mm: add bitmap PMM over E820 usable frames

include/pmm.h

@@ -0,0 +1,53 @@
+/* Nadir physical memory manager
+ *
+ * Runs in 64-bit long mode at Ring 0 (CPL 0) under the stage-2 identity
+ * map; physical addresses are used as-is. Called once from kmain with
+ * interrupts disabled (IF=0), before sti, so no IRQ handler can allocate
+ * concurrently. No faults are possible beyond a programming bug; with no
+ * IDT yet at the call point a wild access would triple-fault.
+ */
+
+#ifndef NADIR_PMM_H
+#define NADIR_PMM_H
+
+#include <stdint.h>
+
+#include "e820.h"
+
+/* 4KB frames over the identity-mapped first 1GB: hard capacity limit of
+ * this allocator (256K frames, 32KB bitmap in .bss). Regions above 1GB
+ * reported by E820 are ignored. */
+#define PMM_FRAME_SIZE 4096U
+#define PMM_MAX_BYTES 0x40000000ULL
+#define PMM_MAX_FRAMES (PMM_MAX_BYTES / PMM_FRAME_SIZE)
+
+/* Memory-map globals: filled by the stage-2 entry (copy from the
+ * stage-1 staging buffer) before kmain runs. pmm_init reads them. */
+extern uint32_t memmap_count;
+extern struct e820_entry memmap_entries[E820_MAX];
+
+/* Build the bitmap from the E820 map: type-1 (usable) frames start free,
+ * everything else starts reserved, then the kernel image, page tables,
+ * staging buffer, low 1MB, stack and VGA regions are reserved.
+ * Preconditions: count <= E820_MAX, entries points at count valid
+ * entries, called once with IF=0 before any alloc/free.
+ * Failure modes: none — an empty/corrupt map just leaves 0 free frames. */
+void pmm_init(uint32_t count, const struct e820_entry *entries);
+
+/* Hand out one 4KB frame: returns its physical base address, or 0 when
+ * out of memory (frame 0 is always reserved, so 0 is never valid).
+ * Preconditions: pmm_init ran. Failure modes: returns 0 on OOM. */
+uint64_t pmm_alloc_frame(void);
+
+/* Return a frame from pmm_alloc_frame to the free pool. Silently ignores
+ * unaligned, out-of-range, or already-free addresses.
+ * Preconditions: pmm_init ran. Failure modes: none (no fault, no panic). */
+void pmm_free_frame(uint64_t addr);
+
+/* Live counters for stats printouts.
+ * Preconditions: pmm_init ran (0 before that). Failure modes: none. */
+uint64_t pmm_total_count(void);
+uint64_t pmm_free_count(void);
+uint64_t pmm_usable_count(void);
+
+#endif /* NADIR_PMM_H */