computer science

Motorola 68451

Motorola 68451 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 Motorola 68451 rather than just read about it. In short: The MC68451 is a Motorola (now Freescale) Memory Management Unit (MMU), which was primarily used in conjunction with the Motorola MC68010 microprocessor. The MC68451 supported a 16 MB address space and provided a MC68000 or a MC68010 with support for memory management and protection of memory against unauthorized access.

Key takeaways

Motorola 68451 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 Motorola 68451 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Motorola 68451 from memory before moving on to harder problems.

Reference excerpt

The MC68451 is a Motorola (now Freescale) Memory Management Unit (MMU), which was primarily used in conjunction with the Motorola MC68010 microprocessor.

The MC68451 supported a 16 MB address space and provided a MC68000 or a MC68010 with support for memory management and protection of memory against unauthorized access. The block size was variable, so it was usually used for segment-based memory management. It supported the mapping of up to 32 memory segments or pages of a variable size from logical to physical addresses. To allow more segments or pages, the simultaneous use of multiple MC68451 MMUs was supported. In combination with a MC68010, the MC68451 permitted the realization of virtual memory. With the earlier MC68000, this was not possible due to the way the MC68000 treated memory access errors, i.e. processor state could not always be properly restored after a page fault; two MC68000s would be required, with the main CPU pausing when it got a memory access error, and the other CPU servicing the page fault. The limitation to 32 segment table entries per MMU made systems based on a MC68010 and a MC68451 slow, as they often had to modify the segment table due to its small size. Motorola made a single-board computer module that demonstrated the combination of 68010 and 68451 for applications requiring virtual memory. H. Berthold AG used 12 MC68451 MMUs together with their UNOS variant vBertOS. Others (e.g. Sun Microsystems, Convergent Technologies) used their own proprietary MMUs instead of the MC68451.

Illustrations

Worked examples

Example 1 — a first encounter with Motorola 68451

Start with the simplest possible case. Write down what Motorola 68451 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 Motorola 68451 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 Motorola 68451 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 Motorola 68451

In research: Motorola 68451 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 Motorola 68451 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: Motorola 68451 is common in secondary-school and first-year university syllabi. It links to neighbouring topics 68k architecture, Computing stubs, Motorola microprocessors, so understanding it makes those chapters shorter.
In everyday life: Look for Motorola 68451 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 Motorola 68451 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Motorola 68451 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 Motorola 68451 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Motorola 68451 in simple terms?

The MC68451 is a Motorola (now Freescale) Memory Management Unit (MMU), which was primarily used in conjunction with the Motorola MC68010 microprocessor. The MC68451 supported a 16 MB address space and provided a MC68000 or a MC68010 with support for memory management and protection of memory again…

Why does Motorola 68451 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 Motorola 68451?

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 Motorola 68451.