-/* This file is derived from source code used in MIT's 6.828
- course. The original copyright notice is reproduced in full
- below. */
-
-/*
- * Copyright (C) 1997 Massachusetts Institute of Technology
- *
- * This software is being provided by the copyright holders under the
- * following license. By obtaining, using and/or copying this software,
- * you agree that you have read, understood, and will comply with the
- * following terms and conditions:
- *
- * Permission to use, copy, modify, distribute, and sell this software
- * and its documentation for any purpose and without fee or royalty is
- * hereby granted, provided that the full text of this NOTICE appears on
- * ALL copies of the software and documentation or portions thereof,
- * including modifications, that you make.
- *
- * THIS SOFTWARE IS PROVIDED "AS IS," AND COPYRIGHT HOLDERS MAKE NO
- * REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED. BY WAY OF EXAMPLE,
- * BUT NOT LIMITATION, COPYRIGHT HOLDERS MAKE NO REPRESENTATIONS OR
- * WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR
- * THAT THE USE OF THE SOFTWARE OR DOCUMENTATION WILL NOT INFRINGE ANY
- * THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS. COPYRIGHT
- * HOLDERS WILL BEAR NO LIABILITY FOR ANY USE OF THIS SOFTWARE OR
- * DOCUMENTATION.
- *
- * The name and trademarks of copyright holders may NOT be used in
- * advertising or publicity pertaining to the software without specific,
- * written prior permission. Title to copyright in this software and any
- * associated documentation will at all times remain with copyright
- * holders. See the file AUTHORS which should have accompanied this software
- * for a list of all copyright holders.
- *
- * This file may be derived from previously copyrighted software. This
- * copyright applies only to those changes made by the copyright
- * holders listed in the AUTHORS file. The rest of this file is covered by
- * the copyright notices, if any, listed below.
- */
-
#include "threads/loader.h"
-
- .intel_syntax noprefix
-
+
#### Kernel loader.
-#### This code should be stored in the first sector of the hard disk.
+#### 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). Then it jumps to the beginning of it,
-#### in real mode. This code switches into protected mode (32-bit
-#### mode) so that all of memory can accessed, loads the kernel into
-#### memory, and jumps to the first byte of the kernel, where start.S
-#### is linked.
-
-/* Flags in control register 0. */
-#define CR0_PE 0x00000001 /* Protection Enable. */
-#define CR0_EM 0x00000004 /* (Floating-point) Emulation. */
-#define CR0_PG 0x80000000 /* Paging. */
-#define CR0_WP 0x00010000 /* Write-Protect enable in kernel mode. */
-
-# Code runs in real mode, which is a 16-bit segment.
-
-.globl start
-start:
+#### 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
-# Disable interrupts.
-# String instructions go upward.
-
- cli
- cld
-
-# Set up data segments.
-
- sub ax, ax
- mov es, ax
- mov ds, ax
-
-# Set up stack segment.
-# Stack grows downward starting from us.
-# We don't ever use the stack so this is strictly speaking
-# unnecessary.
-
- mov ss, ax
- mov sp, 0x7c00
-
-#### Enable A20. Address line 20 is tied to low when the machine
-#### boots, which prevents addressing memory about 1 MB. This code
-#### fixes it.
-
-# Poll status register while busy.
-
-1: in al, 0x64
- test al, 0x2
- jnz 1b
-
-# Send command for writing output port.
-
- mov al, 0xd1
- outb 0x64, al
-
-# Poll status register while busy.
-
-1: in al, 0x64
- test al, 0x2
- jnz 1b
-
-# Enable A20 line.
-
- mov al, 0xdf
- out 0x60, al
-
-#### Get memory size, via interrupt 15h function 88h, which returns CF
-#### clear if successful, with AX = (kB of physical memory) - 1024.
-#### This only works for memory sizes <= 65 MB, which should be fine
-#### for our purposes. We cap memory at 64 MB because that's all we
-#### prepare page tables for, below.
-
- mov ah, 0x88
- int 0x15
- jc panic
- cli # BIOS might have enabled interrupts
- add eax, 1024 # Total kB memory
- cmp eax, 0x10000 # Cap at 64 MB
+# 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 $0x00e3, %ax # 9600 bps, N-8-1.
+ int $0x14 # Destroys AX.
+
+ call puts
+ .string "Pintos loader"
+
+#### 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 eax, 0x10000
-1: shr eax, 2 # Total 4 kB pages
- mov ram_pgs, eax
-
-#### Create temporary page directory and page table and set page
-#### directory base register.
-
-# Create page directory at 64 kB and fill with zeroes.
- mov ax, 0x1000
- mov es, ax
- sub eax, eax
- sub edi, edi
- mov ecx, 0x400
- rep stosd
-
-# Add PDEs to point to PTEs for the first 64 MB of RAM.
-# Also add identical PDEs starting at LOADER_PHYS_BASE.
-# See [IA32-v3] section 3.7.6 for a description of the bits in eax.
-
-# A bug in some versions of GAS prevents us from using the straightforward
-# mov es:[di + LOADER_PHYS_BASE / 1024 / 1024], eax
-# so we calculate the displacement in bx instead.
-
- mov eax, 0x11007
- mov ecx, 0x11
- sub di, di
- mov ebx, LOADER_PHYS_BASE
- shr ebx, 20
-1: mov es:[di], eax
- mov es:[bx + di], eax
- add di, 4
- add eax, 0x1000
- loop 1b
-
-# Set up one-to-map linear to physical map for the first 64 MB of RAM.
-# See [IA32-v3] section 3.7.6 for a description of the bits in eax.
-
- mov ax, 0x1100
- mov es, ax
- mov eax, 0x7
- mov cx, 0x4000
- sub di, di
-1: mov es:[di], eax
- add di, 4
- add eax, 0x1000
- loop 1b
-
-# Set page directory base register.
-
- mov eax, 0x10000
- mov cr3, eax
-
-#### Switch to protected mode.
-
-# Then we point the GDTR to our GDT. Protected mode requires a GDT.
-# We need a data32 prefix to ensure that all 32 bits of the GDT
-# descriptor are loaded (default is to load only 24 bits).
-
- data32 lgdt gdtdesc
-
-# Then we turn on the following bits in CR0:
-# PE (Protect Enable): this turns on protected mode.
-# PG (Paging): turns on paging.
-# WP (Write Protect): if unset, ring 0 code ignores
-# write-protect bits in page tables (!).
-# EM (Emulation): forces floating-point instructions to trap.
-# We don't support floating point.
-
- mov eax, cr0
- or eax, CR0_PE + CR0_PG + CR0_WP + CR0_EM
- mov cr0, eax
-
-# We're now in protected mode in a 16-bit segment. The CPU still has
-# the real-mode code segment cached in cs's segment descriptor. We
-# need to reload cs, and the easiest way is to use a far jump.
-# Because we're not in a 32-bit segment the data32 prefix is needed to
-# jump to a 32-bit offset.
-
- data32 ljmp SEL_KCSEG, 1f + LOADER_PHYS_BASE
-
-# We're now in protected mode in a 32-bit segment.
-
- .code32
-
-# Reload all the other segment registers and the stack pointer to
-# point into our new GDT.
-
-1: mov ax, SEL_KDSEG
- mov ds, ax
- mov es, ax
- mov fs, ax
- mov gs, ax
- mov ss, ax
- mov esp, LOADER_PHYS_BASE + 0x30000
-
-#### Load kernel starting at physical address LOADER_KERN_BASE by
-#### frobbing the IDE controller directly.
-
- mov ebx, 1
- mov edi, LOADER_KERN_BASE + LOADER_PHYS_BASE
-read_sector:
-
-# Poll status register while controller busy.
-
- mov edx, 0x1f7
-1: in al, dx
- test al, 0x80
- jnz 1b
-
-# Read a single sector.
-
- mov edx, 0x1f2
- mov al, 1
- out dx, al
-
-# Sector number to write in low 28 bits.
-# LBA mode, device 0 in top 4 bits.
-
- mov eax, ebx
- and eax, 0x0fffffff
- or eax, 0xe0000000
-
-# Dump eax to ports 0x1f3...0x1f6.
-
- mov ecx, 4
-1: inc dx
- out dx, al
- shr eax, 8
- loop 1b
-
-# READ command to command register.
-
- inc dx
- mov al, 0x20
- out dx, al
-
-# Poll status register while controller busy.
-
-1: in al, dx
- test al, 0x80
- jnz 1b
-
-# Poll status register until data ready.
-
-1: in al, dx
- test al, 0x08
- jz 1b
-
-# Transfer sector.
-
- mov ecx, 256
- mov edx, 0x1f0
- rep insw
-
-# Next sector.
-
- inc ebx
- cmp ebx, KERNEL_LOAD_PAGES*8 + 1
- jnz read_sector
-
-#### Jump to kernel entry point.
-
- mov eax, LOADER_PHYS_BASE + LOADER_KERN_BASE
- call eax
- jmp panic
-
-#### GDT
-
-gdt:
- .quad 0x0000000000000000 # null seg
- .quad 0x00cf9a000000ffff # code seg
- .quad 0x00cf92000000ffff # data seg
-
-gdtdesc:
- .word 0x17 # sizeof (gdt) - 1
- .long gdt + LOADER_PHYS_BASE # address gdt
-
-#### Fatal error.
-#### Print panicmsg (with help from the BIOS) and spin.
-
-panic: .code16 # We only panic in real mode.
- mov si, offset panic_message
- mov ah, 0xe
- sub bh, bh
-1: lodsb
- test al, al
-2: jz 2b # Spin.
- int 0x10
+ 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 "."
+1:
+
+ # Advance memory pointer and disk sector.
+ add $0x20, %ax
+ inc %bx
+ loop next_sector
+
+ call puts
+ .string "\r"
+
+#### Transfer control to the kernel that we loaded. We read the start
+#### address out of the ELF header (see [ELF1]) and convert it from a
+#### 32-bit linear address into a 16:16 segment:offset address for
+#### real mode, then jump to the converted address. The 80x86 doesn't
+#### have an instruction to jump to an absolute segment:offset kept in
+#### registers, so in fact we store the address in a temporary memory
+#### location, then jump indirectly through that location. To save 4
+#### bytes in the loader, we reuse 4 bytes of the loader's code for
+#### this temporary pointer.
+
+ mov $0x2000, %ax
+ mov %ax, %es
+ mov %es:0x18, %dx
+ mov %dx, start
+ movw $0x2000, start + 2
+ ljmp *start
+
+read_failed:
+start:
+ # Disk sector read failed.
+ call puts
+1: .string "\rBad read\r"
+
+ # Notify BIOS that boot failed. See [IntrList].
+ int $0x18
+
+#### Print string subroutine. To save space in the loader, this
+#### subroutine takes its null-terminated string argument from the
+#### code stream just after the call, and then returns to the byte
+#### just after the terminating null. This subroutine preserves all
+#### general-purpose registers.
+
+puts: xchg %si, %ss:(%esp)
+ push %ax
+next_char:
+ mov %cs:(%si), %al
+ inc %si
+ test %al, %al
+ jz 1f
+ call putc
+ jmp next_char
+1: pop %ax
+ xchg %si, %ss:(%esp)
+ ret
+
+#### Character output subroutine. Prints the character in AL to the
+#### VGA display and serial port 0, using BIOS services (see
+#### [IntrList]). Preserves all general-purpose registers.
+####
+#### If called upon to output a carriage return, this subroutine
+#### automatically supplies the following line feed.
+
+putc: pusha
+
+1: sub %bh, %bh # Page 0.
+ mov $0x0e, %ah # Teletype output service.
+ int $0x10
+
+ mov $0x01, %ah # Serial port output service.
+ sub %dx, %dx # Serial port 0.
+ int $0x14 # Destroys AH.
+
+ cmp $'\r', %al
+ jne popa_ret
+ mov $'\n', %al
jmp 1b
-panic_message:
- .ascii "Panic!"
- .byte 0
+#### Sector read subroutine. Takes a drive number in DL (0x80 = hard
+#### disk 0, 0x81 = hard disk 1, ...) and a sector number in EBX, and
+#### reads the specified sector into memory at ES:0000. Returns with
+#### carry set on error, clear otherwise. Preserves all
+#### general-purpose registers.
-#### Physical memory size in 4 kB pages.
-#### This is initialized by the loader and read by the kernel.
- .org LOADER_RAM_PGS - LOADER_BASE
-ram_pgs:
- .long 0
+read_sector:
+ pusha
+ sub %eax, %eax
+ push %eax # LBA sector number [32:63]
+ push %ebx # LBA sector number [0:31]
+ push %es # Buffer segment
+ push %ax # Buffer offset (always 0)
+ push $1 # Number of sectors to read
+ push $16 # Packet size
+ mov $0x42, %ah # Extended read
+ mov %sp, %si # DS:SI -> packet
+ int $0x13 # Error code in CF
+ popa # Pop 16 bytes, preserve flags
+popa_ret:
+ popa
+ ret # Error code still in CF
#### Command-line arguments and their count.
#### This is written by the `pintos' utility and read by the kernel.
#### The loader itself does not do anything with the command line.
.org LOADER_ARG_CNT - LOADER_BASE
-arg_cnt:
- .long 0
+ .fill LOADER_ARG_CNT_LEN, 1, 0
+
.org LOADER_ARGS - LOADER_BASE
-args:
- .fill 0x80, 1, 0
+ .fill LOADER_ARGS_LEN, 1, 0
+
+#### Partition table.
+ .org LOADER_PARTS - LOADER_BASE
+ .fill LOADER_PARTS_LEN, 1, 0
-#### Boot-sector signature.
-#### The BIOS checks that this is set properly.
+#### Boot-sector signature for BIOS inspection.
.org LOADER_SIG - LOADER_BASE
.word 0xaa55
projects / pintos-anon / blobdiff