Move to multi arch support and added memory map

docs/memory-map.md

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+# The memory map
+
+Before a kernel can manage memory, it has to *know what memory exists*: which
+physical address ranges are real RAM it may use, and which are firmware, hardware
+registers, or already occupied. That inventory is the **memory map**, and the
+firmware is the only thing that knows it. This page covers how danos gets that map
+from the firmware and hands it to the kernel — deliberately without dragging UEFI
+into the kernel.
+
+## Why not just pass UEFI's map through?
+
+UEFI hands the loader a perfectly good memory map. The tempting shortcut is to
+forward it to the kernel as-is. We don't, for two reasons:
+
+1. **It would tie the kernel to UEFI.** The kernel would compare against UEFI's
+   memory-type numbers and walk the array using UEFI's variable descriptor stride.
+   That's UEFI vocabulary bleeding across the handoff — and danos wants to boot on
+   systems that have no UEFI at all (a Raspberry Pi describes its memory with a
+   *device tree* instead). See [arch.md](arch.md) for the same "keep the kernel
+   platform-agnostic" principle applied to CPU code.
+2. **We already established the better pattern.** The loader doesn't hand the
+   kernel a raw UEFI GOP either — [`queryFramebuffer`](gop.md) converts it to
+   danos's own `Framebuffer`. The memory map follows the same discipline.
+
+So the boundary is: **each boot path translates its native memory description into
+danos's own neutral format, and the kernel only ever sees that.**
+
+## The neutral format
+
+Defined in `src/root.zig`, the shared loader↔kernel contract:
+
+```zig
+pub const MemoryKind = enum(u32) {
+    usable,       // free RAM the kernel may allocate
+    reserved,     // firmware / MMIO / kernel image — never hand out
+    reclaimable,  // usable once boot-time structures are done with
+    acpi_tables,  // parse, then reclaim
+    acpi_nvs,     // preserve across sleep
+};
+
+pub const MemoryRegion = extern struct {
+    base: u64,    // physical start
+    pages: u64,   // length in page_size (4 KiB) units
+    kind: MemoryKind,
+    _pad: u32 = 0,
+};
+
+pub const MemoryMap = extern struct {
+    regions: usize, // pointer to a [len]MemoryRegion
+    len: usize,
+};
+```
+
+`MemoryKind` is danos's *own* vocabulary — not UEFI's ~15 types, just the
+distinctions the kernel actually acts on. And because danos defines `MemoryRegion`
+itself, `@sizeOf` is authoritative: the kernel walks a plain `[]MemoryRegion` with
+no variable-stride subtlety (that stride problem is a UEFI-ism, and it stays in the
+loader).
+
+`BootInfo` carries it alongside the framebuffer:
+
+```zig
+pub const BootInfo = extern struct {
+    framebuffer: Framebuffer,
+    memory_map: MemoryMap,
+};
+```
+
+## The loader side (UEFI)
+
+Two functions in `src/efi.zig`, called from `exitBootServices`:
+
+- **`classify`** maps each UEFI memory type to a `MemoryKind`:
+  `conventional_memory → usable`; `boot_services_code`/`boot_services_data →
+  reclaimable` (free once we've exited); `acpi_reclaim_memory → acpi_tables`;
+  `acpi_memory_nvs → acpi_nvs`; **everything else → reserved** (the safe default).
+  Our own `loader_data` — the kernel image and these buffers — falls into
+  `reserved`, so it won't be handed out until the kernel deliberately reclaims it.
+- **`convertMemoryMap`** walks the UEFI descriptors (striding by
+  `descriptor_size`, *not* `@sizeOf`), classifies each, and writes danos
+  `MemoryRegion`s into an output buffer, coalescing adjacent same-kind regions.
+
+### The ordering that makes it correct
+
+This is the fiddly part, dictated by two UEFI rules: you can only allocate memory
+*before* `ExitBootServices`, and the memory map is only final *at* the moment you
+exit (its "key" proves you've seen the latest state). So `exitBootServices` does,
+per attempt:
+
+1. `getMemoryMapInfo` to size things, then `allocatePool` **two** LoaderData
+   buffers — one for the raw UEFI map, one for the converted regions. Allocating
+   now, before exit, is mandatory.
+2. `getMemoryMap` then `exitBootServices(key)`. If either fails (allocating can
+   perturb the map and invalidate the key), free both buffers and retry.
+3. **After** the exit succeeds, convert. Conversion is pure computation on memory
+   we already hold — no boot-services calls — so it's safe once services are gone.
+
+Both buffers are `LoaderData`, which survives `ExitBootServices`, so the converted
+array the kernel is pointed at stays valid. (The raw UEFI buffer is just scratch
+for the conversion.)
+
+## The kernel side
+
+The kernel receives a plain array and reads it with zero UEFI knowledge:
+
+```zig
+const mm = boot_info.memory_map;
+const regions = @as([*]const danos.MemoryRegion, @ptrFromInt(mm.regions))[0..mm.len];
+for (regions) |r| {
+    if (r.kind == .usable) usable_pages += r.pages;
+}
+```
+
+Today `kmain` just prints the region count and total usable RAM — enough to prove
+the handoff works. Booted in QEMU with 128 MiB, it reports something like:
+
+```
+mem regions: 35
+usable RAM : 77 MiB
+```
+
+with the balance being `reclaimable` boot-services memory (~44 MiB) and a large
+`reserved` span that is mostly MMIO address space, not RAM. Those figures summing
+back to ~128 MiB is the sanity check that nothing was dropped.
+
+## How Raspberry Pi will fit
+
+No UEFI there, but the boundary is unchanged. The Pi's firmware jumps into the
+kernel with a **device-tree blob**; the AArch64 entry code will parse its
+`/memory` and `/reserved-memory` nodes and produce the *same* `MemoryRegion`
+array. The kernel's memory code — the frame allocator and everything above it —
+never knows the difference.
+
+## What's next (not done here)
+
+This is plumbing plus classification only. Still to come:
+
+- A **physical frame allocator** that consumes `usable` regions and hands out
+  4 KiB frames — the foundation everything else stands on.
+- Reclaiming `reclaimable` regions, and carefully freeing `reserved` `loader_data`
+  (kernel image, these buffers) once the kernel is done reading them.
+- Paging / the kernel's own page tables, then a heap.
+
+See the roadmap in [efi.md](efi.md) for where this sits in the boot flow.