- ### Poll status register while controller busy.
- movl $0x1f7, %edx
-1: inb %dx, %al
- testb $0x80, %al
- jnz 1b
-
- ### Read a single sector.
- movl $0x1f2, %edx
- movb $1, %al
- outb %al, %dx
-
- ### Sector number to write in low 28 bits.
- ### LBA mode, device 0 in top 4 bits.
- movl %ebx, %eax
- andl $0x0fffffff, %eax
- orl $0xe0000000, %eax
-
- ### Dump %eax to ports 0x1f3...0x1f6.
- movl $4, %ecx
-2: incw %dx
- outb %al, %dx
- shrl $8, %eax
- loop 2b
-
- ### READ command to command register.
- incw %dx
- movb $0x20, %al
- outb %al, %dx
-
- ### Poll status register while controller busy.
-1: inb %dx, %al
- testb $0x80, %al
- jnz 1b
-
- ### Poll status register until data ready.
-1: inb %dx, %al
- testb $0x08, %al
- jz 1b
-
- ### Transfer sector.
- movl $512 / 4, %ecx
- movl $0x1f0, %edx
- rep insl
-
- ### Next sector.
- incl %ebx
- cmpl $KERNEL_LOAD_PAGES*8 + 1, %ebx
- jnz read_sector
-
-##### Create temporary page directory and page table, set page
-##### directory pointer, and turn on paging.
-##### FIXME? We could use a single 4 MB page instead of 1024 4 kB pages.
-
- # Create page directory at 64 kB.
- movl $0x10000, %edi
- movl %edi, %cr3
-
- # Fill page directory with zeroes.
- subl %eax, %eax
- movl $0x400, %ecx
- rep stosl
-
- # Set PDEs for 0 and LOADER_PHYS_BASE to point to the
- # page table.
- movl $0x11000 | PG_U | PG_W | PG_P, %eax
- movl %eax, 0x10000
- movl %eax, 0x10000 | (LOADER_PHYS_BASE >> 20)
-
- # Initialize page table.
- movl $PG_U | PG_W | PG_P, %eax
- movl $0x400, %ecx
-1: stosl
- addl $0x1000, %eax
- loop 1b
-
- # Turn on paging and kernel write-protect.
- movl %cr0, %eax
- orl $CR0_PG | CR0_WP, %eax
- movl %eax, %cr0
- jmp 1f
+#### Kernel loader.
+
+#### This code should be stored in the first sector of a hard disk.
+#### When the BIOS runs, it loads this code at physical address
+#### 0x7c00-0x7e00 (512 bytes) and jumps to the beginning of it,
+#### in real mode. The loader loads the kernel into memory and jumps
+#### to its entry point, which is the start function in start.S.
+####
+#### The BIOS passes in the drive that the loader was read from as
+#### DL, with floppy drives numbered 0x00, 0x01, ... and hard drives
+#### numbered 0x80, 0x81, ... We want to support booting a kernel on
+#### a different drive from the loader, so we don't take advantage of
+#### this.
+
+# Runs in real mode, which is a 16-bit segment.
+ .code16
+
+# Set up segment registers.
+# Set stack to grow downward from 60 kB (after boot, the kernel
+# continues to use this stack for its initial thread).
+
+ sub %ax, %ax
+ mov %ax, %ds
+ mov %ax, %ss
+ mov $0xf000, %esp
+
+# Configure serial port so we can report progress without connected VGA.
+# See [IntrList] for details.
+ sub %dx, %dx # Serial port 0.
+ mov $0xe3, %al # 9600 bps, N-8-1.
+ # AH is already 0 (Initialize Port).
+ int $0x14 # Destroys AX.
+
+ call puts
+ .string "PiLo"
+
+#### Read the partition table on each system hard disk and scan for a
+#### partition of type 0x20, which is the type that we use for a
+#### Pintos kernel.
+####
+#### Read [Partitions] for a description of the partition table format
+#### that we parse.
+####
+#### We print out status messages to show the disk and partition being
+#### scanned, e.g. hda1234 as we scan four partitions on the first
+#### hard disk.
+
+ mov $0x80, %dl # Hard disk 0.
+read_mbr:
+ sub %ebx, %ebx # Sector 0.
+ mov $0x2000, %ax # Use 0x20000 for buffer.
+ mov %ax, %es
+ call read_sector
+ jc no_such_drive
+
+ # Print hd[a-z].
+ call puts
+ .string " hd"
+ mov %dl, %al
+ add $'a' - 0x80, %al
+ call putc
+
+ # Check for MBR signature--if not present, it's not a
+ # partitioned hard disk.
+ cmpw $0xaa55, %es:510
+ jne next_drive
+
+ mov $446, %si # Offset of partition table entry 1.
+ mov $'1', %al
+check_partition:
+ # Is it an unused partition?
+ cmpl $0, %es:(%si)
+ je next_partition
+
+ # Print [1-4].
+ call putc
+
+ # Is it a Pintos kernel partition?
+ cmpb $0x20, %es:4(%si)
+ jne next_partition
+
+ # Is it a bootable partition?
+ cmpb $0x80, %es:(%si)
+ je load_kernel
+
+next_partition:
+ # No match for this partition, go on to the next one.
+ add $16, %si # Offset to next partition table entry.
+ inc %al
+ cmp $510, %si
+ jb check_partition
+
+next_drive:
+ # No match on this drive, go on to the next one.
+ inc %dl
+ jnc read_mbr
+
+no_such_drive:
+no_boot_partition:
+ # Didn't find a Pintos kernel partition anywhere, give up.
+ call puts
+ .string "\rNot found\r"
+
+ # Notify BIOS that boot failed. See [IntrList].
+ int $0x18
+
+#### We found a kernel. The kernel's drive is in DL. The partition
+#### table entry for the kernel's partition is at ES:SI. Our job now
+#### is to read the kernel from disk and jump to its start address.
+
+load_kernel:
+ call puts
+ .string "\rLoading"
+
+ # Figure out number of sectors to read. A Pintos kernel is
+ # just an ELF format object, which doesn't have an
+ # easy-to-read field to identify its own size (see [ELF1]).
+ # But we limit Pintos kernels to 512 kB for other reasons, so
+ # it's easy enough to just read the entire contents of the
+ # partition or 512 kB from disk, whichever is smaller.
+ mov %es:12(%si), %ecx # EBP = number of sectors
+ cmp $1024, %ecx # Cap size at 512 kB
+ jbe 1f
+ mov $1024, %cx
+1:
+
+ mov %es:8(%si), %ebx # EBX = first sector
+ mov $0x2000, %ax # Start load address: 0x20000
+
+next_sector:
+ # Read one sector into memory.
+ mov %ax, %es # ES:0000 -> load address
+ call read_sector
+ jc read_failed
+
+ # Print '.' as progress indicator once every 16 sectors == 8 kB.
+ test $15, %bl
+ jnz 1f
+ call puts
+ .string "."