BIOS32

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This page is under construction! This page is a work in progress and may thus be incomplete. Its content may be changed in the near future.

Contents

Introduction

BIOS32 is a method for calling BIOS services from either 16-Bit Real Mode, 16-Bit Protected Mode or 32-Bit Protected Mode. This method can be used to find BIOS32 services and execute BIOS32 methods, but it does not allow you to call "legacy" BIOS calls, directly, like INT 0x10 and INT 0x13 methods. It is mainly used for PNP and PCI services, but there may be other services available, as well. These services are not well documented, and are not necessarily supported on all systems. The BIOS32 API provides a method to query the system for support for a specific service, but it does not provide any method for getting a complete list of supported services.

Interface

Locating

The BIOS32 Service Directory Header is located on a paragraph (16 Byte) boundary in the physical memory region 0xE0000-0xFFFFF. It starts with a 4 byte signature of "_32_" (0x5F32335F)

Structure

Service Directory Header
Offset Size Name Description
0 4 bytes Signature ASCII signature "_32_" (0x5F32335F)
4 4 bytes Entry Point 32 bit physical address (not CS:IP) of BIOS32 calling interface entry point.
8 1 byte Revision Revision of BIOS32. Current 0h
9 1 byte Length Length of structure, in paragraphs
10 1 byte Checksum A summation checksum (1 byte granularity)
11 5 bytes Reserved Reserved for future use, should be 0.

Service Availability

In order to verify that a service is available, you must FAR JUMP to the 32-bit address located at offset 4 in the structure above, with the EBX register set to zero, and the EAX register set to the Component ID for the service that you are looking for. If the service is available, the AL register will contain a zero, EBX will contain the base address for the service, ECX will contain the length of the service in bytes, and EDX will contain the offset (from EBX) for the entry point for the service.

If the service is not supported, the AL register will contain either 0x80 or 0x81. (The documentation is ambiguous on this specific value, so it may be better to check for a non-zero value)

Well Known Services

The services listed below have been found on real hardware, but may not be supported by some (or even most) systems.

Service List
Component ID Name
"$PnP" Plug-n-Play BIOS
"$PCI" PCI BIOS
"$PIR" PCI IRQ Routing Table
"$PMM" POST Memory Manager
"$BC$" PXE Base Code (BC) Rom ID
"$ACF" Plug-n-Play Auto Config
"$BLK" Unknown
"$WDS" Unknown
"$SNY" Unknown
"$SDS" Unknown
"$SDM" Unknown
"$IRT" Unknown
"$CFD" Unknown
"$BBS" Unknown
"$ASF" Unknown
"$$CT" Unknown
"MPTN" Unknown
"PCI " Unknown

See Also

On the Wiki

PCI

External references

Phoenix BIOS 32-bit service proposal

Google Groups post about known services

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