xv6/bootmain.c

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// Boot loader.
//
// The BIOS loads boot sector (bootasm.S) from sector 1 of the disk
// into memory and executes it. The boot sector puts the processor
// in 32-bit mode and calls cmain below, which loads an ELF kernel
// image from the disk starting at sector 2 and then jumps to the
// kernel entry routine.
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#include "types.h"
#include "elf.h"
#include "x86.h"
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#define SECTSIZE 512
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void readseg(uint, uint, uint);
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void
cmain(void)
{
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struct elfhdr *elf;
struct proghdr *ph, *eph;
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void (*entry)(void);
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elf = (struct elfhdr*)0x10000; // scratch space
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// Read 1st page off disk
readseg((uint)elf, SECTSIZE*8, 0);
// Is this an ELF executable?
if(elf->magic != ELF_MAGIC)
goto bad;
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// Load each program segment (ignores ph flags).
ph = (struct proghdr*)((uchar*)elf + elf->phoff);
eph = ph + elf->phnum;
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for(; ph < eph; ph++)
readseg(ph->va, ph->memsz, ph->offset);
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// Call the entry point from the ELF header.
// Does not return!
entry = (void(*)(void))(elf->entry & 0xFFFFFF);
entry();
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bad:
outw(0x8A00, 0x8A00);
outw(0x8A00, 0x8E00);
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for(;;)
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;
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}
void
waitdisk(void)
{
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// Wait for disk ready.
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while((inb(0x1F7) & 0xC0) != 0x40)
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;
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}
// Read a single sector at offset into dst.
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void
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readsect(void *dst, uint offset)
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{
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// 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
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// Read data.
waitdisk();
insl(0x1F0, dst, SECTSIZE/4);
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}
// Read 'count' bytes at 'offset' from kernel into virtual address 'va'.
// Might copy more than asked.
void
readseg(uint va, uint count, uint offset)
{
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uint eva;
va &= 0xFFFFFF;
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eva = va + count;
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// Round down to sector boundary.
va &= ~(SECTSIZE - 1);
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// 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.
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for(; va < eva; va += SECTSIZE, offset++)
readsect((uchar*)va, offset);
}