9-38 Vol. 3A
PROCESSOR MANAGEMENT AND INITIALIZATION
9.11.8.1 Responsibilities of the BIOS
If a BIOS passes the presence test (INT 15H, AX = 0D042H, BL = 0H), it must implement all of the sub-functions
defined in the INT 15H, AX = 0D042H specification. There are no optional functions. BIOS must load the appropriate
update for each processor during system initialization.
A Header Version of an update block containing the value 0FFFFFFFFH indicates that the update block is unused and
available for storing a new update.
The BIOS is responsible for providing a region of non-volatile storage (NVRAM) for each potential processor step-
ping within a system. This storage unit consists of one or more update blocks. An update block is a contiguous
2048-byte block of memory. The BIOS for a single processor system need only provide update blocks to store one
microcode update. If the BIOS for a multiple processor system is intended to support mixed processor steppings,
then the BIOS needs to provide enough update blocks to store each unique microcode update or for each processor
socket on the OEM’s system board.
The BIOS is responsible for managing the NVRAM update blocks. This includes garbage collection, such as
removing microcode updates that exist in NVRAM for which a corresponding processor does not exist in the system.
This specification only provides the mechanism for ensuring security, the uniqueness of an entry, and that stale
entries are not loaded. The actual update block management is implementation specific on a per-BIOS basis.
As an example, the BIOS may use update blocks sequentially in ascending order with CPU signatures sorted versus
the first available block. In addition, garbage collection may be implemented as a setup option to clear all NVRAM
slots or as BIOS code that searches and eliminates unused entries during boot.
NOTES
For IA-32 processors starting with family 0FH and model 03H and Intel 64 processors, the
microcode update may be as large as 16 KBytes. Thus, BIOS must allocate 8 update blocks for each
microcode update. In a MP system, a common microcode update may be sufficient for each socket
in the system.
For IA-32 processors earlier than family 0FH and model 03H, the microcode update is 2 KBytes. An
MP-capable BIOS that supports multiple steppings must allocate a block for each socket in the
system.
A single-processor BIOS that supports variable-sized microcode update and fixed-sized microcode
update must allocate one 16-KByte region and a second region of at least 2 KBytes.
The following algorithm (Example 9-11) describes the steps performed during BIOS initialization used to load the
updates into the processor(s). The algorithm assumes:
•
The BIOS ensures that no update contained within NVRAM has a header version or loader version that does not
match one currently supported by the BIOS.
•
The update contains a correct checksum.
•
The BIOS ensures that (at most) one update exists for each processor stepping.
•
Older update revisions are not allowed to overwrite more recent ones.
These requirements are checked by the BIOS during the execution of the write update function of this interface.
The BIOS sequentially scans through all of the update blocks in NVRAM starting with index 0. The BIOS scans until
it finds an update where the processor fields in the header match the processor signature (extended family,
extended model, type, family, model, and stepping) as well as the platform bits of the current processor.
Example 9-11. Pseudo Code, Checks Required Prior to Loading an Update
For each processor in the system
{
Determine the Processor Signature via CPUID function 1;
Determine the Platform Bits
← 1 << IA32_PLATFORM_ID[52:50];
For (I
← UpdateBlock 0, I < NumOfBlocks; I++)
{
If (Update.Header_Version = 00000001H)
{