An example of an instruction set is the x86 instruction set, which is common to find on computers today. Different computer processors can use almost the same instruction set while still having very different internal design. Both the Intel Pentium and AMD Athlon processors use nearly the same x86 instruction set. An instruction set can be built into the hardware of the processor, or it can be emulated in software, using an interpreter. The hardware design is more efficient and faster for running programs than the emulated software version.

Historically, the first two philosophies to instruction sets were: reduced (RISC) and complex (CISC). The merits and argued performance gains by each philosophy are and have been thoroughly debated.

Complex Instruction Set Computer (CISC) is rooted in the history of computing. Originally there were no compilers and programs had to be coded by hand one instruction at a time. To ease programming more and more instructions were added. Many of these instructions are complicated combination instructions such as loops. In general, more complicated or specialized instructions are inefficient in hardware, and in a typically CISC architecture the best performance can be obtained by using only the most simple instructions from the ISA.

The most well known/commoditized CISC ISAs are the Motorola 68k and Intel x86 architectures.

Reduced Instruction Set Computer (RISC) was realized in the late 1970s by IBM. Researchers discovered that most programs did not take advantage of all the various address modes that could be used with the instructions. By reducing the number of address modes and breaking down multi-cycle instructions into multiple single-cycle instructions several advantages were realized:

The most well known/commoditized RISC ISAs are the PowerPC, ARM, MIPS and SPARC architectures.

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The Instruction Set Architecture (ISA) is the part of the processor that is visible to the programmer or compiler writer. The ISA serves as the boundary between software and hardware. We will briefly describe the instruction sets found in many of the microprocessors used today. The ISA of a processor can be described using 5 catagories:

Where are the operands kept other than in memory?

How many operands are named in a typical instruction.

Can any ALU instruction operand be located in memory? Or must all operands be kept internaly in the CPU?

What operations are provided in the ISA.

What is the type and size of each operand and how is it specified?

Of all the above the most distinguishing factor is the first.

The 3 most common types of ISAs are:

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ISA includes functional definition of operations, modes, and storage locations supported by hardware and precise description of how to invoke, and access them.

 

·         Processors do not understand high level languages which are generally used to write programs now-a-days. Tools like compilers convert these programs into low level binary codes or instructions which the processor can understand and execute. These instructions are defined in the ISA.

·         ISA also define items which are available to programs like data types, registers etc. and memory addressing mode.

Thus the objective ISA is to let software to be executed by the hardware. ISA take programs and make them executable by processor by breaking the program into instructions that the processor can execute.

 

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§  A well-define hardware and software interface 

§  The “contract” between software and hardware 

Ø Functional definition of operations, modes and storage locations supported by hardware 

Ø Precise description of how to invoke and access them. 

§  An ISA is analogous to a human language 

Ø Allows communication 

·        Language : person to person 

·        ISA : hardware to software 

Ø Need to speak the same language/ISA 

Ø Many common aspects 

·        Part of speech: verbs, nouns, etc. 

·        Common operations : calculation, control/ branch, memory 

Ø Many different languages/ISAs, many similarities 

·        Different structure 

Ø Both evolve over time 

§  Key differences: ISAs must be unambiguous 

Ø ISAs are explicitly engineered and extended 

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-part of computer architecture related to programming

-include native data type, instruction,registers, addressing modes,memory architecture,interrup& exception handling & external I/O

  >example R1,R2…,PC

  >example MOV,ADD,INC,AND

-ISA specifies the set of opcodes(machine language), & native commands implemented by a particular processor

WELL KNOWN ISAs

-x86

>based on intel 8086 CPU in 1978

>Intel Family also followed by AMD

>X86-64

   = 64-bit extensions

   =proposed by AMD also followes by INTEL

-ARM

>32-bit &64-bit

>initially by ACORN RISC Machine

>ARM Holding

-MIPS

>32-bit & 64-bit

>by Microprocessor without Interlocked Pipeline Stages (MIPS) Technologies

A GOOD ISA

>last through many implementations

---portability,compatibility\

>used in many different ways

---Servers, Desktop, Laptop,Mobile, Tablet

>provides convenient  function to higher layer

>permit efficient implementation at lower layer

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An ISA defines everything a machine language programmer needs to know in order to program a computer. What an ISA defines differs between ISAs; in general, ISAs define the supported data types, what state there is (such as the main memory and registers) and their semantics (such as the memory consistency and addresing modes), the instruction set (the set of machine instructions that comprises a computer's machine language), and the input/output model.

Examples of instruction set

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EXAMPLES OF INSTRUCTION SET 

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Where are the operands kept other than in memory?

How many operands are named in a typical instruction.

Can any ALU instruction operand be located in memory? Or must all operands be kept internaly in the CPU?

What operations are provided in the ISA.

What is the type and size of each operand and how is it specified?

Of all the above the most distinguishing factor is the first.

The 3 most common types of ISAs are: