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Computer Organization and Architecture/Introduction to Computer Organization and Architecture

Notes
Zero Address Instructions
The locations of the operands in zero address instructions are defined implicitly. These instructions
store operands in a structure called pushdown stack.
We now know that one address instruction uses less number of bits, whereas three address
instructions are uses more number of bits. Similarly, the three address instructions require more
memory access when compared to one address instructions. Thus, three address instructions take
more time to execute instructions when compared to one address instructions.
To reduce the execution time of the instructions, it is advised to refer the operands from the
processor registers instead of referring the operands from the memory.

1. Write an assembly language program to subtract two numbers.

Lab Exercise 2. Write an assembly language program to find the average of two numbers.
7.5 Summary
• A Central Processing unit (CPU) is the main unit of a computer system.

• There are three main components of CPU, namely register set, ALU, and control unit.
• Registers are the temporary storage, which are constructed from flip-flops. They store the
status of the CPU.
• Stack is considered as a memory unit with an address register. It has a stack pointer, which
always points at the top item in the stack.
• If stack is organized as a collection of registers, then stack is considered as register stack. If
implemented in a random access memory attached to the CPU, then stack is considered as
memory stack.
• Instruction format is the layout of bits in an instruction. An instruction includes fields such as
opcode, operands, and addressing mode.
7.6 Keywords

Bus: An electrical conductor that connects all internal computer components to the CPU and the
main memory.
Displacement Value: A value added to the contents of the address register and the resulting value
is used as the address of the operand.
Immediate Field: A field that includes the address offset field for branches, load/store instructions,
and jump target fields.
Multiplexer: A device that can combine several input signals into one output.

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