// Boot loader.
//
// Part of the boot block, 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);
    }
}
