computer science

Runway bus

Runway bus is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Runway bus rather than just read about it. In short: The Runway bus is a front-side bus developed by Hewlett-Packard for use by its PA-RISC microprocessor family. The Runway bus is a 64-bit wide, split transaction, time multiplexed address and data bus running at 120 MHz.

Key takeaways

Runway bus belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Runway bus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Runway bus from memory before moving on to harder problems.

Reference excerpt

The Runway bus is a front-side bus developed by Hewlett-Packard for use by its PA-RISC microprocessor family. The Runway bus is a 64-bit wide, split transaction, time multiplexed address and data bus running at 120 MHz. This scheme was chosen by HP as they determined that a bus using separate address and data wires would have only delivered 20% more bandwidth for a 50% increase in pin count, which would have made microprocessors using the bus more expensive. The Runway bus was introduced with the release of the PA-7200 and was subsequently used by the PA-8000, PA-8200, PA-8500, PA-8600 and PA-8700 microprocessors. Early implementations of the bus used in the PA-7200, PA-8000 and PA-8200 had a theoretical bandwidth of 960 MB/s. Beginning with the PA-8500, the Runway bus was revised to transmit on both rising and falling edges of a 125 MHz clock signal, which increased its theoretical bandwidth to 2 GB/s. The Runway bus was succeeded with the introduction of the PA-8800, which used the Itanium 2 bus. Bus features

64-bit multiplexed address/data 20 bus protocol signals Supports cache coherency Three frequency options (1.0, 0.75 and 0.67 of CPU clock — 0.50 apparently was later added) Parity protection on address/data and control signal Each attached device contains its own arbitrator logic Split transactions, up to six transactions can be pending at once Snooping cache coherency protocol 1-4 processors "glueless" multi-processing (no support chips needed) 768 MB/s sustainable throughput, peak 960 MB/s at 120 MHz Runway+/Runway DDR: On PA-8500, PA-8600 and PA-8700, the bus operates in DDR (double data rate) mode, resulting in a peak bandwidth of about 2.0 GB/s (Runway+ or Runway DDR) with 125 MHz Most machines use the Runway bus to connect the CPUs directly to the IOMMU (Astro, U2/Uturn or Java) and memory. However, the N class and L3000 servers use an interface chip called Dew to bridge the Runway bus to the Merced bus that connects to the IOMMU and memory.

References Bryg, William R.; Chan, Kenneth K.; Fiduccia, Nicholas S. (February 1996). "A High Performance, Low Cost Multiprocessor Bus for Workstations and Midrange Servers" (PDF). Hewlett-Packard Journal. Gwennap, Linley (November 17, 1997). "PA-8500's 1.5M Cache Aids Performance". Microprocessor Report.

Worked examples

Example 1 — a first encounter with Runway bus

Start with the simplest possible case. Write down what Runway bus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Runway bus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Runway bus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Runway bus

In research: Runway bus appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Runway bus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Runway bus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer buses, Hewlett-Packard products, so understanding it makes those chapters shorter.
In everyday life: Look for Runway bus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Runway bus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Runway bus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Runway bus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Runway bus in simple terms?

Why does Runway bus matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Runway bus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Runway bus.