bootmain.c

// Boot loader.
// 
// Part of the boot sector, along with bootasm.S, which calls bootmain().
// bootasm.S has put the processor into protected 32-bit mode.
// bootmain() loads an ELF kernel image from the disk starting at
// sector 1 and then jumps to the kernel entry routine.

#include "types.h"
#include "elf.h"
#include "x86.h"
#include "memlayout.h"

#define SECTSIZE  512

void readseg (uchar *, uint, uint);

void
bootmain (void)
{
    struct elfhdr *elf;
    struct proghdr *ph, *eph;
    void (*entry) (void);
    uchar *pa;

    elf = (struct elfhdr *) 0x10000;    // scratch space

    // Read 1st page off disk
    readseg ((uchar *) elf, 4096, 0);

    // Is this an ELF executable?
    if (elf->magic != ELF_MAGIC)
        return;                 // let bootasm.S handle error

    // Load each program segment (ignores ph flags).
    ph = (struct proghdr *) ((uchar *) elf + elf->phoff);
    eph = ph + elf->phnum;
    for (; ph < eph; ph++)
    {
        pa = (uchar *) ph->paddr;
        readseg (pa, ph->filesz, ph->off);
        if (ph->memsz > ph->filesz)
            stosb (pa + ph->filesz, 0, ph->memsz - ph->filesz);
    }

    // Call the entry point from the ELF header.
    // Does not return!
    entry = (void (*)(void)) (elf->entry);
    entry ();
}

void
waitdisk (void)
{
    // Wait for disk ready.
    while ((inb (0x1F7) & 0xC0) != 0x40)
        ;
}

// Read a single sector at offset into dst.
void
readsect (void *dst, uint offset)
{
    // Issue command.
    waitdisk ();
    outb (0x1F2, 1);            // count = 1
    outb (0x1F3, offset);
    outb (0x1F4, offset >> 8);
    outb (0x1F5, offset >> 16);
    outb (0x1F6, (offset >> 24) | 0xE0);
    outb (0x1F7, 0x20);         // cmd 0x20 - read sectors

    // Read data.
    waitdisk ();
    insl (0x1F0, dst, SECTSIZE / 4);
}

// Read 'count' bytes at 'offset' from kernel into physical address 'pa'.
// Might copy more than asked.
void
readseg (uchar * pa, uint count, uint offset)
{
    uchar *epa;

    epa = pa + count;

    // Round down to sector boundary.
    pa -= offset % SECTSIZE;

    // Translate from bytes to sectors; kernel starts at sector 1.
    offset = (offset / SECTSIZE) + 1;

    // If this is too slow, we could read lots of sectors at a time.
    // We'd write more to memory than asked, but it doesn't matter --
    // we load in increasing order.
    for (; pa < epa; pa += SECTSIZE, offset++)
        readsect (pa, offset);
}