Recap: Registers

A register is a single byte (or word) memory element.

Once written to (clocked) it remembers the input byte and outputs the same number regardless of further changes at the D input…

…until the next clock input of course.

CLK

D0-7 Q0-7D0-7 Q0-7

CLK

8 8

Interconnecting Registers

Interconnecting registers in a computer is like building roads to connect towns on a map:

Silly wayn towns require (n2-n)/2 roads

A

C

B

E

D

Interconnecting Registers

Interconnecting registers in a computer is like building roads to connect towns on a map:

Sensible wayFar fewer

roads, (but lesser

capacity)

A B

C

E

D

Interconnecting Registers

CLK

D0-7 Q0-7

CLK1

8 8

CLK

D0-7 Q0-7

CLK2

8 8

CLK

D0-7 Q0-7

CLK3

8 8

8

Register Connections

Data can be written from any register to any other register

Limitations Only one data transfer can happen at once In general, this is how computers work. They can only do a small number of very

simple operations… …and they can only do one of them at a time. But, they do it very quickly!

Bi-directional Connection

Connections from the common bus to the registers are bi-directional.

CLK

D0-7 Q0-7

8 8

CLK

Input / Output

8

Set of 8 digitally controlled switches

Input / OutputControl

Tri-State Ports

This arrangement is often known as a tri-state port.

Each input/output line can have three possible states: When being read from (input/output is low)

Low-Impedance, Logic 1 (or high) Low-Impedance, Logic 0 (or low)

Otherwise (input/output is high) High-Impedance (or floating, open-circuit)

Impedance

A simple switch can be thought of of having two states:

Low impedance (on) High impedance (off)

High impedance (tens of M)No current flows

Low impedance (tens of )Current flows

Equivalent Circuits for Tri-State Port

Low Impedance Logic 0

(Low)

High Impedance

Low Impedance Logic 1

(High)

+5 V

0 V

+5 V

0 V

+5 V

0 V

I/OI/O I/O

Three ports sharing a common bus.

Port 1 is in the high state, ports 2 and 3 are in the high impedance

states.

+5 V

0 V

I/O1 I/O2 I/O3

What NOT to do…

Three ports sharing a common bus.

Port 1 is in the high state, ports 2 is in the high impedance

state, port 3 is in low state.

Current

Only one port can be in a low-impedance (output) state at any time

+5 V

0 V

I/O1 I/O2 I/O3

Tri-State Ports and a Common Bus

Only one register can be read from at a time.

i.e. all tri-state ports except one must be high-impedance.

Any register can read and store the contents of any other.

A I/OCLK

I/O

8

CLKA

I/OA

CLKB

I/OB

CLKC

I/OC

SystemBus

B I/OCLK

I/O

C I/OCLK

I/O

Example

To store the contents of register A in register B…

8

CLKA

I/OA

CLKB

I/OB

SystemBus

CLKA

I/OA

CLKB

I/OB

A I/OCLK

I/O

B I/OCLK

I/O

B is triggered to store input from bus

A – Outputs contents to bus

Simplifying the ALU with a Bus

ALU

A B S

COUT F

Single input byte

Control inputs

Result Carry output

Working register

D Q

CLK 8

8

Before8 data input lines

8 data outputs

ALU

A B S

COUT F

Control inputs

Carry output

Working register

8

8

8

Bus

AfterOne set of 8 shared data input/output

lines

Example of ALU Operation To add the contents of

register R1 to the contents of register R2 and place the result in register R3:

W R1 R2 R3

System Bus8

registers

ALU

Carry output

A B

S

COUT F

8

8

R1 outputs to the bus, W inputs and stores.

R2 outputs to the bus, the ALU adds and stores the result in W.

W outputs to the bus, R3 inputs and stores.

ALU Operation Notes

Although the ALU performs calculations instantaneously (nearly), this addition requires three separate operations

However, it is now possible to perform ALU operations on any pair of register contents

Despite this flexibility, the required connections are relatively simple

Automating the ALU To perform calculations like the previous

example, the following control signals must be generated in sequence:

Input / Output select signals to the registers Clock (latch enable) signals to the registers Selection inputs to the ALU

To perform a predetermined set of operations, the corresponding sequence of control signals must be stored.

This is called a program.

A Victorian Computer Control signals = piano key

presses Sequences of key presses are

stored as holes on a paper roll. This is exactly how early

computer programs were stored too.

Our First Computer

W R

System Bus 8

ALU

Carry output

A B

S

COUT F

8

8

To registers’ input/output

and clock inputs

Sequence of control signal combinations

Summary The use of tri-state ports and a common

system bus hugely simplifies register interconnections.

With an ALU and a few registers, relatively elaborate operations can be performed.

The necessary control signals could be recorded for automatic operation.

Of course, we don’t really use paper tape. Instead we use program memory.