ACPI BIOS implementations that directly access certain system hardware resources from AML code cannot be synchronized with operating system access to the same resources, which can cause the operating system to become unstable or to stop responding. This article describes issues related to BIOS AML firmware accessing system board resources, and the changes implemented to address these issues in the Windows XP, Windows Server 2003, and future versions of the operating system.

Introduction

During the development of the Windows XP operating system, Microsoft discovered many ACPI BIOS implementations that directly access and attempt to manipulate system hardware resources from BIOS ASL code. At run time, system board resources must not be simultaneously accessed or modified by both the BIOS and the operating system, because these accesses cannot be synchronized with operating system access to the same system resources. As a result, BIOS read or write access to these resources can cause adverse effects, ranging from general instability to causing the system to stop responding.

Historically, error conditions of this nature might have been construed by the operating system as an unrecoverable error, resulting in a stop error (blue screen) and system shutdown. In an effort to improve the end user experience and increase system reliability, Windows XP has been re-designed as described in this article.

Changes in Windows XP

While developing and testing Windows XP a list of system board resources was identified that, when accessed from BIOS AML code, proved to be the most problematic. This article refers to the list of addresses associated with these resources as the "blocked ports list." Table 1 lists affected system resources and their associated I/O addresses.

The ACPI AML interpreter in the Windows XP kernel monitors all attempts by BIOS AML code to read from or write to the specific addresses on the blocked ports list. When a read or write access is detected at any of these addresses, the following actions will occur:

Windows XP BIOS Compatibility Determined Using _OSI

The specific actions taken by the operating system when read or write accesses to these resources are detected depends on the version of ACPI interface that the BIOS indicates it supports. This is determined through the BIOS use of the _OSI method as described in this article.

For complete details about the _OSI method, see How to Identify Windows Versions in ACPI Using _OSI.

The _OSI method is being introduced with the Windows XP operating system. BIOS ASL code can test for the level of features supported in the current Windows operating system by passing a string to the _OSI method. The operating system returns TRUE for any string that represents a feature set that it can support. Windows XP, for example, returns TRUE for the string "Windows 2001".

By passing the string "Windows 2001" into the _OSI method, the BIOS indicates to the operating system that the BIOS is aware of and compatible with the ACPI implementation and feature set of Windows XP. Windows XP will then reject all I/O reads or writes from BIOS ASL code to addresses on the blocked ports list and generate an error in the system event log.

Windows XP and Legacy BIOS ASL Implementations

For those BIOS implementations that are compatible with versions of Windows released before Windows XP, the operating system will allow the read or write access to succeed in most cases. Accesses to the PIC and cascaded PIC, and to the PIC Edge/Level Control Registers, are always blocked. In all cases an error will be written to the system event log.

To help ensure system stability on systems with legacy BIOS ASL implementations, Windows XP attempts to synchronize accesses to resources whenever possible. For example:

Accessing BIOS Non-volatile Memory (CMOS NVRAM)

With these changes in Windows XP, BIOS developers may be looking for alternative methods to access CMOS NVRAM. The following examples present some possible solutions.

Reading from CMOS NVRAM

This example ASL code defines an operation region in the system memory address space. During POST, BIOS code copies the data from CMOS to this memory area. ASL code can then use fields to read the CMOS data.

Each CMnn field in the example corresponds to 8 bits of CMOS. The BIOS engineer is responsible for defining these fields appropriately. Each field of this memory can be any combination of bits.

During system boot, BIOS POST code should update this memory area with the value written in the CMOS before handing off to the operating system. Once the operating system has switched to ACPI mode, any attempt to read CMOS from ASL code should be done by reading from these memory locations and not from CMOS NVRAM I/O ports.

// Declare a memory operation region for 255 bytes of CMOS. 
// First 0 to 127 bytes reflects CMOS access from IO ports 0x70 
// and 0x71 and the subsequent bytes reflect CMOS access from IO 
// ports 0x72 to 0x73.  BIOS code should properly set up the base 
// address of the beginning of this memory range at offset during 
// BIOS POST.

OperationRegion (CMRM, SystemMemory, offset, 255)  //Operation Region 

Field (CMRM, AnyAcc, NoLock, Preserve) {   //Field
    //  Memory corresponding to addresses 0x70, 0x71
    CM00,    8,
    CM01,    8,
    CM02,    8,
    .
    .
    CM7f,     8,     

    // Memory corresponding to addresses 0x72, 0x73
    CM80,    8,
    .
    .
    CMFF
    }   // End of CMOS field

The following is an example of ASL code which reads from CMOS offset 52 bits 4 to 7 and returns the value read.

// This example returns bits 4 to 7 of CMOS offset 52

Method (RDCM) {
    ShiftRight (CM52, 4, Local0)
    Return (Local0) 
    }

Writing to CMOS NVRAM

BIOS code should write to CMOS NVRAM by generating a system management interrupt (SMI). AML code can generate a SMI by writing a specific value to the SMI command port. AML code can pass the CMOS offset and value to be written through the NVRAM memory operation region. The BIOS SMI handles the writes to CMOS, and also updates the memory area pointed by the CMRM operation region to reflect the correct CMOS contents.

Affected System Resources and Addresses

Table 1 lists the system resources and associated I/O addresses that should not be directly accessed by BIOS AML code (the "blocked ports" list).

Table 1 Blocked I/O Port Addresses and System Board Resources

*Read or write accesses to these ports are always blocked, regardless of the BIOS use of the _OSI method.

Call to Action

BIOS developers should design their BIOS code according to the guidelines in this article: