Describe different types of instruction

Instructions available in a processor may be broadly classified into six groups.

This are-

(i) Data transfer instructions

(ii) Arithmetic instructions

(iii) Logical instructions

(iv) Program control instructions

(v) System control instructions

(vi) I/O instructions

(i) Data transfer instructions: Data transfer instructions are concerned primarily with data transfers between the processor and main memory. Typically an ideal instruction set must be able to handle the following transfers:

• Register to register
•  Register to memory
• Memory to register     
•  Memory to memory

(ii) Arithmetic instructions: All instruction sets typically include ADD and SUBTRACT instructions.

Example: LEA Z, A2; Load the address of Z into the address register A2.

(iii) Logical instructions: Invariably, the instruction sets of all contemporary processors include instructions to perform Boolean AND, OR, NOT and EXCLUSIVE-OR operations on a bit-by-bit basis.

Example: LSR.W #3,D5 – This instruction performs a left logical shift of the low-order, 16 bits of the data register D5 by three places.

(iv) Program control instructions: In a conventional computer, instructions are always executed in the same order they are presented. Instructions that perform this are called program control instructions. These instructions may be classified into four groups:

• ·     Unconditional branch instructions
•       Conditional branch instructions
•        Subroutine call instructions
•        Interrupt handling instructions

Example: MOV AB X, R7; Move the address of X register into R7.

(v) System control instructions: With the advent of low cost VLSI microprocessors, designing systems with several processors is feasible. Such a multiprocessor system has significant advantage, such as high processing speed and more reliability.

Example: LOOP1       TAS           TEST

                                      BNE           LOOP1

                                      CLR.B       TEST

(vi) I/O instructions: I/O instructions allow a processor to perform input and output operations. An input instruction allows a peripheral to transfer a word to either a CPU register to memory. Similarly, an output instruction enables a processor to transfer a word into the buffer register of a peripheral device.

Example: IN, OUT.