REGISTERS AND REGISTER

TRANSFERS

Basic Symbols

1

EKT221 - DIGITAL ELECTRONIC II

Outlines

Register Transfer Language

Type of Registers

Basic Symbols

Register Transfer

Arithmetic Microoperations

HIGH LEVEL LANGUAGE

Example : C+, VB, JAVA

ASSEMBLY LANGUAGE

Example : uP and uC

OPCODE

MICROCODE

Mircocode (Micro-operations):

Operations executed on data

stored in registers, performed in

one clock cycle

Register Transfer Language

(RTL):

Symbolic notation used to

describe micro-operations

Register Transfer Language (RTL)

RTL

• to represent registers

• specify the operations

on their contents

RTL

Is an algebraic notation used to define machine

level operations

It is not executed by a computer

It is used to explain how the computer works.

Example:

In 68000 assembly language instruction

ADD #3, D2

is defined in RTL as

[D2] [D2] + 3

Register Transfer Language (RTL)

Types of Registers

AR : Address Registers

A register holds an address for memory

unit

DR: Data Registers

PC : Program Counter

IR : Instruction Register

Rn : n indicates the Register number

e.g. R2 is Register 2

Block Diagram of Registers

R

PC(H)

Register

16 bit Register

7

8 bit Register

6 5 4 3 2 1 0

Bit 7 Bit 0

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Bit 16 Bit 0

PC(L)

8 bit = 1 byte

H = High order byte L = Low order byte

PC(H) = PC(15:8) PC(L) = PC(7:0)

BASIC SYMBOLS

R followed by a number is referring to a

register:

R2 = second register/register no. 2

R2

BASIC SYMBOLS

M refers to Memory with addresses in square

braces:

Direct Addressing :

M[10] = contents of memory address 10

In this example,

M[10] refers to 10111011

10000000

10111011

11111111

9

10

11

Address Content

MEMORY

BASIC SYMBOLS

In-direct Addressing :

M[R3] = content of the memory address

in R3

10000000

10111011

11111111

15

16

17

Address Content

MEMORY

10000000

00000110

00001111

1

2

3

Address Content

REGISTER

00001111

= 15

Ans : M[R3] refers to 10000000

BASIC SYMBOLS

Arrow pointing to the right shows transfer of

data :

R4 R3 = Stores the value of R3 into R4

The word transfer is misleading, since it implies

that data is moved from one location to

another. In fact, the data is copied from one

location to another since it also still resides in

register R3

=> Contents of the source REG do not change.

Only change the contents of destination REG

Destination Source

BASIC SYMBOLS

A comma represents simultaneous transfer:

R1 R2 , R6 R7

Stores R2 into R1 and at the same time stores

R7 into R6.

BASIC SYMBOLS

A colon indicates the termination of function:

K : R1 R2

If K=1, then stores R2 into R1.

K1K2’: R3 R2

If K=1 and K2=0, then stores R2 into R3.

K: a control signal generated in the control unit, 0 or 1

BASIC SYMBOLS

Parenthesis (bracket) indicates part of the register.

R8(1) = bit 1of R8

R8

7 6 5 34 2 1 0

1 0 1 11 0 0 0

Bit Position

Content

MSB LSB

LSB : Least Significant Bit

MSB : Most Significant Bit

BASIC SYMBOLS

Parenthesis indicates part of the register.

R3(7:0) = the least significant byte of R3

Note : 1 byte = 8 bit

R3

7 6 5 34 2 1 0

1 0 1 11 0 0 0

14..915 8

10..1

Summary

Symbol Description Example

Square brackets Specifies an address for

memory

M[R2]

Capital Letters

& Numerals

Denotes a register AR, IR, PC, R2

Parentheses Denotes part of a

register

R2(1), R2(7:0),

PC(L)

Arrow Denotes Transfer of data R1 R2

Colon Denotes termination of

control function

K:

Comma Separates simultaneous

transfers

R1 R2, R3 R2

ARITHMETIC MICROOPERATIONS

The basic arithmetic microoperations areAddition

Subtraction

Increment

Decrement

The additional arithmetic microoperationsareAdd with carry

Subtract with borrow

Transfer/Load

etc. …

Arithmetic Microoperations

MATHEMATICAL AND

LOGICAL SYMBOLS

Addition is indicated by the + sign:

R1 R2 + R3

Add R2 and R3, stores in R1

R2 R4 + R1

Add R4 and R1, stores in R2

Example 1 :

Example 2 :

MATHEMATICAL AND

LOGICAL SYMBOLS

Subtraction is handled not with the minus

sign but with complementing.

1’s complement :

2’s complement :

R5 R3+R4

R5 R3+R4+1

R3 minus R4 in 1’s

complement

R3 minus R4 in 2’s

complement

Note : Computers use the 2’s complement for negative integer

numbers in all arithmetic operations.

+ Addition

- Subtraction

* Multiplication

/ Division

Example: R2 R1+R2

Example: R2 R1+R2’+1

Example: R2 R1*R2

Example: R2 R1/R2

ARITHMETIC MICROOPERATIONS

Summary of Typical Arithmetic Microoperations

(Page 547)

Arithmetic Microoperations

ARITHMETIC MICROOPERATIONS

Note : Any register may be specified for source 1, source 2, or

destination.

MICROOPERATIONS

Microoperations - The operations performed

on data stored in registers or in memory.

The functions built into registers are

examples of microoperations

Shift

Load

Clear

Increment

…

MICROOPERATIONS

The microoperations most often encountered in digital systems are of four types:

Register transfer microoperations

Transfer binary data from one REG to another

Arithmetic microoperations

Perform arithmetic on data in REG

Logic microoperations

Perform bit manipulation on data in REG

Shift microoperations

Shift data in REG

REGISTER TRANSFER

Copying the contents of one register to

another is a register transfer.

A register transfer is indicated as

R2 R1 In this case the contents of register R1 are copied

(loaded) into register R2.

A simultaneous transfer of all bits from the source R1

to the destination register R2, during one clock pulse.

Note that this is a non-destructive; i.e. the contents of

R1 are not altered by copying (loading) them to R2

REGISTER TRANSFER

A register transfer such as

R3 R5Implies that the digital system has

the data lines from the source register (R5) to

the destination register (R3)

Parallel load in the destination register (R3)

All bits are loaded simultaneously on clock pulse

(same source)

Control lines to perform the action

Control the transfer or clock cycles

CONDITIONAL REGISTER TRANSFER

Conditional Statement

Using control signal to control the transfer

Can be symbolized by if-then statement

if (K1

= 1) then (R2 R1)

In RTL we can write it as:

K1

: R2 R1

The transfer operation be executed only if K1=1

A subscripted letter followed by a colon is a conditional

K1: control signal

CONDITIONAL REGISTER TRANSFER

R2R1

K1

CLK

n

K1

CLK

Transfer occurs here

n = no of lines = no of bits

Transfer occurs in parallel

K1 : R2 R1

Load control input

K1 :R5 R2

CONDITIONAL REGISTER TRANSFER

Content of R2 will be stored in R5 when

condition K1 occurs

R5

R2

K1

Example:

SIMULTANEOUS OPERATIONS

If two or more operations are to occur

simultaneously, they are separated with

commas

K: R3 R5 , AR IR

If the control function K = 1, load the

contents of R5 into R3, and at the same

time (positive clock edge), load the

contents of register IR into register AR

Register Transfer

COMPLEX LOGICAL SYMBOLS

Content of R5 will be stored in R4 only IF

both condition K1 and condition K2 are true:

K1K2: R4 R5

Register Transfer

If K1=1 and K2=1, then stores R5 into R4

COMPLEX LOGICAL SYMBOLS

Content of R5 will be stored in R4 IF

either condition K1 or condition K2 were true, + sign would be used:

(K1+K2) : R4 R5

NOTE:

In (K1 + K2), “+” means “OR”

In R1 ← R1 + R3, “+” means “plus”

Register Transfer

COMPLEX LOGICAL SYMBOLS

If – then – else is implemented with commas and multiple colons.

If K1 true, then stores R4 into R6, else if K2 true, then stores R5 into R6, else store R7 into R6.

K1 : R6 R4, K1K2 : R6 R5, K1K2: R6 R7

Register Transfer

Thank you