1 files changed, 121 insertions, 0 deletions
diff --git a/bootmain.c b/bootmain.c
new file mode 100644
index 0000000..79d769c
--- /dev/null
+++ b/bootmain.c
@@ -0,0 +1,121 @@
+#include <types.h>
+#include <elf.h>
+#include <x86.h>
+
+/**********************************************************************
+ * This a dirt simple boot loader, whose sole job is to boot
+ * an elf kernel image from the first IDE hard disk.
+ *
+ * DISK LAYOUT
+ *  * This program(boot.S and main.c) is the bootloader.  It should
+ *    be stored in the first sector of the disk.
+ * 
+ *  * The 2nd sector onward holds the kernel image.
+ *	
+ *  * The kernel image must be in ELF format.
+ *
+ * BOOT UP STEPS	
+ *  * when the CPU boots it loads the BIOS into memory and executes it
+ *
+ *  * the BIOS intializes devices, sets of the interrupt routines, and
+ *    reads the first sector of the boot device(e.g., hard-drive) 
+ *    into memory and jumps to it.
+ *
+ *  * Assuming this boot loader is stored in the first sector of the
+ *    hard-drive, this code takes over...
+ *
+ *  * control starts in bootloader.S -- which sets up protected mode,
+ *    and a stack so C code then run, then calls cmain()
+ *
+ *  * cmain() in this file takes over, reads in the kernel and jumps to it.
+ **********************************************************************/
+
+#define SECTSIZE	512
+#define ELFHDR		((struct Elf *) 0x10000) // scratch space
+
+void readsect(void*, uint32_t);
+void readseg(uint32_t, uint32_t, uint32_t);
+
+void
+cmain(void)
+{
+	struct Proghdr *ph, *eph;
+
+	// read 1st page off disk
+	readseg((uint32_t) ELFHDR, SECTSIZE*8, 0);
+
+	// is this a valid ELF?
+	if (ELFHDR->e_magic != ELF_MAGIC)
+		goto bad;
+
+	// load each program segment (ignores ph flags)
+	ph = (struct Proghdr *) ((uint8_t *) ELFHDR + ELFHDR->e_phoff);
+	eph = ph + ELFHDR->e_phnum;
+	for (; ph < eph; ph++)
+		readseg(ph->p_va, ph->p_memsz, ph->p_offset);
+
+	// call the entry point from the ELF header
+	// note: does not return!
+	((void (*)(void)) (ELFHDR->e_entry & 0xFFFFFF))();
+
+bad:
+	outw(0x8A00, 0x8A00);
+	outw(0x8A00, 0x8E00);
+	while (1)
+		/* do nothing */;
+}
+
+// Read 'count' bytes at 'offset' from kernel into virtual address 'va'.
+// Might copy more than asked
+void
+readseg(uint32_t va, uint32_t count, uint32_t offset)
+{
+	uint32_t end_va;
+
+	va &= 0xFFFFFF;
+	end_va = va + count;
+	
+	// round down to sector boundary
+	va &= ~(SECTSIZE - 1);
+
+	// translate from bytes to sectors, and 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.
+	while (va < end_va) {
+		readsect((uint8_t*) va, offset);
+		va += SECTSIZE;
+		offset++;
+	}
+}
+
+void
+waitdisk(void)
+{
+	// wait for disk reaady
+	while ((inb(0x1F7) & 0xC0) != 0x40)
+		/* do nothing */;
+}
+
+void
+readsect(void *dst, uint32_t offset)
+{
+	// wait for disk to be ready
+	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
+
+	// wait for disk to be ready
+	waitdisk();
+
+	// read a sector
+	insl(0x1F0, dst, SECTSIZE/4);
+}
+