120 lines
2.8 KiB
C
120 lines
2.8 KiB
C
// This a dirt simple boot loader, whose sole job is to boot
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// an elf kernel image from the first IDE hard disk.
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//
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// DISK LAYOUT
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// * This program(boot.S and main.c) is the bootloader. It should
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// be stored in the first sector of the disk.
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//
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// * The 2nd sector onward holds the kernel image.
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//
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// * The kernel image must be in ELF format.
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//
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// BOOT UP STEPS
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// * when the CPU boots it loads the BIOS into memory and executes it
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//
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// * the BIOS intializes devices, sets of the interrupt routines, and
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// reads the first sector of the boot device(e.g., hard-drive)
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// into memory and jumps to it.
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//
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// * Assuming this boot loader is stored in the first sector of the
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// hard-drive, this code takes over...
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//
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// * control starts in bootloader.S -- which sets up protected mode,
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// and a stack so C code then run, then calls cmain()
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//
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// * cmain() in this file takes over,
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// reads in the kernel and jumps to it.
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#include "types.h"
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#include "elf.h"
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#include "x86.h"
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#define SECTSIZE 512
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#define ELFHDR ((struct elfhdr*) 0x10000) // scratch space
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void readsect(void*, uint);
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void readseg(uint, uint, uint);
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void
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cmain(void)
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{
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struct proghdr *ph, *eph;
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// read 1st page off disk
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readseg((uint) ELFHDR, SECTSIZE*8, 0);
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// is this a valid ELF?
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if(ELFHDR->magic != ELF_MAGIC)
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goto bad;
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// load each program segment (ignores ph flags)
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ph = (struct proghdr*) ((uchar*) ELFHDR + ELFHDR->phoff);
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eph = ph + ELFHDR->phnum;
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for(; ph < eph; ph++)
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readseg(ph->va, ph->memsz, ph->offset);
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// call the entry point from the ELF header
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// note: does not return!
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((void(*)(void)) (ELFHDR->entry & 0xFFFFFF))();
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bad:
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outw(0x8A00, 0x8A00);
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outw(0x8A00, 0x8E00);
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for(;;)
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;
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}
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// Read 'count' bytes at 'offset' from kernel into virtual address 'va'.
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// Might copy more than asked
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void
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readseg(uint va, uint count, uint offset)
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{
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uint end_va;
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va &= 0xFFFFFF;
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end_va = va + count;
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// round down to sector boundary
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va &= ~(SECTSIZE - 1);
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// translate from bytes to sectors, and kernel starts at sector 1
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offset = (offset / SECTSIZE) + 1;
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// If this is too slow, we could read lots of sectors at a time.
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// We'd write more to memory than asked, but it doesn't matter --
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// we load in increasing order.
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while(va < end_va) {
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readsect((uchar*) va, offset);
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va += SECTSIZE;
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offset++;
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}
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}
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void
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waitdisk(void)
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{
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// wait for disk reaady
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while((inb(0x1F7) & 0xC0) != 0x40)
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;
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}
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void
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readsect(void *dst, uint offset)
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{
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// wait for disk to be ready
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waitdisk();
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outb(0x1F2, 1); // count = 1
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outb(0x1F3, offset);
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outb(0x1F4, offset >> 8);
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outb(0x1F5, offset >> 16);
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outb(0x1F6, (offset >> 24) | 0xE0);
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outb(0x1F7, 0x20); // cmd 0x20 - read sectors
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// wait for disk to be ready
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waitdisk();
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// read a sector
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insl(0x1F0, dst, SECTSIZE/4);
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}
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