98 lines
2.2 KiB
C
98 lines
2.2 KiB
C
// Boot loader.
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//
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// The BIOS loads boot sector (bootasm.S) from sector 1 of the disk
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// into memory and executes it. The boot sector puts the processor
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// in 32-bit mode and calls cmain below, which loads an ELF kernel
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// image from the disk starting at sector 2 and then jumps to the
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// kernel entry routine.
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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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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 elfhdr *elf;
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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
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readseg((uint)elf, SECTSIZE*8, 0);
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// Is this an ELF executable?
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if(elf->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*)elf + elf->phoff);
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eph = ph + elf->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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// Does not return!
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entry = (void(*)(void))(elf->entry & 0xFFFFFF);
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entry();
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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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void
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waitdisk(void)
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{
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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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}
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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.
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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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// Read data.
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waitdisk();
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insl(0x1F0, dst, SECTSIZE/4);
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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 eva;
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va &= 0xFFFFFF;
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eva = 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; 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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for(; va < eva; va += SECTSIZE, offset++)
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readsect((uchar*)va, offset);
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}
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