76 votes

Why does the 80486 take longer to execute simple instructions than complex ones?

TL;DR: It's the pipeline. The 80486 contains parallel operating stages for decoding, operand fetch, execution and write back. So while an ADD reg,reg does take 3 clocks to perform, as it did in the ...
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  • 166k
75 votes
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Why did 1950s-60s computers have such wide words?

And if you go back further, e.g. to the ENIAC, you'll see a word size of 40 bits. And if you go back even further, to mechanical calculators, you'll see word sizes determined by the number of decimal ...
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  • 20.8k
69 votes

What was the rationale behind 36 bit computer architectures?

Was there some particular design theory or constraint that made a 36 bit word size attractive for early computers? Beside integer arithmetic, 36 bit words work quite fine with two different byte ...
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  • 166k
51 votes

Does the industry continue to produce outdated architecture CPUs with leading-edge process?

Manufacturing simple processors on newer semiconductor processes is done. But not quite to that extreme. Let's consider your proposed 8086 done in a 14 nm process. Let's say we do it in CMOS, and ...
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47 votes
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On what computer did "JUMP" mean "don't jump?"

PDP-10. It had a very orthogonal instruction set based around mnemonics with suffixes, and depending on the operation, sometimes no suffix meant a NOP. http://pdp10.nocrew.org/docs/instruction-set/...
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  • 9,386
45 votes

Why did ones' complement decline in popularity?

I imagine representations other than two’s complement (ones’ complement, sign/magnitude...) declined in popularity because two’s complement is simpler to implement; in particular: addition, ...
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43 votes

Does the industry continue to produce outdated architecture CPUs with leading-edge process?

TL;DR: Older CPUs have been shrunk to smaller sizes but not in the same way as modern design, simply as there is no gain in doing so. Details: Does the industry continue to produce outdated ...
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  • 166k
40 votes
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Distinction between scientific and business computing

In practice, "scientific computing" meant floating-point number-crunching like physics simulations, and "business" computing meant I/O-oriented record processing, such as doing the ...
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  • 9,386
40 votes
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What is the purpose of mirrored memory regions in NES's CPU memory map?

It is not intentionally mirrored, it is just a side effect of making the address decoding hardware for RAM as simple and cheap as possible with a single common 74LS139 chip used for the task, when an ...
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  • 16k
38 votes

Why did DEC develop Alpha instead of continuing with MIPS?

The main thrust of the marketing behind the DEC Alpha was its 64-bit microprocessor architecture. They got there years before potential competitors, including MIPS. At the time DEC was shipping the 64-...
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38 votes
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What was the rationale behind 32-bit computer architectures?

there some particular design theory or constraint that made a 32-bit word size attractive for IBM to migrate to? It all comes down to the most basic data type, addressing constrains and, less ...
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  • 166k
30 votes
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Did the 68000 separate A/D registers save circuitry?

Each of the 68K series CPUs had dedicated address-generation hardware which was wired more directly to the A registers and had only limited access to the D registers. Conversely, the main ALU was ...
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  • 16k
28 votes

Why did DEC develop Alpha instead of continuing with MIPS?

and would have improved the chances of a single RISC architecture having wide enough industry support to achieve critical mass rather than being outcompeted by x86. It's important to remember that in ...
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  • 1,445
28 votes
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Which computers did Donald Knuth "mix" together to get MIX?

Section 1.3.1 of The Art of Computer Programming says the following: MIX is the world's first polyunsaturated computer. Like most machines, it has an identifying number—the 1009. This number was ...
25 votes

What was the rationale behind 36 bit computer architectures?

36 bit word size attractive Many sizes have been tried, but fundamentally, this results in a certain precision; from Wikpedia on 36-bit Early binary computers aimed at the same market ...
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  • 3,087
24 votes
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Were there any 8-bit CPUs with 24-bit addressing?

Were there any 8-bit CPUs with 24-bit addressing? Not many. Most prominent and best fitting examples would be WDC 65816 of 1983 Hitachi 64180 of 1985 / Zilog Z180 of 1985 (only 19/20 bit) eZ80 of ...
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  • 166k
23 votes
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Why the change in layout of single and double precision in floating-point registers?

When you have a small transistor budget, it is considerably easier to design your circuitry around a single representation format - the most capable one - and treat converting other formats to and ...
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  • 16k
23 votes

Does the industry continue to produce outdated architecture CPUs with leading-edge process?

They're not quite cutting-edge process sizes, but there are current 8051-core micro-controllers that do much better than the 1981 original's 12 MHz on 3.5-μm process silicon. For example, the Cast ...
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  • 17.1k
21 votes

What was the rationale behind 36 bit computer architectures?

The key point made by Wikipedia seems to be: Prior to the introduction of computers, the state of the art in precision scientific and engineering calculation was the ten-digit, electrically powered,...
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  • 20k
21 votes
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Why does the 80486 take longer to execute simple instructions than complex ones?

First, it is not true that the 486 executes instructions in a single cycle. The 80486 is a pipelined architecture, so it's more accurate to say that most instructions can start one cycle after the ...
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21 votes

Any ways to have an architecture with a different base than base16?

Computers do not in general have "a base 16 architecture". They are binary, i.e., base 2. The base derives from the number of states a storage element can have. Almost exclusively, we use ...
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  • 27.6k
21 votes

Why did 1950s-60s computers have such wide words?

Longer words mean more bits can be processed at once. An 8 bit processor can perform a 32 bit calculation, but it has to do it in 4 stages of 8 bits each. A 32 bit processor can do it in one stage. ...
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  • 13.7k
20 votes
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Why did ones' complement decline in popularity?

Two's complement is generally simpler to implement in hardware than ones' complement, except for one thing: if one wants a "live" readout of register values using one set of lights for positive ...
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  • 26.1k
19 votes

Was the IBM 5100 ever used for codebreaking?

The 5100 had programmable microcode which could be used to implement crypto-specific opcodes1,2,3. Then there's the whole John Titor4,5 thing... 1 Such as population count 2 I'm unaware of anyone ...
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  • 8,727
18 votes
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What was the rationale behind 36 bit computer architectures?

I'm going to address the power of 2 part of the question. Keep in mind that before microprocessors, computers were assembled by hand. Increasing the number of bits in a computer was really a big ...
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  • 13.8k
18 votes
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Why did Commodore not upgrade OCS straightforwardly to 32-bit architecture?

One could just as easily ask why it was not extended to 64 bits, or even wider. Well, if you ignore backwards-compatibility and cost, yes, it could have been. I'll pretend we can ignore backwards-...
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  • 8,864
17 votes

Why did ones' complement decline in popularity?

But I find that modern day examples of computers that use ones complement rather hard to come across. I can only think of the Unisys Clearpath here - and even they are Itaniums at hardware level by ...
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  • 166k
17 votes
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DEC Alpha: why no 8/16-bit load/stores?

Support for byte writes throughout a memory system is expensive. Among other things, if one wishes to use error-corrected memory that can correct single-bit errors, a memory that can be written in ...
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  • 26.1k
17 votes

Why did DEC develop Alpha instead of continuing with MIPS?

DEC's use of MIPS was only ever as basically a stop-gap. Before they used MIPS, DEC had started work on a project called Prism. It was intended to be their first commercial RISC processor. In June of ...
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  • 3,587
16 votes

What was the rationale behind 36 bit computer architectures?

Wiki page 36-bit shows some reasons (all copied from the page): "This was long enough to represent positive and negative integers to an accuracy of ten decimal digits (35 bits would have been the ...
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