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Questions tagged [instruction-set]

For questions regarding the instruction sets of microprocessors.

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3answers
4k views

Why does an instruction include the address of the next instruction on the IBM 650?

The IBM 650 seems to be a load-store machine. One advantage of a load-store machine is that the instruction can be shorter because there's less pressure to pack more information into it. But the IBM ...
2
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1answer
206 views

Which CPUs had instructions leaving data registers in an unspecified state?

When an ALU performs a floating point division operation using the non-restoring or the SRT algorithm, it maintains the current value of the "remainder" (in quotes, because it is not a true non-...
11
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1answer
561 views

What undocumented 8085 instructions is Steven Morse referring to in “In The Beginning”?

In S. P. Morse's 1980 allegory, "In The Beginning", he writes And Intel said, "Let there be an 8085 with an oscillator on the same chip as the processor, and let an on-chip system controller ...
10
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3answers
766 views

What is the instruction set of the Z4?

I am able to find a few instructions, such as: Fin (presumably "Fine", as in the end of a musical score, ends a program), Fin', (a conditional Fin), St (possibly "Start" -- the need for this is ...
8
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2answers
535 views

Instruction set support for multiplication with a constant

Before integer multipliers in silicon, several cpus had some support for multiplication. For instance SPARCv7 has the MULScc multiply-step instruction (several other cpus also have this). As far as ...
9
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3answers
606 views

How to keep the instruction prefetcher filled up

My understanding is that the Intel 8088 has this buffer which reads ahead in the instruction stream whenever it has a spare bus cycle or two, so that when the time comes to execute that instruction, ...
6
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2answers
822 views

What is the purpose of the ω register of the БЭСМ-6?

This page describing the БЭСМ-6 instruction set refers to a value called ω, which is stored inside a Mode Register. It appears to be set by certain kinds of instructions, so that ω can tell you which ...
10
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2answers
606 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, ...
13
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8answers
951 views

Uses for the halt instruction?

What was the halt instruction in early CPUs such as the Z80 and 8080 used for? Here's a description of the Z80 instruction: The HALT instruction suspends CPU operation until a interrupt or reset ...
22
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4answers
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Why does the Z80 include the RLD and RRD instructions?

The Z80 has an instruction RLD, which apparently treats the lower 4 bits in the accumulator and the full 8 bits in (HL) as a twelve bit integer which it then rotates left by 4 bits. The carry flag ...
8
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3answers
759 views

Intel 8080 - Behaviour of the carry bit when comparing a value with 0

I'm in the process of writing an emulator for the Intel 8080. The description of the CMP instruction from Intel's 8080 programming manual (see page 20) says the following: The specified byte is ...
11
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1answer
659 views

Nintendo 64 microcode format

The Nintendo 64 GPU ('Reality Coprocessor') had microcode that could be loaded at runtime. Several standard microcodes were supplied with the development kit, and the easily findable documentation ...
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2answers
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Why are first four x86 GPRs named in such unintuitive order?

On x86 the first four general-purpose registers are named AX, CX, DX, BX. It would be quite intuitive if their indices (those used in instruction encoding) were in alphabetical order, but instead of ...
4
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1answer
460 views

Are there any articles elucidating the history of the POPCOUNT instruction?

Figuring out how many bits in a group of bits are set to 1, known as computing "population count", Hamming weight, or "bit summation", among others, has various applications. It is also fairly cheap ...
32
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5answers
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Executable ASCII files before x86?

I've known about a technique allowing to bootstrap arbitrary 16-bit x86 code from a subset of instructions representable as printable ASCII bytes since the early 1990s. The first example of an ASCII ...
17
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3answers
774 views

PDP-11 instruction set inconsistencies

The wiki page PDP-11 architecture has a cryptic paragraph with no references or examples: Inconsistent instructions Over the life of the PDP-11, subtle differences arose in the implementation ...
22
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1answer
2k views

What is the relative code density of 8-bit microprocessors?

When RAM is at a premium, as it was in the old days, a greater code density of an instruction set can be a substantial advantage. (Click saver: Code density refers loosely to how many microprocessor ...
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0answers
302 views

What are some programs known to take advantage of Intel MMX instructions? [closed]

Although MMX techology, introduced in 1997, is long superseded by SSE series of extensions, it's still included in every modern x86 CPU for backwards compatibility. What are some examples of ...
9
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1answer
283 views

Was there ever a division instruction with sign of the remainder following the sign of the quotient?

A document about the precursor to ANDF (Architecture Neutral Distribution Format) mentions that two of its integer division/remainder primitives "are those generally implemented directly by processor ...
10
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1answer
293 views

Timing and microcode in the PDP-11/40

I've been trying to figure out how the sequence of microcode instructions as described in the schematics and ROM listing in the PDP-11/40 (KD11-A Processor) relates to the timings given in Appendix C ...
11
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2answers
474 views

Was the PDP-11 coroutine instruction actually used?

In a typical description of the coroutine mechanism it usually mentioned that the PDP-11 instruction set provided a way to effect the coroutine switch by a single instruction, namely JSR PC,@(SP)+. ...
40
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2answers
5k views

Why did the PDP-11 include a JMP instruction?

The PDP-11's program counter was addressable in two ways: as a general purpose register or as a memory location. Still, the PDP-11's instruction set included separate instructions for moving a new ...
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2answers
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What is the instruction set of the PDP-7?

The PDP-7 was a bookshelf-sized "minicomputer", with keyboard, magnetic and paper tape and printer I/O. According to Wikipedia, the first version of UNIX (then named Unics) was programmed in assembly ...
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2answers
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How did the Z80 instruction set differ from the 8080?

The Zilog Z80 microprocessor, known for its use in the ZX Spectrum, was designed to be a backwards-compatible extension to the Intel 8080 processor. It introduced several new instructions to the 8080'...