Micro Operation (Computer Architecture)

In computer CPU the micro-operations are detailed low-level instructions which is used in some designs to implement complex machine instructions. Usually, micro-operations perform basic operations on data stored in one or more register, including transferring data between registers or between registers and external buses of the CPU and performing arithmetic or logical operations on registers. The execution of micro-operations is performed under control of the CPU’s control unit which decides on their execution while performing various optimizations such as reordering, fusion and caching of the operations.

The operations executed on data stored in registers are called micro-operations. A micro-operation is an elementary operation performed on the information stored in one or more registers. The functions built into registers are examples of micro operations, They are Shift, Load, Clear, Increment, add, subtract, complement, and, or, xor etc ..

There are Four Types of Micro Operations

1. Register transfer micro-operations

2. Arithmetic micro-operations

3. Logic micro-operations

4. Shift micro operations

1) Register transfer micro operation

Register transfer language is a kind of intermediate representation (IR) that is very close to assembly language, such as that which is used in a compiler. It is used to describe data flow at the register transfer level of an architecture. It is a convenient tool for describing the internal organization of digital computers in concise and precise manner. These types of micro operations are used to transfer binary information from one register to another. Often the names indicate function:

MAR – Memory Address Register
PC - Program Counter
IR - Instruction Register

In this case the contents of register R1 are copied (loaded) into register R2

2) Arithmetic Micro-Operations

These micro-operations are used to perform some arithmetic operations on numeric data stored in the registers. A micro operation is an elementary operation performed on the information stored in one or more registers. The micro operation in digital computers is of 4 types: Addition, Subtraction, Increment, and Decrement

Addition:-

R1 = A + B

R3 = R1 + R2

Subtraction: -

R1 = R2 – R3

Subtraction operation also can be performed on 2’s complement R = R1 + R2’ + 1

Increment:-

R1 = R1 + 1, R2 = R2 + 1

Decrement:-

R1 = R1 – 1, R2 = R2 – 1

3) Logic Micro-Operations

These micro operations are used to perform bit style operations or manipulations on non-numeric data. In computer CPU, micro-operations are the functional or atomic operations of a processor. Logic micro operations are bit-wise operations, i.e., they work on the individual bits of data. These are useful for bit manipulations on binary data and also useful for making logical decisions based on the bit value.

R1 = R1 XOR R2

R2 = R3 AND R4

4) Shift Micro-Operations

These are used for serial transfer of data. That means we can shift the contents of the register to the left or right. In the shift left operation the serial input transfers a bit to the right most position and vice versa.

There are three types of shifts as follows:
a) Logical Shift

It transfers 0 through the serial input. The symbol "shl" is used for logical shift left and "shr" is used for logical shift right.

              R1 ← she R1
              R1 ← she R1

The register symbol must be same on both sides of arrows.

b) Circular Shift

This circulates or rotates the bits of register around the two ends without any loss of data or contents. In this, the serial output of the shift register is connected to its serial input. "cil" and "cir" is used for circular shift left and right respectively.

              R1 = cir R1
              R2 = cil R2

c) Arithmetic Shift

This shifts a signed binary number to left or right. An arithmetic shift left multiplies a signed binary number by 2 and shift left divides the number by 2. Arithmetic shift micro-operation leaves the sign bit unchanged because the signed number remains same when it is multiplied or divided by 2.

R ← ashl R (arithmetic shift left R (register))

R ← ashr R (arithmetic shift right R (register))