Source kernel/frame.h

// SafeC Standard Library — Physical Frame Allocator
// Bitmap-based physical memory frame allocator. Freestanding-safe.
#pragma once

// Each bit represents one 4K frame. 1 = allocated, 0 = free.
// 4096 unsigned int entries = 4096 * 32 = 131072 frames = 512 MB.
#define FRAME_BITMAP_SIZE 4096

namespace std {

struct FrameAllocator {
    unsigned int  bitmap[FRAME_BITMAP_SIZE];
    unsigned long total_frames;  // total number of physical frames
    unsigned long used_frames;   // currently allocated frames
    unsigned long base;          // physical address of frame 0

    // Initialize for `total_frames` frames starting at physical 0 (clears bitmap).
    void          init(unsigned long total_frames);

    // Initialize for `total_frames` frames starting at physical `base_phys`
    // (4 KiB aligned) — e.g. the RAM base on boards where RAM isn't at 0.
    void          init_at(unsigned long base_phys, unsigned long total_frames);

    // Physical address of frame number `frame`: base + frame * 4096.
    unsigned long frame_addr(unsigned long frame) const;

    // Allocate a single frame. Returns frame number, or -1 if OOM.
    long long     alloc();

    // Free a frame by frame number.
    void          free(unsigned long frame);

    // Check if a frame is allocated. Returns 1 if allocated.
    int           is_used(unsigned long frame) const;

    // Mark a range of frames as used (e.g., for kernel image).
    void          mark_range(unsigned long start, unsigned long count);

    // Return the number of free frames.
    unsigned long free_count() const;

    // Internal: count trailing zeros in a 32-bit word.
    int           ctz32_(unsigned int x) const;
};

} // namespace std