Instruction storage and usage
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Instruction Storage and Usage. Instructions are stored in a memory called instruction memory

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Instruction Storage and Usage

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Instruction storage and usage

Instruction Storage and Usage

  • Instructions are stored in a memory called instruction memory

  • To execute each instruction, we get the instruction out of memory, store in a register called instruction register (may be more than one single register of n-bits depending on the number of words in an instruction), and carry out operations on data path to achieve the desired result

  • It may take more than one cycle to execute an instruction

  • Each cycle is called phase (or state) in instruction execution

  • Actions in one cycle is stored in one nano-instruction

  • For each instruction (micro-instruction per book), multiple nano instructions, one for each phase, are stored in nano_inst_mem

  • Instructions are fetched one at a time pointed by a counter, called program counter

  • For each instruction, nano instructions are fetched from nano memory, using a counter called nano-program counter


Adding program memory

Inst

Mem

WIAD

Nano

Inst

Mem

INAD

PC

IAddr

Control

NPC

INST

NAddr

IIAD

WNAD

Input

Output

RA1

RA2

M-

Reg

WOUT

Reg

File

OpCode

EMR

EALU

EI

S

H

I

F

T

A

L

U

WM

Mem

Unit

Cond

Code

RM

WR

Data

Addr

SC

WA

EM

WDAD

Adding Program Memory

  • Program memory has

    • A PC

    • Memory

    • Mechanism to change PC


Adding conditional branches

Inst

Mem

Nano

Inst

Mem

IIAD

INAD

PC

INST

Control

NPC

IAddr

NAddr

WIAD

WNAD

Input

Output

RA1

RA2

M-

Reg

WOUT

Reg

File

OpCode

EMR

EALU

EI

S

H

I

F

T

A

L

U

WM

Mem

Unit

Cond

Code

RM

WR

Data

Addr

SC

WA

EM

WDAD

Adding Conditional Branches

  • ALU generate status bits like

    • carry, overflow, sign, and zero

    • zero=1 means result from ALU is zero

  • These bits can be used to change flow of program

    • A new address is generated using CCs


Conditional branch instruction

Inst

Mem

Nano

Inst

Mem

IIAD

INAD

PC

INST

Control

NPC

IAddr

NAddr

WIAD

WNAD

Input

Output

RA1

RA2

M-

Reg

WOUT

Reg

File

OpCode

EMR

EALU

EI

S

H

I

F

T

A

L

U

WM

Mem

Unit

Cond

Code

RM

WR

Data

Addr

SC

WA

EM

WDAD

Conditional Branch Instruction

  • Program change flow

  • For example

    • If.. Then‚Ķ Else

  • Condition codes determine flow

  • PC is changed


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