Unit-4
Instruction Codes
A computer instruction is a binary code that determines the micro-
operations in a sequence for a computer. They are saved in the memory
along with the information. Each computer has its specific group of
instructions.
They can be categorized into two elements as Operation codes
(Opcodes) and Address. Opcodes specify the operation for specific
instructions. An address determines the registers or the areas that can be
used for that operation. Operands are definite elements of a computer
instruction that show what information is to be operated on.
The following block diagram shows the instruction format for a
basic computer.
There are three parts of the Instruction Format, which are as
follows -
Addressing Modes
,Instructions that define the address of a definite memory location are
known as memory reference instructions. The method in which a target
address or effective address is recognized within the instruction is
known as addressing mode.
The address field for instruction can be represented in two different
ways are as follows −
Direct Addressing − It uses the address of the operand.
Indirect Addressing − It facilitates the address as a pointer to the
operand.
The address of the operand or the target address is called the effective
address.
Effective Address (EA) − It defines the address that can be executed as a
target address for a branch type instruction or the address that can be
used directly to create an operand for a computation type instruction,
without creating any changes.
Opcodes
An opcode is a collection of bits that represents the basic operations
including add, subtract, multiply, complement, and shift. The total
number of operations provided through the computer determines the
number of bits needed for the opcode. The minimum bits accessible to
the opcode should be n for 2n operations. These operations are
implemented on information that is saved in processor registers or
memory.
Address
The address is represented as the location where a specific instruction is
constructed in the memory. The address bits of an instruction code is
used as an operand and not as an address. In such methods, the
, instruction has an immediate operand. If the second part has an address,
the instruction is referred to have a direct address.
There is another possibility in the second part including the address of
the operand. This is referred to as an indirect address. In the instruction
code, one bit can signify if the direct or indirect address is executed.
The figure shows a diagram showing direct and indirect addresses.
Common Bus System
Instruction Codes
A computer instruction is a binary code that determines the micro-
operations in a sequence for a computer. They are saved in the memory
along with the information. Each computer has its specific group of
instructions.
They can be categorized into two elements as Operation codes
(Opcodes) and Address. Opcodes specify the operation for specific
instructions. An address determines the registers or the areas that can be
used for that operation. Operands are definite elements of a computer
instruction that show what information is to be operated on.
The following block diagram shows the instruction format for a
basic computer.
There are three parts of the Instruction Format, which are as
follows -
Addressing Modes
,Instructions that define the address of a definite memory location are
known as memory reference instructions. The method in which a target
address or effective address is recognized within the instruction is
known as addressing mode.
The address field for instruction can be represented in two different
ways are as follows −
Direct Addressing − It uses the address of the operand.
Indirect Addressing − It facilitates the address as a pointer to the
operand.
The address of the operand or the target address is called the effective
address.
Effective Address (EA) − It defines the address that can be executed as a
target address for a branch type instruction or the address that can be
used directly to create an operand for a computation type instruction,
without creating any changes.
Opcodes
An opcode is a collection of bits that represents the basic operations
including add, subtract, multiply, complement, and shift. The total
number of operations provided through the computer determines the
number of bits needed for the opcode. The minimum bits accessible to
the opcode should be n for 2n operations. These operations are
implemented on information that is saved in processor registers or
memory.
Address
The address is represented as the location where a specific instruction is
constructed in the memory. The address bits of an instruction code is
used as an operand and not as an address. In such methods, the
, instruction has an immediate operand. If the second part has an address,
the instruction is referred to have a direct address.
There is another possibility in the second part including the address of
the operand. This is referred to as an indirect address. In the instruction
code, one bit can signify if the direct or indirect address is executed.
The figure shows a diagram showing direct and indirect addresses.
Common Bus System
