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84 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 ...
Stephen Kitt's user avatar
70 votes
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Why was desktop CPU frequency so low in the late 1990s?

As an extremely simplistic approximation, frequency is related to the size of transistors. Smaller transistors and wire interconnects have smaller capacitance. Which makes them able to toggle faster ...
Grabul's user avatar
  • 3,647
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 ...
RETRAC's user avatar
  • 13.7k
61 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 ...
Stephen Kitt's user avatar
61 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, ...
Stephen Kitt's user avatar
58 votes

Why did they create PC games relying on a fixed CPU frequency instead of a clock function?

Every game I've seen that assumed a fixed CPU frequency in some sense came from one of two eras: First, games designed for the original IBM PC and its clones (what Turbo buttons were invented to ...
ssokolow's user avatar
  • 7,065
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.) ...
Artelius's user avatar
  • 1,030
57 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Does the industry continue to produce outdated architecture CPUs with leading-edge process?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 ...
RETRAC's user avatar
  • 13.7k
49 votes

What motivated stack being invented originally?

The Stack, as we know it today, was developed by Friedrich Bauer and Klaus Samelson as part of their work on the Munich PERM and ALGOL. ALGOL-58 was in turn the first language to use a stack. They ...
Raffzahn's user avatar
  • 224k
48 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 ...
supercat's user avatar
  • 36.6k
48 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 ...
Raffzahn's user avatar
  • 224k
48 votes

What was the last x86 processor that didn't have a microcode layer?

The original 8086 was microcoded; x86 has never been hard-wired. The P6 microarchitecture, first seen in the Pentium Pro, was the first Intel design to buffer a RISC-esque translation of the x86 ...
Tommy's user avatar
  • 37.3k
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 ...
Raffzahn's user avatar
  • 224k
46 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 ...
Stephen Kitt's user avatar
46 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 ...
Biff Iam's user avatar
  • 2,209
45 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? :)) ...
Raffzahn's user avatar
  • 224k
45 votes

Why was desktop CPU frequency so low in the late 1990s?

A MOSFET transistor has a switching time that's related to the size of the transistors. Basically the gate forms a capacitor with the substrate, and you have to load and unload the capacitor so the ...
dirkt's user avatar
  • 27.9k
43 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, ...
Harper - Reinstate Monica's user avatar
42 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 ...
Raffzahn's user avatar
  • 224k
41 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-...
cjs's user avatar
  • 26.6k
40 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 ...
Tommy's user avatar
  • 37.3k
40 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., ...
manassehkatz-Moving 2 Codidact's user avatar
39 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 ...
Raffzahn's user avatar
  • 224k
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.
Raffzahn's user avatar
  • 224k
39 votes
Accepted

Did underclocking the early Z80 chips improve yield?

That would be a rather unusual move. The 2.5 MHz Z80 is well within Mostek's (*1) base technology of 1976 (*2). In fact, already the the very first mask set of 1976 yielded 20% good for 4 MHz (*3). So ...
Raffzahn's user avatar
  • 224k
36 votes
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What was the release date of the MOS 8502?

I just talked to Bil Herd (the designer of the C128 hardware), and he said, "It was made specifically for the C128, design mods done by Dave DiOrio," and so was essentially "released&...
cjs's user avatar
  • 26.6k
34 votes

Why was desktop CPU frequency so low in the late 1990s?

Even during World War II, a 1GHz oscillator would not have been impractical to build. Indeed, the Soviets used an oscillator operating at around that frequency immediately after World war II in order ...
supercat's user avatar
  • 36.6k
34 votes

Why did they create PC games relying on a fixed CPU frequency instead of a clock function?

A typical clock function in the 1980s was not very high resolution. MS-DOS function 2CH apparently reported 100ths of a second, but was actually based on the 18.2 Hz timer interrupt, so it wasn't ...
manassehkatz-Moving 2 Codidact's user avatar
34 votes

How could early computers perform data operations before John von Neumann proposed the concept of ALU?

What von Neumann proposed was the idea of the ALU as a subsystem of an electronic computer, conceptually separate from the memory and input/output subsystems. The concept of hardware for doing ...
John Dallman's user avatar
  • 13.3k
33 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 ...
Stephen Kitt's user avatar

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