# The kernel heap

The [frame allocator](frame-allocator.md) hands out fixed 4 KiB physical frames;
the [VMM](paging.md) maps pages into virtual addresses. The **kernel heap** sits on
top of both to provide what the rest of the kernel actually wants: `alloc(n)` /
`free(p)` for arbitrary byte sizes. It's the first real consumer of `map()`, and
the thing that unlocks dynamic data structures — lists, hash maps, driver state,
eventually a process table.

It's generic kernel code (`system/kernel/heap.zig`): the allocator logic is
architecture-neutral, using `arch.mapPage` and the frame allocator underneath.

## A growable free-list allocator

The algorithm is a classic **first-fit free list**:

- The heap owns a virtual region. Free space is tracked as an **address-ordered
  singly linked list** of free blocks; each block begins with a 16-byte header
  (`size`, and a `next` link used while free).
- **alloc(n)** walks the list for the first block big enough. If the block is much
  larger it's **split** — the front becomes the allocation, the remainder stays
  free. If nothing fits, the heap **grows** (below) and the search retries.
- **free(p)** finds the block header just before `p` and inserts it back into the
  list, **coalescing** with the physically adjacent free blocks on either side so
  the space can be reused as one region rather than fragmenting away.

Allocations are 16-byte aligned; larger alignments aren't supported yet (the
`std.mem.Allocator` `alloc` returns `null` for them).

## Growing on demand

The heap lives in the **higher half** of the address space (virtual base
`0xFFFF_8000_0000_0000`) — unmapped, well clear of the identity-mapped low half,
and leaving the low half free for a future user address space. (That base is
x86_64-canonical; another architecture would pick its own.)

When the free list can't satisfy a request, `grow` extends the mapped region: it
pulls fresh frames from the [frame allocator](frame-allocator.md) and `map`s each
onto the end of the heap, then adds the new span as a free block (coalescing with
the current tail). So the heap starts at one page and expands page-by-page as
demand requires, up to a cap. This is exactly what the VMM's on-demand `map` was
built for.

## A std.mem.Allocator

The heap is exposed as a **`std.mem.Allocator`** (`heap.allocator()`), Zig's
standard allocator interface. That's a deliberate multiplier: it means the whole of
Zig's standard library — `ArrayList`, `AutoHashMap`, `std.fmt.allocPrint`, and the
rest — works directly on the kernel heap, no bespoke containers required.

## Verifying it

The `heap` test (see [testing.md](testing.md)) exercises the allocator end to end:

```
[PASS] alloc 4096 bytes
[PASS] heap memory is writable and reads back
[PASS] freed block is reused              <- free list + coalescing works
[PASS] many allocations (heap growth) stay valid   <- grow() maps fresh frames
[PASS] std.ArrayList on the kernel heap    <- std containers work on it
```

The "freed block is reused" check (free then re-alloc returns the same address) is
the proof that free and the free list actually work, not just alloc; "heap growth"
forces allocation past the initial page so `grow`/`map` runs; and the `ArrayList`
check is the std-integration payoff.

## What's next (not done here)

- **Thread/interrupt safety.** The heap assumes a single caller — no lock yet.
  It's safe now (nothing allocates from interrupt handlers), but threads or an
  allocating IRQ handler will need a lock (or `cli` around the critical section).
- **Larger alignments** than 16 (for page-aligned buffers, DMA regions).
- **`resize`/`remap` in place**, so growing an `ArrayList` needn't always copy.
- **Reclaiming empty tail pages** back to the frame allocator when the heap shrinks.
