99 votes
Accepted

How much better was DEC Alpha than contemporaneous x86?

The Alpha team set out to create a high-performance architecture, planned to last for 25 years and allow for 1000-fold performance increase over those 25 years. So they placed some long bets, starting ...
Stephen Kitt's user avatar
86 votes

Were there 8086 coprocessors other than the 8087?

Not all of the original co-processors were for floating point math. Intel itself offered an I/O coprocessor for the 8088 and 8086 called the 8089. Part of the reason it didn't do as well as the 8087 ...
mschaef's user avatar
  • 4,836
77 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
  • 223k
72 votes

How much better was DEC Alpha than contemporaneous x86?

Stephen Kitt has done what seems to me an excellent job of outlining features and when they were introduced. I'll take a slightly different tack, instead picking a single point in time, and pointing ...
Jerry Coffin's user avatar
  • 4,842
61 votes

Were there ever 12-, 24-, 48-, etc bit processors?

Certainly. The DEC PDP-8 family was 12-bit, and so was the Intersil 6100, a single-chip CMOS implementation of the PDP-8 ISA. There have been many 24-bit DSP-type processors, from Motorola, ...
Dave Tweed's user avatar
  • 1,754
50 votes
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Why did CPU designers in the 70s prioritize reducing pin count?

Existing Machinery. Reasoning about the usage of existing packages Adding a few hundred transistors for multiplexing is approximately free compared to buying production machinery for several millions -...
Raffzahn's user avatar
  • 223k
50 votes
Accepted

Why are different emulators needed to run platforms that use 6502 assembly code?

The 6502 CPU is just one piece of the puzzle Emulators emulate entire machines, not merely CPUs. Even the likes of QEMU emulate an entire generic computer. It helps if you think of the Apple II and ...
520's user avatar
  • 596
42 votes
Accepted

How was microcode implemented in retro processors?

How was microcode implemented in retro processors such as the Z80 or 8080? None of these chips (likewise 6800 and 6502) use microcode the same way as it's used today. The decoding isn't as strictly ...
Raffzahn's user avatar
  • 223k
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
  • 223k
40 votes
Accepted

Were there ever 12-, 24-, 48-, etc bit processors?

specifically after the 8-bit byte became the industry standard? There's no clear point of time where the 8-bit byte became a standard, since it's still just a de facto standard nowadays¹. However ...
phuclv's user avatar
  • 3,592
32 votes

Why are different emulators needed to run platforms that use 6502 assembly code?

There are many answers to this and none might satisfy you. First of all, an emulator doesn't just do a CPU, but a machine. The same way you can't run an NES game on an Apple II. So while one emulator ...
Raffzahn's user avatar
  • 223k
32 votes

How could the Intel 4004 address 640 bytes if it was only 4-bit?

There may only be four pins coming out from the chip but it allowed for more than sixteen values because it multiplexed 4n-bit values onto those four pins based on timing. The instructions for ...
paxdiablo's user avatar
  • 4,742
31 votes
Accepted

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 ...
tofro's user avatar
  • 35k
30 votes
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How did old computers address far more than 64K of memory despite only having a 16 bit address bus?

I don't know details of the Sharp PC-G830 specifically but the technique used to address more than 64K with a 16-bit address bus is called "bank switching". This involves setting up some ...
jwh20's user avatar
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30 votes
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Were there any enhancement chips that vastly outperformed the main CPU?

I think the SA-1 chip in various late-era Super Nintendo cartridges (eg. Kirby Super Star, Kirby's Dream Land 3, Super Mario RPG) would qualify. Here's what Wikipedia has to say about it (emphasis ...
ssokolow's user avatar
  • 6,765
27 votes

Were there 8086 coprocessors other than the 8087?

Alan Cox mentions in this post having seen a hard drive interface that plugged into the 8087 socket (for computers with no expansion slots). I've checked various issues of Amstrad PC magazine. PPC ...
john_e's user avatar
  • 7,658
27 votes
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Intel processor transistor type evolution

Here’s the list of main technologies used: 4004: 10µm PMOS; 4040: 10µm PMOS; 8008: 10µm PMOS; 8080: 6µm NMOS (faster than PMOS, and TTL-compatible); 8085: 3.2µm NMOS, then HMOS (“H” variants); 8086: ...
Stephen Kitt's user avatar
26 votes
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Uses for the halt instruction?

The HALT condition does not (at least on retro CPUs) consume considerably less power than normal execution does. One very obvious use case is synchronizing program flow with external (hardware) events....
tofro's user avatar
  • 35k
25 votes

Were there 8086 coprocessors other than the 8087?

I don't think there ever were any incompatible co-processors which used the same sockets and I/O mechanisms as the Intel co-processors. There were other incompatible co-processors, at least for the ...
Stephen Kitt's user avatar
22 votes

Why did CPU designers in the 70s prioritize reducing pin count?

Coincidentally, I found this explanation reading through Microprocessor Interfacing Techniques 3rd Ed 1979 by Zaks and Lesea page 16: The Standard Microprocessor System Throughout this book, ...
TTL's user avatar
  • 229
22 votes
Accepted

What was the first microprocessor to support full virtualization?

Full, hardware-assisted virtualisation, with the intention of supporting hypervisors running operating systems without requiring para-virtualisation, was added to micro-processors relatively recently. ...
Stephen Kitt's user avatar
21 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
Accepted

The Slowest Microprocessor

Any CPU that is fully-static can be clocked down to 0 Hz and still function. Such CPUs are in wide use today. Here are some examples: RCA 1802 Used in a few early micros, as an embedded processor, ...
Alex Hajnal's user avatar
  • 9,350
18 votes

Uses for the halt instruction?

What was the halt instruction in early CPUs such as the Z80 and 8080 used for? Stopping the system in a known state to allow a clean restart/react to external sources. It's a very useful feature for ...
Raffzahn's user avatar
  • 223k
18 votes
Accepted

Can we express the instructions to the Analytical Engine in terms of assembler or machine code?

Yes. In fact, it is a very simple system in machine language terms. The key to understanding the system is to look at the physical construction of the part you saw. This is what we would today would ...
Maury Markowitz's user avatar
18 votes
Accepted

What happened to the 65832?

In the 1988 Report on the 65c832, Mensch described the 65c832 as a back-burner project with an uncertain timeline: Since WDC is not a gigantic conglomerate, it has limited resources. If all ...
Michael Graf's user avatar
  • 10.1k
18 votes

The Slowest Microprocessor

Going with the spirit of the question. A quick glance at Wikipedia's Microprocessor Chronology we find: PPS-4 from Rockwell at 200 kHz PPS-8 from Rockwell at 256 kHz TMS 1000 from TI at 400 kHz PPS-...
Will Hartung's user avatar
  • 12.3k
18 votes

How could the Intel 4004 address 640 bytes if it was only 4-bit?

The 4004 used the data bus for both addresses and data. The memory control outputs were used for bank selection. Here's how it works: At the beginning of the instruction, the CPU raises SYNC to let ...
JeremyP's user avatar
  • 11.8k
17 votes

Were there 8086 coprocessors other than the 8087?

In addition to the i8087 & i8089, intel had the i80130 and i80150 "Operating System Coprocessors". These were single-chip bundled timers and irq controllers that had a subset of the iRMX-86 ('130)...
KJ Seefried's user avatar
  • 1,765
17 votes

Why did CPU designers in the 70s prioritize reducing pin count?

Your question assumes that CPUs with more than 40 pins were a rarity in the 1970s, but this was common for early 16-bit CPUs. Both the TI TMS9900 and the Motorola 68000 had 16-bit external data busses,...
Brian H's user avatar
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