IT253: Computer Organization

Lecture 4:

Instruction Set Architecture

Tonga Institute of Higher Education

Instruction Set Architecture: ISA

Instruction Set Architecture

• The ISA describes the level of the hardware/software interface. A level which was developed before any other level.

• It is the way to communicate between hardware and software

• An ISA describes the instructions that are able to execute simple commands on a processor

ISA

• When you compile a program, it changes your source code into a list of instructions which work on your processor type

• There are many types of ISA’s used. • Each processor has their own ISA• We will learn a type of ISA called MIPS,

which is used by many schools to teach, and also by many types of processors in use.

ISA

• The ISA level is used to go between high level languages and hardware.

• People who make processors talk to people who make compilers to decide what the ISA should be like.

• Two important issues to consider in ISA design– Can the set of instructions be implemented

efficiently – Will it be easy for chip designers and compiler

designers

Thinking about ISAsRequirements for an ISA

What operations do we need?

What should be made in Hardware?

int x = 5;for (int k = 0; k < 15; k++) { System.out.println(k);}

if (x > k) { int [] array = new int[5]; }

int z = (x | 7) / 56;

Design for an ISA

• What do we need for a good ISA?– Operations (add, subtract,multiply)– Explicit operands (constants – 1,2,3)– Storage (where the variables live)– Memory Address (how we get to variables)– Type of variable (int, double)

• This means we need,– Arithmetic and logic instructions– Data transfer instructions (load, store)– Control transfer (getting to different places in the program)

ISA Interface

• An ISA is a way to define an interface – a way for the software to run on the hardware

• A good interface will last a long time– Efficient– Used in different ways (versatile)– Easy to use

Typical ISA interface

Efficiency in instructionsFor a simple command like

C = A + BWhat instructions to we need to go through

A two address ISAA 2 address ISA uses two explicit operands, and one implicit. That means that the commands look like:

add tmp1,a -- In ISAtmp1 = tmp1 + a -- in high level

A three address ISA

• An ISA with three explicit operands.

Using Registers

• Registers are memory inside the processor that is used to store variables the CPU is currently working with

• It is where the numbers for operations like add and subtract are stored

• All computers use registers• They are very fast because they exist on the

CPU. • In fact, there could be no CPU without the

registers

Registers

• A register is a place to hold variables needed for an instruction

• Like memory, but much smaller.

• Usually there are 32, 32-bit registers.

• Each register will use 32 bits to save data

• Sometimes there are special registers for floating point numbers

3 Address ISA with Registers

• Just like 3 address ISA, but now we can use registers instead of saying each variable

A Load/Store ISA

• We have seen an “add” instruction. It is similar for subtracting, multiplying, dividing

• What if we need to use memory?• Load and store commands allow a programmer

to move data to and from memory.• It is the only way to actually use memory in order

to save data for a longer period of time• Memory in this case means the cache, RAM or

virtual memory

Load/Store ISA

• Example of Load and Storing ISA commands

Registers

• Registers can hold data like integers.• This means they can also hold addresses

of memory (an address is an integer, like a pointer)

Control Operations

• "Control flow" is the word used to describe the order the instructions are executed in

• You can change the order with if/else, for loops and functions (methods)

int main() { cout << “hello” << endl; int x = GetNumber(); cout << x << endl;}

int GetNumber() { return 7;}

public static void main(String[] args) { System.out.println(“hello”); int x = GetNumber(); System.out.println(x);}

public static int GetNumber() { return 7;}

Control Operations

• If there is a function, the “control flow” of the program changes.

• The program jumps out of "int main()" and into "int GetNumber()"

• You can also change control flow with conditionals (if statements), loops (for and while)

Control Flow: Conditionals

• It is a very common action to compare two values and take action on the result

if (x == 6) { then do this; }

else if (x == 7) { do that; }

• This is called a “branch” statement.

• It is also known as a conditional.

• It is also an if statement.

• Know all those names!

Control Flow: ExampleC++/: if (a == b) {Java c = 1; } else { c = 2; }

MIPS: // we assume $s1 = a, $s2 = b, $s3 = c

beq $s1, $s2, then // if a==b goto “then”li $s3,2 // c = 2;j done // jump to done

then: li $s3,1 // then c = 1; done: // exit program

ISA Commands we need

ISA

• We have seen typical 2 and 3 address instruction set architectures.

• These were only examples of the form of an ISA. A real ISA, like MIPS, will fill in the missing details

• An ISA needs registers, operands and operations

• It needs operations like add, load and store• It needs control operations like jumps and

branching

ISA

• But all these are only commands written in text. A CPU can only understand binary

• The key to this problem is that an ISA can be directly translated into binary.

• This means each instruction has a special binary code.

• Each register has a special binary code

Making instructions machine readable

• Programmers are able to read and write commands like the following (in text)– add r1,r2,6

• A processor needs a series of bits to work with• The goal is to translate an instruction to a 32 bit

binary number. • It will be like floating point, where certain parts of

the instruction are saved in certain parts of the 32 bit integer

Examples of this translation

Machine Codes

Machine Code

• Let’s look at one example of the machine code translation

Op: This is where we tell whether it is a add, subtract, load, store, and so on. There is a special binary code for each operationSince it is a 6 bit field, we have 6 bits to save the code.

For example: Add = 000001

Rs1 = the address of the first register (choose 1 out of 32)Rd = the address of the register of the destinationImmediate = a constant integer value

Summary for ISA

• ISA is the language that computers use to allow software to communicate with hardware

• We need certain types of instructions to make a complete ISA

• These instructions will perform operations, load memory, execute conditionals

• Once we have an ISA, the computer must convert each instruction to a machine readable format (it has to be binary!)

• This is our introduction for the MIPS ISA