bootmain.c

   1 // Boot loader.
   2 //
   3 // Part of the boot sector, along with bootasm.S, which calls bootmain().
   4 // bootasm.S has put the processor into protected 32-bit mode.
   5 // bootmain() loads an ELF kernel image from the disk starting at
   6 // sector 1 and then jumps to the kernel entry routine.
   7
   8 #include "types.h"
   9 #include "elf.h"
  10 #include "x86.h"
  11
  12 #define SECTSIZE  512
  13
  14 void readseg(uchar*, uint, uint);
  15
  16 void
  17 bootmain(void)
  18 {
  19   struct elfhdr *elf;
  20   struct proghdr *ph, *eph;
  21   void (*entry)(void);
  22   uchar* va;
  23
  24   elf = (struct elfhdr*)0x10000;  // scratch space
  25
  26   // Read 1st page off disk
  27   readseg((uchar*)elf, 4096, 0);
  28
  29   // Is this an ELF executable?
  30   if(elf->magic != ELF_MAGIC)
  31     return;  // let bootasm.S handle error
  32
  33   // Load each program segment (ignores ph flags).
  34   ph = (struct proghdr*)((uchar*)elf + elf->phoff);
  35   eph = ph + elf->phnum;
  36   for(; ph < eph; ph++){
  37     va = (uchar*)ph->va;
  38     readseg(va, ph->filesz, ph->offset);
  39     if(ph->memsz > ph->filesz)
  40       stosb(va + ph->filesz, 0, ph->memsz - ph->filesz);
  41   }
  42
  43   // Call the entry point from the ELF header.
  44   // Does not return!
  45   entry = (void(*)(void))(elf->entry);
  46   entry();
  47 }
  48
  49 void
  50 waitdisk(void)
  51 {
  52   // Wait for disk ready.
  53   while((inb(0x1F7) & 0xC0) != 0x40)
  54     ;
  55 }
  56
  57 // Read a single sector at offset into dst.
  58 void
  59 readsect(void *dst, uint offset)
  60 {
  61   // Issue command.
  62   waitdisk();
  63   outb(0x1F2, 1);   // count = 1
  64   outb(0x1F3, offset);
  65   outb(0x1F4, offset >> 8);
  66   outb(0x1F5, offset >> 16);
  67   outb(0x1F6, (offset >> 24) | 0xE0);
  68   outb(0x1F7, 0x20);  // cmd 0x20 - read sectors
  69
  70   // Read data.
  71   waitdisk();
  72   insl(0x1F0, dst, SECTSIZE/4);
  73 }
  74
  75 // Read 'count' bytes at 'offset' from kernel into virtual address 'va'.
  76 // Might copy more than asked.
  77 void
  78 readseg(uchar* va, uint count, uint offset)
  79 {
  80   uchar* eva;
  81
  82   eva = va + count;
  83
  84   // Round down to sector boundary.
  85   va -= offset % SECTSIZE;
  86
  87   // Translate from bytes to sectors; kernel starts at sector 1.
  88   offset = (offset / SECTSIZE) + 1;
  89
  90   // If this is too slow, we could read lots of sectors at a time.
  91   // We'd write more to memory than asked, but it doesn't matter --
  92   // we load in increasing order.
  93   for(; va < eva; va += SECTSIZE, offset++)
  94     readsect(va, offset);
  95 }