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Questions tagged [architecture]

For questions about computer architecture: the logical structure of a computer system, distinct from particular implementations.

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9 answers
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What was the rationale behind 36 bit computer architectures?

Was there some particular design theory or constraint that made a 36 bit word size attractive for early computers? As opposed to the various power-of-2 word sizes which seem to have won out?
Mark Harrison's user avatar
49 votes
10 answers
11k views

Why did 1950s-60s computers have such wide words?

Modern general-purpose computers typically have a 64-bit word size, but looking back in time, we see narrower CPUs. In the early 80s, the 68000 dealt with 32-bit addresses but the ALU was only 16 bits ...
rwallace's user avatar
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49 votes
2 answers
12k views

Why does the 80486 take longer to execute simple instructions than complex ones?

The 80486 processor can execute many instructions in a single cycle, such as a register-to-register add instruction (ADD EAX, EBX, for example), which one would generally assume is fairly complex, ...
occipita's user avatar
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38 votes
4 answers
8k views

What motivated stack being invented originally?

In the very early days (the earlier the better! Babbage maybe?) when something like a stack was developed, what motivated it originally? I am aware that these days it makes many features possible, ...
BipedalJoe's user avatar
36 votes
8 answers
9k views

Does the industry continue to produce outdated architecture CPUs with leading-edge process?

Intel has named the i7-8086K in honor of the 8086 processor, though itself it is a 64-bit processor. And we still see in embedded systems or MIL-SPEC platforms there are old CPUs like the 80386 ...
Schezuk's user avatar
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36 votes
8 answers
6k views

Why did DEC develop Alpha instead of continuing with MIPS?

I have been rereading a fascinating discussion about why DEC replaced VAX with Alpha based on talks with people who were there at the time; in a nutshell, in the opinion of the VAX engineers, it was ...
rwallace's user avatar
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34 votes
2 answers
10k views

On what computer did "JUMP" mean "don't jump?"

This question triggered a brain cell. I remember a computer architecture, possibly one that was known for its beautifully symmetric instruction set, with an assortment of distinct mnemonics for ...
Solomon Slow's user avatar
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29 votes
6 answers
10k views

Why did ones' complement decline in popularity?

Many early computers use ones' complement to represent some kind of signed integer. Examples include the PDP-1, the CDC-6600, and many other popular computers. The C standard is obviously written with ...
Omar and Lorraine's user avatar
23 votes
3 answers
5k views

Distinction between scientific and business computing

Wikipedia's page on the IBM System/360 family claims that a distinction once existed between business and scientific computers. The model 44 in particular was designed for scientific work and was set ...
Q3El58's user avatar
  • 333
21 votes
3 answers
8k views

What was the rationale behind 32-bit computer architectures?

Though today various power-of-2 word sizes seems to be the norm, back in the 50-60s multiple-of-6 word sizes was more popular and was required by Department of Defense(DOD) in particular. 36-bit ...
Schezuk's user avatar
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20 votes
4 answers
6k views

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

In UK secondary education, there's a model called the fetch-execute cycle, which describes how computers work. (See: Isaac CS; Bitesize GCSE, Higher; Teach CS.) As I understand it: The processor has ...
wizzwizz4's user avatar
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20 votes
4 answers
6k views

Were there any 8-bit CPUs with 24-bit addressing?

Or was that something that didn't appear until later CPUs (around the time of the 286 maybe)? Also, how would I go about researching this on my own? It's not exactly something I can look up on ...
Anthony's user avatar
  • 455
20 votes
2 answers
2k views

Did the 68000 separate A/D registers save circuitry?

The Motorola 68000 has sixteen integer registers, which was considered a very generous complement at the time it was introduced. They are divided into address and data registers, eight each. Many ...
rwallace's user avatar
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19 votes
3 answers
5k views

What is the purpose of mirrored memory regions in NES's CPU memory map? [duplicate]

[Please see answers to this related question as well] I've started reading the "official" NES Documentation and in page ten, it says that "memory locations $0000-$07FF are mirrored ...
anmomu92's user avatar
  • 193
19 votes
1 answer
2k views

In what sense would the Atari 3200 have been a 10-bit machine?

The Atari 3200 was a canceled project that would have produced a compatible successor to the 2600. Little seems to be known about it; about the best reference I have been able to find is https://...
rwallace's user avatar
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19 votes
3 answers
2k views

What is the history of the PDP-11 MARK instruction?

The PDP-11 MARK instruction was intended to be used as part of the standard PDP-11 subroutine return convention. MARK facilitated the stack clean up procedures involved in subroutine exit. To use it, ...
Leo B.'s user avatar
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18 votes
5 answers
5k views

Why did old consoles have special RAM dedicated for a specific task?

Even in the PlayStation/Saturn era, they had like little RAM chips which were dedicated to just "sound", or "video", or "general". Since they still needed to have the RAM ...
Supernintendo Chalmers's user avatar
18 votes
4 answers
4k views

What was the first CPU with exposed pipeline?

Quoting from Programming for Performance exercise: early versions of the MIPS processor had an "exposed pipeline" (that is, the assembly language programmer needed to know the latencies of ...
Leo B.'s user avatar
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18 votes
2 answers
3k views

Why was there a need for separate I/O address space in addition to a memory address space already?

I was reading through PCI and PCIe configuration access mechanism in Chapter 3 (page 96) of PCIe System Architecture (Mindshare series). As a solution to prevent locking (in case of multiple threads) ...
analogkp's user avatar
  • 183
18 votes
1 answer
2k views

What did it cost the 8086 to support unaligned access?

The Intel 8086 supported unaligned loads and stores of 16-bit data, e.g. mov ax, foo was guaranteed to work even if foo was odd. What did this cost, in terms of performance and chip area, compared to ...
rwallace's user avatar
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17 votes
2 answers
2k views

DEC Alpha: why no 8/16-bit load/stores?

The first version of the DEC Alpha had no load/store instructions for 8 or 16-bit values; if you wanted to deal with data of such sizes, you had to do it by shifting and masking values in registers as ...
rwallace's user avatar
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16 votes
2 answers
1k views

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

On a recent trip to the London Science Museum I saw Babbage’s Analytical Engine. Apparently this had an ALU (or equivalent). I can build an ALU out of logic gates but I can’t conceptualise how to do ...
hawkeye's user avatar
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15 votes
4 answers
6k views

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

According to Wikipedia, John von Neumann proposed the Arithmetic and Logic Unit concept in 1945. Mathematician John von Neumann proposed the ALU concept in 1945 in a report on the foundations for a ...
Noob_Guy's user avatar
  • 693
15 votes
2 answers
3k views

Which computers did Donald Knuth "mix" together to get MIX?

The MIX was a computer design that Donald Knuth used to illustrate computer instruction sets in his magnum opus The Art of Computer Programming. MIX's model number is 1009, which was derived by ...
DrSheldon's user avatar
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14 votes
7 answers
5k views

Which CPUs have implemented trap on signed integer overflow?

All mainstream microprocessors from the 4004 on, have implemented signed integer arithmetic with twos complement and silent wraparound on overflow (by which I mean that the CPU itself will not trap, ...
rwallace's user avatar
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14 votes
2 answers
1k views

Why did Commodore not upgrade OCS straightforwardly to 32-bit architecture?

This is somewhat of a speculative question... Amiga's 1985 Original Chip Set is inherently a 16bit architecture (the data bus is 16bit, registers are 16bit, the blitter itself is 16 state machines ...
user180940's user avatar
  • 3,726
13 votes
2 answers
1k views

When did the natural number of branch delay slots become greater than 1?

Some early RISC CPUs had branch delay slots, the theory being that this would make the CPU both cheaper and faster; you could omit some interlock circuitry, and at the same time, in some cases, ...
rwallace's user avatar
  • 62.7k
11 votes
4 answers
9k views

Was the IBM 5100 ever used for codebreaking?

There's a rumor that there's something embedded in the IBM 5100 which makes it useful for codebreaking. What is the legitimacy of that claim?
MisterGeeky's user avatar
11 votes
6 answers
3k views

What made the ENIAC "programmable"?

The ENIAC was the first programmable, electronic, general-purpose digital computer. However, it was programmed by "rewiring", and this is what I do not understand. When we say "...
Noob_Guy's user avatar
  • 693
10 votes
4 answers
2k views

Why the change in layout of single and double precision in floating-point registers?

The Intel 8087 supported both single and double precision floating point, but only in memory; the on-chip registers were purely double precision. (Strictly speaking they were actually 80-bit extended ...
rwallace's user avatar
  • 62.7k
10 votes
1 answer
1k views

Did the IBM 1401 have much better code density than the 360?

Reading an old thread https://groups.google.com/g/alt.folklore.computers/c/53C2adEQ5jE I see a surprising claim: I told an IBM salesman once that IBM had so bollixed up the architecture that our ...
rwallace's user avatar
  • 62.7k
10 votes
4 answers
463 views

What was the value of the HP/1000 repeated indirection capability?

The HP/1000 was considered like a 16-bit "expansion" of the 12-bit DEC PDP-8.  Its addressing mode for loads and stores similarly used pages, base page and current page, and allowed for ...
Erik Eidt's user avatar
  • 3,357
10 votes
2 answers
547 views

Did any core-memory computers have a read-and-erase instruction?

Magnetic core, the primary form of computer memory from the mid-fifties to the early seventies or thereabouts, had the slightly awkward property that reading it erased it, so every time the CPU ...
rwallace's user avatar
  • 62.7k
9 votes
2 answers
1k views

Why was ones-complement integers implemented? [duplicate]

Wikipedia’s article on ones’ complement mentions large brands using it in their hardware for integer arithmetic into the late 1980s. This is surely for backwards compatibility? According to the ...
Captain Giraffe's user avatar
8 votes
1 answer
2k views

Did any CPU ever expose load delays?

There have been CPUs with exposed branch delays, such as early MIPS: What was the first CPU with exposed pipeline? (Later MIPS kept the delay slots from the early MIPS, though by that time, it wasn't ...
rwallace's user avatar
  • 62.7k
8 votes
1 answer
795 views

Were there any working computers using residue number systems?

Wikipedia says: A residue numeral system (RNS) is a numeral system representing integers by their values modulo several pairwise coprime integers called the moduli. Bit widths of each of those "...
Leo B.'s user avatar
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7 votes
5 answers
2k views

Pulling my hair out over a 4-bit architecture, there ought to be some law here somewhere, is there? [closed]

I am trying to build the most simple CPU possible. And doing it on bread boards. There is a popular movement, one of the biggest protagonists of that being a certain Ben Eater, who is a good teacher, ...
Gunther Schadow's user avatar
7 votes
4 answers
497 views

Were there computers that used processing-in-memory or smart RAM?

Recently there is a huge interest in "processing-in-memory" solutions, where they put some CPU, AI processor or even a DBMS inside a RAM chip. Indeed, current computers appear to be very ...
jhnlmn's user avatar
  • 211
6 votes
2 answers
492 views

Did any pre-microprocessor CPUs have a clock near the toggle speed of the logic ICs used?

This question is NOT about single chip CPUs, but such that were built from many SSI/MSI logic ICs or using a bitslice configuration. A 74F... or 74S... type TTL gate or flipflop, if you provide them ...
rackandboneman's user avatar
5 votes
2 answers
459 views

Data length module of 3.2^n confusion in IBM system/360 architecture

I'm currently reading about the IBM system/360 architecture and there's a part that has me very confused: The decision on basic format (which affected character size, word size, instruction field, ...
KetDog's user avatar
  • 405
5 votes
0 answers
215 views

Why did the Burroughs 205 not use bi-quinary like the IBM 650?

The IBM 650, one of the first general-purpose digital computers, designed in the early fifties, used decimal digits with bi-quinary representation for reasons discussed here: Why did the IBM 650 use ...
rwallace's user avatar
  • 62.7k
4 votes
7 answers
2k views

Any ways to have an architecture with a different base than base16? [closed]

I was reading this Wikipedia entry and thought if any other Computer/or Architecture was ever made with a different base system than base16. And if not, any ways to do it? (maybe by making one from ...
Nordine Lotfi's user avatar
4 votes
2 answers
3k views

How exactly do all control signals in 6502 work?

I'm trying to make a 6502 replica in Logisim. I want to know what exactly each control signal in 6502, how the clock cycles work and additionally I would like to see an example of these control ...
Senijs's user avatar
  • 41
3 votes
2 answers
443 views

Can S-100 cards attach to the ZX machines?

S-100 was, as I understand, a very popular way to connect various computer equipment together in the past. Also, from what I understand, S-100 closely matches the same signals as the Intel 8080, as ...
Omar and Lorraine's user avatar
3 votes
1 answer
1k views

How does the SNES (Super Nintendo) calculate the address of a character?

The SNES has a PPU (Picture Processing Unit) which comprises VRAM, OAM (Object Attribute Memory), and CGRAM (Color Graphics/Palette RAM). These are used to represent tiles, tile maps; attributes for ...
AlphaCentauri's user avatar
3 votes
1 answer
221 views

How are the index registers on the Censor 932 accessed?

The Censor 932 CPU architecture has one instruction type that takes a 3-bit "index register" field, but looking at the rest of the instruction set, I can't see any instructions that read, write, or ...
Vatine's user avatar
  • 681
2 votes
2 answers
274 views

How the I/O performance of legacy mainframes/minicomputers in different architectures is measured and benchmarked comparatively?

Powerful in computing power, mainframes were irreplaceable thanks to the capability of processing numerous transactions rather than doing the math. While computing power is measured in FLOPS and MIPS, ...
Schezuk's user avatar
  • 3,754
1 vote
0 answers
262 views

Ancient browser media photo [closed]

Perhaps 15 years ago, I saw a photo which looked professional showing multiple media types all with labels for either Netscape v4 or the integrated Mozilla suite (which became SeaMonkey). At the very ...
Mark Morgan Lloyd's user avatar
-2 votes
1 answer
227 views

How did Jon von Neumann come up with the idea of the Von Neumann architecture? [duplicate]

The Von Neumann architecture has the following components: A processing unit with both an arithmetic logic unit and processor registers A control unit that includes an instruction register and ...
Noob_Guy's user avatar
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