory can never be handed out.

Verifying it

kmain brings the allocator up and self-tests it. Booted in QEMU with 128 MiB:

/system/kernel: frame allocator online
  free frames: 30520 (119 MiB)      <- matches the map's 119 MiB usable
  alloc x3   : 0x3000 0x4000 0x5000 <- frame 0 reserved, bitmap at 0x1000, AP trampoline at 0x2000
  after free : 30520 frames free    <- three freed, count restored

(The AP-trampoline page is claimed with allocBelow right after init, before the demo allocations — hence they start at 0x3000.)

The free frames MiB agreeing with the memory map's usable RAM, the three distinct consecutive addresses, and the count returning to its start after freeing are the three signals that init, alloc and free are all correct.

Boot-services memory comes pre-reclaimed

The UEFI boot-services memory (~44 MiB) is defunct and free once ExitBootServices runs, taking usable RAM from ~76 MiB up to ~119 MiB. The frame allocator does nothing special to get it: the loader already classified it as usable (see memory-map.md), so it's just part of the usable regions init frees. Keeping that boot-protocol knowledge on the loader side is deliberate — the kernel has no notion of "reclaimable" or of UEFI at all.

The one live piece in that memory is the boot stack the kernel starts on; the loader leaves the single region containing it reserved, so init won't hand it out. A later step will move task 0 onto a kernel-owned stack, freeing that last ~1 MiB region too (and giving user mode the clean stack it wants).

What's next (partly done since)

• Contiguous allocation — done: allocContiguous scans for a run of clear bits, with an optional physical ceiling for DMA (dma_alloc is its user), and allocBelow serves the SMP trampoline.
• A kernel stack for task 0 — still open: the boot processor's idle task runs on the boot stack to this day, so that region can't be freed.
• Freeing the reserved loader_data (the boot-time map buffers) — still open: the bitmap deliberately tracks those frames so they can be freed, but nothing frees them yet.