UEFI与固件程序设计
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同济大学软件学院
Legacy BIOS Interface
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Legacy BIOS provides OS with I/O routines via x86 software
interrupts
– Normally for OS booting
– Can also be used at runtime by legacy OSes (DOS, Windows 3.x,
Windows 9X)
Typical software interrupts used by legacy OS loaders
– INT 10h – Video display
– INT 13h – Floppy/Hard disk I/O
– INT 15h – Extended memory support, Report memory map, …
– INT 16h – Keyboard input
– INT 19h – Bootstrap loader
– INT 1Ah – System date and time, PCI BIOS real mode interface
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Boot OS under Legacy BIOS
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BIOS identifies all IPL(Initial Program Load) devices in the system and
determines the boot order during POST
– BAIDs (BIOS Aware IPL Device)
– PnP ISA/PCI Cards
Legacy ISA/PCI cards may hook INT 19h to control the boot process
BIOS invokes INT 19h after POST to start booting OS
The INT 19h handler attempts to boot from IPL devices sequentially in
the boot order, if failed then next
Boot flow when normally booting from hard disk
– Hard disk is partitioned to up to 4 primary partitions so that multiple
OSes can co-exist
– The first sector (cylinder 0, head 0, sector 1) referred to as MBR (Main
Boot Record) contains OS independent bootstrap code
– INT 19h handler relies on INT 13h to read MBR into 0:7C00h
– INT 19h handler verifies the validity of MBR
– INT 19h handler transfers the control to the MBR bootstrap code by
jumping to 0:7C00h
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Hard Disk Partition and MBR Layout
Legacy Master Boot Record
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Hard Disk Partition and MBR Layout
Offset
Length
(byte)
Field
Description
0
1
Boot Indicator
1
1
Starting Head
2
1
Starting Sector
3
1
Starting Cylinder
4
1
System ID
5
1
Ending Head
6
1
Ending Sector
7
1
Ending Cylinder
10-bit number, with a maximum value of 1023
8
4
Relative Sectors
The offset from the beginning of the disk to
the beginning of the volume, counting by
sectors.
12
4
Total Sectors
The total number of sectors in the volume
00 – inactive
80 - active
Bit 0~5 sector number
Bit 6~7 upper two bits of Starting Cylinder
10-bit number, with a maximum value of 1023
Defines the volume type
Bit 0~5 sector number
Bit 6~7 upper two bits of Ending Cylinder
Partition Table Entry
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What Does MBR Do?
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Replicate itself to a memory range at lower address to
make room for the coming OS bootstrap code
Parse the partition table to find the active partition
Load the first sector of the active partition to memory at
0:7C00h
Verify the validity of the sector as a bootstrap sector
Transfer the control to the OS bootstrap sector by
jumping to 0:7C00h
OS specific bootstrap process starts
Refer to UEFI Specification Revision 2.2 section 5.2
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UEFI Boot Mechanisms
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File level boot
– EFI_SIMPLE_FILE_SYSTEM_PROTOCOL
– EFI_LOAD_FILE_PROTOCOL
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Boot options are defined via Boot#### variables
– Two types of boot options
 UEFI OS boot loaders
 Other UEFI applications
– System firmware and OS installer can set to a known set of boot options
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UEFI boot manager attempts to load and execute boot options in the order
defined by the BootOrder variable
– If a boot option failed and the device path points directly to an
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL device and specify no exact file, then
search for a default application to boot per removable media boot behavior
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If no valid boot options are present, a default behavior is defined
– Enumerate all removable media devices followed by all fixed media devices
– Removable media boot behavior applies to the devices support
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL
– Otherwise, attempt to boot the device via the EFI_LOAD_FILE_PROTOCOL
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Removable media boot behavior
– \EFI\BOOT\BOOT{machine type short-name}.EFI
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EFI GUIDed Partition Table (GPT)
Partitioning Scheme
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GPT Advantages over MBR
Partition Table
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64-bit Logical Block Addressing.
Supports many partitions.
Uses a primary and backup table for redundancy.
Uses version number and size fields for future expansion.
Uses CRC32 fields for improved data integrity.
Defines a GUID for uniquely identifying each partition.
Uses a GUID and attributes to define partition content
type.
Each partition contains a 36 Unicode character human
readable name.
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UEFI System Partition
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Persistent store on large mass storage media types for
storing UEFI OS boot loader images, drivers/applications
to extend platform firmware capabilities and other data
files
Infrastructure to increase platform value-add without
significantly growing the need for nonvolatile platform
memory
Place can be shared by firmware developers, OEMs,
OSVs and third parties
Based on FA32 file system
Identification
– Legacy MBR partitioning scheme: new partition type 0xef
– GPT: C12A7328-F81F-11d2-BA4B-00A0C93EC93B
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