Introduction

•Setting a translated base address for a downstream range to fall within an address range defined for upstream forwarding. This would cause the 21555 to respond as a target on the secondary bus to a downstream transaction that it has initiated as a master. The transaction would then be forwarded back to the primary bus. The address on the primary bus depends on the translated base address value for that upstream range.

•Setting a translated base address for an upstream range to fall within an address range for downstream forwarding. This results in similar behavior described in the previous condition, but in the opposite direction.

•Enabling I/O subtractive decoding in both directions. When an I/O transaction is subtractively decoded on the primary bus and forwarded downstream by the 21555, and no target responds on the secondary bus, the 21555 subtractively decodes the transaction on the secondary bus and forwards it back upstream. Since there is no address translation for subtractively decoded I/O transactions, this results in the 21555 forwarding the transaction downstream and upstream forever.

•Enabling VGA decoding in both directions. Refer to subtractive I/O decoding in the previous bullet. Again, there is the case of a non translated I/O address decoded by the 21555 on both interfaces as a target and forwarded to the opposite interface.

2.4.2Transaction Forwarding

When using the indirect I/O transaction generation mechanism, the low two bits of the I/O address in the I/O Address register must match the byte enables as described in the PCI Local Bus Specification, Revision 2.2. The 21555 does not correct any discrepancies between the byte enables and address bits [1:0].

2.4.3ROM Access

Parallel and SROM access mechanisms do not accommodate multiple masters. That is, when more than one master attempts to access the ROM during the same time period, wrong data may be returned or written to the ROM. There is no semaphore method to guarantee atomicity of the ROM address, data, and control register accesses.

This also applies to a parallel ROM access through the Primary Expansion ROM BAR at the same time a secondary bus master might be accessing ROM registers.

21555 Non-Transparent PCI-to-PCI Bridge User Manual

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Intel 21555 user manual Transaction Forwarding, ROM Access
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21555 specifications

The Intel 21555 is a prominent microprocessor developed by Intel, designed to cater to a variety of computing needs. This processor marks a significant step forward in Intel's lineup and underscores the company's commitment to advancing technology in personal computing, enterprise solutions, and beyond.

One of the standout features of the Intel 21555 is its advanced architecture. It utilizes a multi-core design, enabling improved performance through parallel processing. This architecture allows multiple applications to run seamlessly without a decline in speed. The cores are built on a cutting-edge manufacturing process that enhances energy efficiency while maintaining high clock speeds.

The Intel 21555 supports a wide range of technologies, including Intel Turbo Boost, which enables dynamic adjustments to the processor’s performance based on workload demands. This feature allows the processor to accelerate its speed during intensive tasks, providing users with a responsive experience when it matters the most.

Another key characteristic of the Intel 21555 is its support for integrated graphics. With Intel UHD Graphics technology, users can enjoy enhanced visuals for everyday tasks such as video playback, web browsing, and light gaming. This eliminates the need for a separate graphics card for many users, particularly in home office or light gaming scenarios.

Security is a significant focus in the design of the Intel 21555. It includes built-in hardware-based security features like Intel Trusted Execution Technology and Secure Boot. These features help protect against various types of threats, ensuring that user data remains secure from malicious attacks.

The Intel 21555 is also optimized for use with Intel's platform technologies, including Intel Optane memory and Intel Rapid Storage Technology. These technologies work together to deliver faster boot times and improved system responsiveness, making computing more efficient for users.

Furthermore, the Intel 21555 is designed to support virtualization technologies, allowing multiple operating systems to run concurrently without compromising performance. This is particularly useful for developers and businesses that rely on virtualization for testing and development environments.

In summary, the Intel 21555 is a powerful and versatile processor that reflects Intel's ongoing innovation in the computing space. With its multi-core architecture, enhanced graphics capabilities, strong security features, and advanced technologies, it stands out as an excellent choice for a wide range of applications, serving both casual users and professionals alike.
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