Control Unit Operation

Group 1

Michael BouizzaRolando Abreu

Carlos De CossioRicardo Urena

Michael HernandezRobert Romano

Sun Li Yang

Micro-Operations

A computer executes a program

Fetch/execute cycle

Each cycle has a number of steps

Called micro-operations

Atomic operation of CPU

Constituent Elements of Program Execution

Fetch - 4 Registers

Memory Address Register (MAR)

Memory Buffer Register (MBR)

Program Counter (PC)

Instruction Register (IR)

Fetch Sequence Address of next instruction is in PC Address (MAR) is placed on address bus Control unit issues READ command Result appears on data bus Data from data bus copied into MBR PC incremented by 1 Data (instruction) moved from MBR to IR MBR is now free for further data fetches

Fetch Sequence (symbolic)

t1: MAR <- (PC) t2: MBR <- (memory) PC <- (PC) +1 t3: IR <- (MBR) (tx = time unit/clock cycle)or t1: MAR <- (PC) t2: MBR <- (memory) t3: PC <- (PC) +1 IR <- (MBR)

Rules for Clock Cycle Grouping

Indirect Cycle

Interrupt Cycle

Execute Cycle (ADD)

ADD R1, X

Execute Cycle (ISZ)

ISZ X - increment and skip if zero

Execute Cycle (BSA)

BSA X - Branch and Save Address

Address of instruction following BSA is saved in X

Execution continues from X+1

Instruction Cycle Each phase decomposed into sequence of

elementary micro-operations Such as fetch, indirect, and interrupt cycles. Execute cycle

One sequence of micro-operations for each opcode

Assume new 2-bit register Instruction cycle code (ICC) designates which part

of cycle processor is in▪ 00: Fetch▪ 01: Indirect▪ 10: Execute▪ 11: Interrupt

Data Flow - Fetch Cycle

Data Flow – Indirect Cycle

Data Flow – Interrupt Cycle

Execute Cycle

Can take many forms, depending on the instruction located in the IR.

May Involve transferring data among registers, read/write from memory or I/O, and

This cycle may involve transferring data among registers, read or write from memory or I/O.

Functional Requirements

Define basic elements of processor Describe micro-operations processor

performs Determine functions control unit

must perform

CPU with the System Bus

Basic Elements of a Processor ALU Registers Internal data paths External data paths Control Unit

Types of Micro-operation Transfer data between registers

Transfer data from register to external

Transfer data from external to register

Perform arithmetic or logical ops

Functions of Control Unit

Sequencing Causing the CPU to step through a series

of micro-operations

Execution Causing the performance of each micro-

op

Control Signals

Control Signals Output

Electronic Signals that execute instructions

In CPU Causes Data Transfer Executes Instructions

In Control Bus Sends instructions out to I/O devices

Ex: Modem, Soundcard & Video card Sends signals to Memory

Modern I/O Controllers

Serial ATA Raid Controller on PCI Express

Control Signal Sequence – Fetch

MAR to PC Opens Communication Between PC and

MAR MBR to Memory

Steps Listed: Opens Communication Between MAR

and Address Bus Memory gets Read Control Signal Communication open Between Data Bus

and MBR

Data Paths and Control Signals

Internal Organization

Single internal bus Gates used in order to move through the buss Control Signals will send out electronics signals

to guide the transfer of information from the external systems bus

Temporary storage in registers are used for the proper functionality of ALU

CPU with Internal Bus

Intel 8085 CPU Block Diagram

Intel 8085 Pin Configuration

Intel 8085 OUT InstructionTiming Diagram

Hardwired Implementation

The control unit inputs signals are a combination of: Opcode Bits. Other signals such as interrupts, or

condition codes. Timing info (T1 to Tn). These signals

are essential for timing for proper sequencing through instruction cycles.

Control Unit with Decoded Inputs

Hardwired Implementation

Instruction register the part of the CPU’s control unit that stores the instruction currently being executed or decoded.

Op-code causes different control signals for each different instruction

Unique logic for each op-code Decoder takes encoded input and

produces single output n binary inputs and 2n outputs

Hardwired Implementation

Clock Repetitive sequence of pulses Useful for measuring duration of micro-

ops Must be long enough to allow signal

propagation Different control signals at different

times within instruction cycle Need a counter with different control

signals for t1, t2 etc.

Problems With Hard Wired Designs Complex sequencing & micro-

operation logic Difficult to design and test Inflexible design Difficult to add new instructions

Questions

What are a micro operations ? Why should you not read and write from the

same register at the same time ? what are the basic elements of a processor ? what happens when interupts are enabled ? Name 3 sub-cycles for the instruction cycles

More Questions

How many types of micro-operation? What are the inputs of the control signals? List modern I/O Devices What sends an electronic signals that organizes

various processor unit to execute an instruction ? What are advantages of hardware

implementation ?

References http://cnx.org/content/m29366/latest/

http://en.wikipedia.org/wiki/Arithmetic_logic_unit

http://www.cs.toronto.edu/~hehner/csc258/comp.html\

http://www.seas.upenn.edu/~palsetia/cit595s08/Lectures08/control1.pdf

http://www.hc11.demon.nl/thrsim11/68hc11/