// Physical memory allocator, intended to allocate
// memory for user processes, kernel stacks, page table pages,
// and pipe buffers. Allocates 4096-byte pages.

#include "mem.h"
#include "console.h"
#include "cpu/memlayout.h"
#include "cpu/x86.h"
#include <stdint.h>

struct run {
    struct run *next;
};

struct {
    struct run *freelist;
} kmem;

void freerange(void *vstart, void *vend) {
    char *p;
    p = (char *)PGROUNDUP((uintptr_t)vstart);
    for (; p + PGSIZE <= (char *)vend; p += PGSIZE)
        kfree(p);
}

void *memset(void *dst, unsigned c, uint64_t n) {
    if ((uintptr_t)dst % 4 == 0 && n % 4 == 0) {
        c &= 0xFF;
        stosl(dst, (c << 24) | (c << 16) | (c << 8) | c, n / 4);
    } else
        stosb(dst, c, n);
    return dst;
}

// Free the page of physical memory pointed at by v,
// which normally should have been returned by a
// call to kalloc().  (The exception is when
// initializing the allocator; see kinit above.)
void kfree(void *v) {
    struct run *r;

    if ((uintptr_t)v % PGSIZE || V2P(v) >= PHYSTOP)
        panic("kfree");

    // Fill with junk to catch dangling refs.
    memset(v, 1, PGSIZE);

    r = v;
    r->next = kmem.freelist;
    kmem.freelist = r;
}

// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
// Returns 0 if the memory cannot be allocated.
void *kalloc(void) {
    struct run *r;

    r = kmem.freelist;
    if (r)
        kmem.freelist = r->next;
    return (char *)r;
}