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281 votes
Accepted

Why does trying to break into the NT 3.1 kernel reboot my 486DX4 machine?

Short explation The Windows NT 3.1 kernel is incompatible with enhanced 486 processors. Specifically, it is incompatible with 486 processors providing the CPUID instructions. Kernel debugging works ...
Michael Karcher's user avatar
78 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 ...
Raffzahn's user avatar
  • 226k
35 votes

Why did x86 support self-modifying code in the 80s and 90s?

Raffzahn has the main idea: once self-modifying code had been supported in the past, it had to continue to be supported in perpetuity. But just to go back a little further, https://stackoverflow.com/...
Nate Eldredge's user avatar
33 votes
Accepted

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
28 votes
Accepted

Detecting the version of i486 CPU

The most comprehensive pre-CPUID x86 CPU detection code I’m aware of is in TMi0SDGL (Too Much in One So Don’t Get Lost). It includes source code. It will use model-specific MSRs, BIOS services etc. to ...
Stephen Kitt's user avatar
28 votes
Accepted

How were the test registers used on the i386 and the i486?

The 486 test registers are described in the i486 Processor Programmer’s Reference Manual, starting on page 10-8. The 386 test registers are a subset. Registers TR6 and TR7 provide access to the TLB. ...
Stephen Kitt's user avatar
27 votes
Accepted

Did 486 SMP systems provide Total Store Ordering?

BRIEF: most i486 multi-processors were probably TSO. But system vendors could definitely build non-TSO systems, and were definitely considering doing so. ---+ DETAIL ---++ SMP Marketplace ~1991 (not ...
Krazy Glew's user avatar
24 votes
Accepted

Were there 486SX revisions without an FPU on the die?

Yes, Intel did make 486SX CPUs with no FPU on the die. You can see the difference in die shots, e.g. from this CPU-World.com thread: early 486SXs are nearly indistinguishable from 486DXs, but later ...
Stephen Kitt's user avatar
22 votes
Accepted

What's the story behind the "mysterious" 486DX3?

The short answer is no, there never was a DX3 part. 486 CPUs from Intel have a varied naming history, mostly because of trademark concerns: Intel wanted to stop its competitors selling CPUs with ...
Stephen Kitt's user avatar
22 votes

Why did x86 support self-modifying code in the 80s and 90s?

The support for self-modifying code was not specifically built as a feature, so it was not specifically allowed or prevented. It just was possible due to the architecture of the CPU and the system. To ...
Justme's user avatar
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22 votes
Accepted

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 ...
mcleod_ideafix's user avatar
20 votes
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How did AMD's 386 and 486 perform like Intel's newer generation?

The Am386 and Am486 were designed as clock-for-clock equivalents of the corresponding Intel CPUs, based on reverse-engineering and AMD’s previous second-source licenses — at least the Am386 even used ...
Stephen Kitt's user avatar
20 votes

Detecting the version of i486 CPU

If your goal is to display as much information as possible, I'd take a look at Chapter 4: System and Equipment Detection of The Undocumented PC by Frank van Gilluwe. (Starting at page 95 in the first ...
ssokolow's user avatar
  • 7,105
19 votes

Why did x86 support self-modifying code in the 80s and 90s?

I'm going to dispute the basis of the question, to at least a minimal degree. The original 8086 and 8088 both included prefetched instruction queues. If you modified an instruction that was already in ...
Jerry Coffin's user avatar
  • 5,173
19 votes
Accepted

What sequence of instructions is the equivalent to `fcomip` on i486?

This is actually documented in present-day Intel manuals. See the Intel 64 and IA-32 Architectures Software Developer’s Manual, volume 1, section 8.1.4, "Branching and Conditional Moves on ...
Nate Eldredge's user avatar
18 votes

Did 486 SMP systems provide Total Store Ordering?

The first standard for x86-based multi-processing systems was Intel’s MultiProcessor Specification, first published in 1994 – so there weren’t really any “typical” 486 SMP systems before then. (There ...
Stephen Kitt's user avatar
15 votes
Accepted

Why did x86 support self-modifying code in the 80s and 90s?

Self-modifying code was one of the few ways one could invoke a dynamically-chosen software interrupt. High-level language toolchains targetting the MS-DOS platform sometimes provided an ‘invoke ...
user3840170's user avatar
  • 23.1k
15 votes

How do I slow down the emulation in VMware and make it go "proper fullscreen" for Windows 3.11?

VMWare is not an emulator these days, but a hypervisor. Windows 3.1 is running directly on the hardware with nothing underneath it, just fenced in by hardware-level protection and translation. If ...
RETRAC's user avatar
  • 13.8k
13 votes

In what year was my old, long-lost IBM PC made and which form factor/model was it?

I've found out the right model. Turns out it wasn't a "ValuePoint" at all. It was an IBM "Personal Computer 330", but at least I remembered the "466DX2" part right... I ...
Hopeless Nostalgic's user avatar
12 votes

Why did x86 support self-modifying code in the 80s and 90s?

Preface: Avoiding '(self) modified' code is impossible on computers allowing to load code (see section at the End) The Question Backward compatibility, is the obvious general answer. But backward ...
Raffzahn's user avatar
  • 226k
10 votes

How did AMD's 386 and 486 perform like Intel's newer generation?

Is it really so simple as just getting to higher clock speeds than Intel did? Yes, it is that simple. Up and including the 486 AMD's CPUs were developed close to Intels devices, supported by in ...
Raffzahn's user avatar
  • 226k
9 votes

Why did Socket 3 have more pins than needed for the 486?

Socket 3 did not have more pins than needed. It was designed to support the Pentium OverDrive CPUs that Intel released in the mid-1990s. These CPUs could have up to 237 pins, while the Socket 3 ...
Brian H's user avatar
  • 60.8k
8 votes

How do I slow down the emulation in VMware and make it go "proper fullscreen" for Windows 3.11?

You might consider DOSBox (free). It is more oriented towards emulating retro hardware, and has some speed control options. It tries to 'figure out' what the speed control should be, but there is a ...
ziggurat29's user avatar
7 votes
Accepted

In what year was my old, long-lost IBM PC made and which form factor/model was it?

EDIT 2020-12-01: While I stand by my reasoning, my conclusion below was incorrect. Memory is a funny thing, isn't it? Anyway, @HopelessNostalgic provided the correct answer, please upvote that one ...
snips-n-snails's user avatar
7 votes
Accepted

Bad floppy drive in T1910CS 486 laptop

As far as I know this laptop can’t easily be connected to a network, which leaves three options for getting software onto it: transfer over the serial or parallel ports, using an appropriate cable ...
Stephen Kitt's user avatar
6 votes

Bad floppy drive in T1910CS 486 laptop

Use the following manual for disassembly: http://www.minuszerodegrees.net/manuals/Toshiba/Other/Toshiba%20T1950%20T1950CS%20T1950CT%20-%20Maintenance%20Manual.pdf You have to unscrew about 30 screws. ...
techgeek's user avatar
  • 101
6 votes

Why did x86 support self-modifying code in the 80s and 90s?

"Real" self-modifying code is seldom used on x86. There are some exceptions, like the int86() example in Raffzahn answer but in most cases, it is not quite self modifying as some 8bits CPUs ...
Grabul's user avatar
  • 3,657
5 votes

Why did x86 support self-modifying code in the 80s and 90s?

A couple decades later, the answer would be obvious: JavaScript JIT compilers. But the 486 predates them. It predates Javascript JIT compilers, but there were plenty of other virtual machines for ...
occipita's user avatar
  • 2,382
3 votes

Why did x86 support self-modifying code in the 80s and 90s?

I first learned to compute in the late sixties. Here are some personal memories of what ordinary programmers were doing at the time. Back in the seventies, many machines had (or appeared to have) a ...
Richard Kirk's user avatar
2 votes

Bad floppy drive in T1910CS 486 laptop

There is a USB HDD adapter which has usual laptop and desktop IDE ports (2.5" and 3.5" HDD) Also SATA port and USB. You can copy stuff in each of them when connected to a modern PC. Make ...
Petri-fied's user avatar

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