The document discusses computer architecture, focusing on instruction codes and their components, which include the operation part and address part. It explains different addressing modes (immediate, direct, indirect) and details the instruction cycle phases: fetch, execute, indirect, and interrupt. Additionally, it highlights the importance of the control unit, instruction register, and processor registers in executing instructions stored in memory.

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Computer Architecture and
Design
Computer Architecture and Design
Computer System
Engineering Department

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Instruction Code
Embedded System Development
Lecture 1

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Instruction code
 An instruction code is a group of bits that tells
the computer to perform a specific operation part.
 A computer instruction is a binary code that
specifies a sequence of micro-operations for the
computer.
 Instruction codes and data are stored in memory.
 Computer read each instruction from memory and
places it in a control register.
 Control unit interprets the binary code of the
instruction and proceeds to execute it by issuing a
sequence of micro-operations.

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Instruction code
 It is divided into two parts operation part and Address
part.
 Operation code
 Group of bits
 Operations
 add
 subtract
 shift
 complement.
 The number of bits required for the operation code
depends on the total number of operations available in
the computer.
 2n
distinct operations  n bit operation code

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Instruction code

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Instruction code

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Instruction code (Addressing)
 Three Addressing Modes used for
address portion of the instruction
code.
Immediate: the operand is given in
the address portion (constant).
Direct: the address points to the
operand stored in the memory.
Indirect: the address points to the
pointer (another address) stored in
the memory that references the
operand in memory.

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Instruction code (Addressing)

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Computer Instruction
 Computer instructions are
normally stored in consecutive
memory locations and executed
sequentially one at a time.
 Control unit reads an instruction
from a specific address in memory
and executes it.
 Needs counter to calculate the
address of the next instruction
after execution of the current
instruction is completed.

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Computer Instruction
 Instruction Register IR is used in
the control unit for storing the
instruction code after it is read from
memory.
 The computer needs processor
registers or Accumulator
register AC for manipulating data
and a register for holding a memory
address.

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Computer Instruction

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Instruction format (Memory ref inst)

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Instruction format (Register ref inst)

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Instruction format (input - output ref inst)

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Timing and control

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Instruction cycle
 Phases of Instruction Cycle :
1- Fetch cycle
2- Execute cycle
3- Indirect cycle
4- Interrupt cycle
 Instruction cycle repeats
indefinitely unless a HALT
instruction is encountered.

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Fetch cycle
c0T0: MAR PC
←
c0T1: MBR MAR, PC PC+1
← ←
c0T2: OPR MBR(OP),
←
I MBR(I)
←

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Flow chart Instruction cycle

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Execution of Memory Ref Inst

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Execution of Branch and save return
Address (BSA)
M[AR] PC, PC
AR+1

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Complete Computer Description

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ALLAH HAFIZ