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75 votes
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Intel CPU bug in the '90s

I suspect your teacher was referring to the FDIV Pentium bug, which led to a large outcry in the media at the time and for which Intel issued a recall. This bug caused floating-point division to ...
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63 votes
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Why didn't early single-chip CPUs support multiplication instructions

Fast multiplier circuits as used today take enormous amounts of logic, far beyond what would have been cost-effective (or perhaps even possible) in the mid-70s for an inexpensive microprocessor. Even ...
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57 votes

Can an x86 CPU running in real mode be considered to be basically an 8086 CPU?

An x86 CPU running in real mode is intended to be backwards-compatible with an 8086 or 8088, but there do end up being a number of differences, for example: newer CPUs run faster (in general); newer ...
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57 votes

Why didn't early single-chip CPUs support multiplication instructions

You don't need it Multiplying two arbitrary bytes together has limited practical value. (If you want to multiply by a constant you can hardcode the optimal sequence of instructions to do so.) ...
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55 votes

What was the last x86 CPU that did not have the x87 floating-point unit built in?

As far as I’m aware, the last FPU-less x86-compatible CPU which could still be considered general-purpose is the Vortex86SX, released in 2007 and still available now. This is a Pentium-class CPU, ...
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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Does the industry continue to produce outdated architecture CPUs with leading-edge process?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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49 votes

What does the "x" in "x86" represent?

The term x86 is shorthand for 80x86, which was used to refer to any member of the family 8086 (and also, incidently, 8088), 80186, 80286, etc. Things have since gotten a bit muddled by the fact that ...
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47 votes
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Will PC-DOS run faster on 4 or 8 core modern machines?

No, DOS won't use any additional CPU (*1) ever. (Though it might run faster due them new CPUs being faster) Quite the same way as DOS doesn't take advantage of the extended memory or additional ...
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45 votes
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Why did Intel abandon unified CPU cache?

I’m not sure the separate cache was “obviously better” back when the Intel designers were working on the 80486, at least, not to the designers in question. But “better” might not even have been much ...
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44 votes
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Z80 CPU address lines not stable

It appears that A0 through A6 operate correctly, but A7 though A9 (I've not tested the rest of the upper bits) are only active on the clock edge. Doesn't that exactly look like refresh cycles? :)) ...
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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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41 votes

Does any computer resemble the model taught in UK secondary education?

Is there a computer that works like this? Erm, next to all? FETCH-DECODE-EXECUTE is the basic execution cycle for each and every digital CPU. No matter if a Turing Machine or a Pentium. I suspect the ...
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40 votes

Why is the processor instruction called "move", not "copy"?

Besides the matter of semantics and personal taste, there’s a much more practical reason: some instructions sets claim to be copyrighted, as the Wikipedia Z80 article states: Because Intel claimed ...
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39 votes
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How did people program for Consoles with multiple CPUs?

It varies machine to machine; at the simplest end is the Neo Geo — its 68000 and Z80 have completely independent buses. You write one program for the 68000 and one for the Z80 and a single pipe of ...
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39 votes

What does the "x" in "x86" represent?

x is meant as wildcard, so this represents all CPUs able to run 8086 compatible code.
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39 votes
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Why did post-8008 CPUs not keep the on-chip stack idea?

In short, to better support interrupts, because interrupts were arguably broken (or at least very limited in usability) on the 8008. The direct answer to the question of why you'd move the stack off-...
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38 votes
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Why did some CPUs use two Read/Write lines, and others just one?

What reasons would CPU designers have for choosing these different approaches? It depends on what the designers intended to mark a valid bus cycle, which is the 'leading' signal for decoding. In a ...
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38 votes
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How much did IBM save by limiting the PC to 4.77 MHz?

Worshipping at the altar of color clock Back in that day, everything was built around the NTSC color clock frequency of 3.579545 MHz. Everything from the Atari VCS to the C64 made ample use of it, ...
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35 votes
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How was the 80186 incompatible with the IBM PC?

The main issue with the 80186 isn’t with the CPU core itself, but with its integrated peripherals: they aren’t compatible with those used in the IBM PC, and they aren’t integrated in the same way ...
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34 votes
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Why is the processor instruction called "move", not "copy"?

Because "move" is the typical necessary function It isn't always this way, of course, but especially with earlier CPUs, there were limited destinations for data from a particular operation - e.g., ...
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32 votes
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Why did Socket 3 have more pins than needed for the 486?

The extra pins were forward-planning, on both Socket 2 and Socket 3. Most of the extra pins are used for power (Vcc) and ground (Vss), which is useful to provide more power to a CPU. The other pins ...
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31 votes
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Can the two CPUs in a Commodore 128 run at the same time?

No, they cannot. They share both the data and the address bus of the C128, so they can only run exclusively at any one point in time. The address bus is apparently directly connected, the data bus ...
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31 votes
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How did the CP1600 CPU supposedly make looping faster?

TL;DR: No. The CDP1600 did not have any unusual/speedy loop mode, but its predecessor, the DG Nova mini computer, had two very innovative and increment/decrement instruction, using a core memory ...
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30 votes

Does the Intel 8086 CPU have user mode and kernel mode?

No. There is no mechanism for any privilege levels or protection in 8086. As a consequence, there is nothing special about OS code, and thus user applications are allowed to do everything, including ...
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28 votes

How much did IBM save by limiting the PC to 4.77 MHz?

For many kinds of parts, there's a substantial gap between specified maximum/minimum timings and typical timings. The 8088 specification requires that the clock be high for a minimum of 69 ns every ...
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28 votes

Intel CPU bug in the '90s

Stephen Kitt has already provided a good answer regarding the FDIV bug. I'll fill in some details about Intel employing logicians: Because of this bug, Intel had to replace a lot of processors, which ...
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28 votes
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What are the screws for on the UltraSPARC?

They are used for attaching a heat sink.
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28 votes

Do instruction exercisers exist for 8086 and 68K (and other) CPUs?

Throwing some things out there that might fit the bill: 6502: AllSuiteA which assembles into a single binary and exits, leaving a failure or success code in memory; 6502 and 65C02: Klaus Dormann's ...
27 votes

Does the Intel 8085 CPU use real memory addresses?

The 8085 is effectively the same as the 8080 microprocessor. The 8080 has a flat 16-bit address space and no segment registers. So yes, the 8085 uses real memory addresses without any translation.
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27 votes
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Did any RISC CPU ever take more than one clock cycle per instruction?

Classic RISC CPUs like ARM ... instructions execute in one cycle ... This assumption is not correct. The ARM-2 CPU (VL86C010, one of the first ARM CPUs) took: Only one cycle for most operations (as ...
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