Best note

1. Haramaya University
College of Computing and Informatics
Department of Computer Science
Computer Organization and Architecture(COA)
04/05/2025 1

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Chapter - 5
Basic Computer Organization and Design

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Outline
Instruction Codes
Computer Registers
Computer Instructions
Timing and Control
Design of Basic Computer

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Instruction Codes
o A program is a set of instructions that
specify the operations, operands, and the
sequence by which processing has to occur.
o A computer instruction is a binary code
that specifies a sequence of micro-
operations for the computer.
o An instruction code is a group of bits
that instruct the computer to perform a
specific operation.
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Instruction Codes
The user of a computer can control the process by means
of a program
A program is a set of instructions that specify the
operations, operand, the sequence(control)
An instruction is a binary code that specifies a sequence
of micro operations.
Instruction codes together with data are stored in
memory(Stored Program Concept)
Instruction Cycle: The computer reads each instruction
from memory and places it in a control register. The
control then interprets the binary code of the instruction
and proceeds to execute it by issuing a sequence of
micro operations.

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Instruction Codes…
o Instruction format:
o Two major components of an instruction code:
o The operation code (opcode)
o Address
o An opcode specifies the operation for the instruction.
o An address specifies the registers and/or locations
in memory to use for that operation
Opcode
Fig: Instruction Format
11 0
15 14
I Address
12
Addressing
mode
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Instruction Codes…
o Address Mode:
o The address field of an instruction represents
o Direct address: the address in memory of the data to
use
(the address of the operand)
oIndirect address: the address in memory of the
address in memory of the data to use
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Basic Computer Registers
Fig: Basic Processor
registers
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Addressing Mode
Immediate operand address: The second part of an
instruction code(address field) specifies an operand
 Operand is specified in the instruction itself.
Direct operand address: The second part of an instruction
code specifies the address of an operand
Indirect operand address :The bits in the second part of the
instruction designate an address of a memory word in
which the address of the operand is found
one bit of the instruction code is used to distinguish
between a direct and an indirect address
Effective Address: The operand address in computation-
type instruction or the target address in a branch-type
instruction

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Computer Registers
Basic computer registers and memory
Data Register(DR) : hold the operand(Data) read from
memory
Accumulator Register(AC) : general purpose processing
register
Instruction Register(IR) : hold the instruction read from
memory
Temporary Register(TR) : hold a temporary data during
processing
Address Register(AR) : hold a memory address, 12 bit.

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Cont…
Program Counter(PC) : Hold the address of the next
instruction to be read from memory after the current
instruction is executed.
Instruction words are read and executed in sequence
unless a branch instruction is encountered.
A branch instruction calls for a transfer to a
nonconsecutive instruction in the program.
The address part of a branch instruction is transferred to
PC to become the address of the next instruction.
To read instruction, memory read cycle is initiated, and
PC is incremented by one (next instruction fetch)

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Cont…
Input Register(INPR) : receive an 8-bit
character from an input device
Output Register(OUTR) : hold an 8-bit
character for an output device

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Basic Computer Registers…
Fig: Registers
Connected to
Common
Bus
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Cont…
Common Bus System
The basic computer has eight registers, a memory unit,
and a control unit
Paths must be provided to transfer information from one
register to another and between memory and registers
A more efficient scheme for transferring information in a
system with many registers is to use a common bus
The connection of the registers and memory of the basic
computer to a common bus system

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Computer Instructions
o The basic computer has three instruction
code formats:
15
011

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Computer Instructions
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Computer Instructions…
Fig: Basic Computer
Instructions
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Computer Instructions…
o Instruction Types:
o Functional Instructions
o Arithmetic, logic, and shift instructions
o ADD, CMA, INC, CIR, CIL, AND, CLA
o Transfer Instructions
oData transfers between the main memory
and the processor registers
o LDA, STA
o Control Instructions
oProgram sequencing and control
o BUN, BSA, ISZ
o Input/Output Instructions
oInput and output
oINP, OUT
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Timing and Control
o Control Unit:
o Control units are implemented in one of
two ways:
o Hardwired Control
oCU is made up of sequential and combinational
circuits to generate the control signals
o Microprogrammed Control
oA control memory on the processor contains
microprograms that activate the necessary
control signals
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Cont…
Two major types of control organization
Hardwired Control
 The control logic is implemented with gates, decoders, and other
digital circuits,
 Fast operation.
 Wiring change (if the design has to be modified)
Micro programmed Control
 The control information is stored in a control memory, and the
control memory is programmed to initiate the required sequence of
micro operations.
 Any required change can be done by updating the micro program in
control memory.
 Slow operation

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Instruction Cycle
o In the basic computer each instruction
cycle consists:
i. Fetch an instruction from memory
ii. Decode the instruction
iii. Read the effective address from memory if the
instruction has an indirect address.
iv. Execute the instruction.
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Design of Basic Computer
The basic computer consists of the following hardware
components
1. A memory unit with 4096 words of 16 bits
2. Eight registers: AR, PC, DR, AC, IR, TR, OUTR, INPR
3. Seven F/Fs: I, S, E, R, IEN, FGI, and FGO
4. Two decoder in control unit: 3 x 8 operation decoder, 4
x 16 timing decoder
5. A 16-bit common bus
6. Control Logic Gates:
7. Adder and Logic circuit connected to the AC input

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Input-Output and Interrupts…
o External events can force the computer to quit
the normal program flow to react fast on the
events.
o Such event that requires fast response by the
computer is called interrupt.
o interrupt routine: The collection of
instructions that have to be executed to
respond to the interrupt
o IEN (Interrupt-enable flip-flop)
o can be set and cleared by instructions
o when cleared, the computer cannot be interrupted
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24. Questions
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