engineering

Motorola 68881

Motorola 68881 is a engineering 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 68881 rather than just read about it. In short: The Motorola 68881 and Motorola 68882 are floating-point units (FPUs) used in some computer systems in conjunction with Motorola's 32-bit 68020 or 68030 microprocessors. These coprocessors are external chips, designed before floating point math became standard on CPUs.

Key takeaways

Motorola 68881 belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Motorola 68881 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Motorola 68881 from memory before moving on to harder problems.

Reference excerpt

The Motorola 68881 and Motorola 68882 are floating-point units (FPUs) used in some computer systems in conjunction with Motorola's 32-bit 68020 or 68030 microprocessors. These coprocessors are external chips, designed before floating point math became standard on CPUs. The Motorola 68881 was introduced in 1984. The 68882 is a higher performance version produced later.

Overview

The 68020 and 68030 CPUs were designed with the separate 68881 chip in mind. Their instruction sets reserved the "F-line" instructions – that is, all opcodes beginning with the hexadecimal digit "F" could either be forwarded to an external coprocessor or be used as "traps" which would throw an exception, handing control to the computer's operating system. If an FPU is not present in the system, the OS would then either call an FPU emulator to execute the instruction's equivalent using 68020 integer-based software code, return an error to the program, terminate the program, or crash and require a reboot.

Architecture

The 68881 has eight 80-bit data registers (a 64-bit mantissa plus a sign bit, and a 15-bit signed exponent). It allows seven different modes of numeric representation, including single-precision floating point, double-precision floating point, extended-precision floating point, integers as 8-, 16- and 32-bit quantities and a floating-point Binary-coded decimal format. The binary floating point formats are as defined by the IEEE 754 floating-point standard. It was designed specifically for floating-point math and is not a general-purpose CPU. For example, when an instruction requires any address calculations, the main CPU handles them before the 68881 takes control. The CPU/FPU pair are designed such that both can run at the same time. When the CPU encounters a 68881 instruction, it hands the FPU all operands needed for that instruction, and then the FPU releases the CPU to go on and execute the next instruction.

68882

The 68882 is an improved version of the 68881, with better pipelining, and eventually available at higher clock speeds. Its instruction set is exactly the same. Motorola claimed in some marketing literature that it executes some instructions 40% faster than a 68881 at the same clock speed; the typical performance improvement is more modest. The 68882 is pin compatible with the 68881 and can be used as a direct replacement in most systems. The most important software incompatibility is that the 68882 uses a larger FSAVE state frame, which affects UNIX and other preemptive multitasking OSes that had to be modified to allocate more space for it.

Usage The 68881 or 68882 were used in the Sun Microsystems Sun-3 workstations, IBM RT PC workstations, Macintosh II family, NeXT Computer, X68000, Amiga 3000, Convergent Technologies MightyFrame, Atari Mega STE, TT, and Falcon. Some third-party Amiga and Atari products, including the CMI PAMC-500, used the 68881 or 68882 as a memory-mapped peripheral to the 68000.

Versions

68881 155,000 transistors on-chip 12 MHz version 16 MHz version ran at 160 kFLOPS 20 MHz version ran at 192 kFLOPS 25 MHz version ran at 240 kFLOPS

68882 176,000 transistors on-chip 25 MHz version ran at 264 kFLOPS 33 MHz version ran at 352 kFLOPS 40 MHz version ran at 422 kFLOPS 50 MHz version ran at 528 kFLOPS These statistics came from the comp.sys.m68k FAQ. No statistics are listed for the 16 MHz and 20 MHz 68882, though these chips were indeed produced.

Legacy Starting with the Motorola 68040, floating point support was included in the CPU itself.

References

Notes freescale.com - Motorola MC68000 Family Programmer's Reference Manual

Illustrations

Worked examples

Example 1 — a first encounter with Motorola 68881

Start with the simplest possible case. Write down what Motorola 68881 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 68881 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 68881 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 68881

In research: Motorola 68881 appears in engineering 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 68881 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 68881 is common in secondary-school and first-year university syllabi. It links to neighbouring topics 68k architecture, Coprocessors, Floating point, so understanding it makes those chapters shorter.
In everyday life: Look for Motorola 68881 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 68881 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Motorola 68881 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 68881 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Motorola 68881 in simple terms?

Why does Motorola 68881 matter?

Because it connects several engineering 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 68881?

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 68881.