84
votes
Accepted
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 ...
- 118k
70
votes
Accepted
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 ...
- 3,447
63
votes
Accepted
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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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 ...
- 118k
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 ...
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57
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, ...
- 118k
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.)
...
- 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Does the industry continue to produce outdated architecture CPUs with leading-edge process?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 ...
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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 ...
- 34.7k
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 ...
- 213k
48
votes
Accepted
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 ...
- 213k
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 ...
- 213k
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 ...
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46
votes
Accepted
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 ...
- 118k
45
votes
Accepted
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? :))
...
- 213k
45
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 ...
- 2,159
44
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 ...
- 26k
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 ...
- 213k
41
votes
Accepted
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, ...
40
votes
Accepted
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 ...
- 35.9k
40
votes
Accepted
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., ...
40
votes
Accepted
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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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.
- 213k
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 ...
- 213k
38
votes
Accepted
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 ...
- 213k
36
votes
Accepted
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&...
- 24.6k
35
votes
Accepted
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 ...
- 118k
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 ...
- 34.7k
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 ...
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 ...
- 12.4k
Only top scored, non community-wiki answers of a minimum length are eligible