63 lines
2.4 KiB
Markdown
63 lines
2.4 KiB
Markdown
---
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categories:
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- Computer Architecture
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tags: [CPU]
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---
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# instruction set architectures
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We know that the [ALU](Arithmetic_Logic_Unit.md) is
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responsible for the "execute" stage of the
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[fetch, decode, execute](Fetch_decode_execute.md)
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cycle, implementing the most basic binary operations such as adding two numbers.
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Accross different machines and CPU types there can be differences in how the
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given instruction, say "add", is implemented. Not all computers will execute the
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instruction in the same way.
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A family of CPUs that do implement the core instructions of the ALU in the same
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way are said to share an **instruction set architecture** (ISA). Software that
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is built for a certain ISA works on any CPU that implements that ISA.
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> Note that this is a logical architecture rather than a physical architecture.
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> Like ISA processors may work very differently on the hardware level whilst
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> still adhering to the same ISA
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## ARM and x86
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The two main processor architectures are ARM and x86. x86 processors were first
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introduced by Intel but are also made by AMD.
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### x86
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The name stems from Intel's internal naming conventions - every new processor
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they introduced would have numerical designation ending with "86". Later they
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would adopt names such as "Pentium", "Celeron" etc. Regardless, these processors
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still fall under the "x86" family.
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Over time, new instructions have been added to the x86 architecture but they all
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maintain backwards compatibility with preceding generations.
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There have been different, successive generations of x86 corresponding to their
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[word-size](Signed_and_unsigned_numbers.md):
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16-bit, 32-bit, 64-bit. Word size here just means how many bits the processor
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can work with at a time.
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A 34-bit processor can operate on values that are 32 bits in length. This means
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it has 32bit registers, a 32-bit data bus, a 32-bit address bus and so on.
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AMD introduced the first 64-bit processor, not Intel but they later caught up. A
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64-bit x86 processor is often referred to as _x64_ or _x86-64_.
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### ARM
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Whilst x86 processors dominate the personal computer and server space, on mobile
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a different instruction set architecture predominates: ARM. The ARM company
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develops the architecture and standard which it licenses it to other companies
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to implment.
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They are preferred on mobile because they have reduced power consumption and
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lower costs compared to x86.
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With ARM processors the processor is on a single chip that contains the CPU as
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well as memory and other hardware.
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