Intel MultiProcessor manual Posted Memory Write, Multiprocessor Interrupt Control

Hardware Specification

operations over its internal shared memory bus, if it is AT compatible. Operating system and software developers must ensure that data is aligned if locked access is required, because lock operations on misaligned data are not guaranteed to work on all platforms.

3.5 Posted Memory Write

When controlling I/O devices, it is important that memory and I/O operations be carried out in the order programmed. Intel-compatible processors do not buffer I/O writes; thus, strict ordering among I/O operations is enforced by the processors.

To optimize memory performance, processors and chipsets often implement write buffers and writeback caches. Intel-compatible processors guarantee processor ordering on all internal cache and write buffer accesses. However, chipsets must also guarantee processor ordering on all external memory accesses.

For systems based on the integrated APIC, posting of memory writes may result in spurious interrupts for memory-mapped I/O devices using level-triggered interrupts. I/O device drivers must serialize instructions to ensure that the device interrupt clear command reaches the device before the EOI command reaches the APIC and handles the spurious interrupt in case one occurs.

3.6 Multiprocessor Interrupt Control

In an MP-compliant system, interrupts are controlled through the APIC. The following sections describe the APIC architecture and the three interrupt modes allowed in an MP-compliant system.

3.6.1APIC Architecture

The Intel Advanced Programmable Interrupt Controller (APIC) is based on a distributed architecture. Interrupt control functions are distributed between two basic functional units: the local unit and the I/O unit. The local and I/O units communicate through a bus called the ICC bus. The I/O unit senses an interrupt input, addresses it to a local unit, and sends it over the ICC bus. The local unit that is addressed accepts the message sent by the I/O unit.

In an MP-compliant system, one local APIC per CPU is required. Depending on the total number of interrupt lines in an MP system, one or more I/O APICs may be used. The bus interrupt line assignments can be implementation-specific and can be defined by the MP configuration table described in Chapter 4.

The Intel 82489DX APIC is a “discrete APIC” implementation. The programming interface of the 82489DX APIC units serves as the base of the MP specification. Each APIC has a version register that contains the version number of a specific APIC implementation. The version register of the 82489DX family has a version number of “0 x,” where x is a four-bit hexadecimal number. Version number “1 x” refers to Pentium processors with integrated APICs, such as the Pentium 735\90 and 815\100 processors, and x is a four-bit hexadecimal number.

The integrated APIC maintains the same programming interface as the 82489DX APIC. Table 3-2 describes the features specific to the integrated APIC.

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