Registers & Shift Registers: Definition, Function & Examples Video

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  • 0:04 What Are Registers?
  • 0:47 What Are Shift Registers?
  • 1:52 Shift Register Types &…
  • 3:48 Shift Register Application
  • 4:31 Lesson Summary
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Lesson Transcript
Instructor: Lyna Griffin

Lyna has tutored undergraduate Information Management Systems and Database Development. She has a Bachelor's degree in Electrical Engineering and a Masters degree in Information Technology.

In this lesson you will learn about and understand the basic functions of registers and shift registers. You will be introduced to different types, understand their operations, and learn about their various applications.

What Are Registers?

A register is basically a storage space for units of memory that are used to transfer data for immediate use by the CPU (Central Processing Unit) for data processing. Also known as memory registers, they can actually form part of the computer processor as a processor register. The register is large enough to hold any kind of data, such as dates, instruction sets, storage addresses, bits, sequences, and characters. Some instruction sets are partly formed by registers. Types of registers include memory address register, memory buffer register, input output address register, input output buffer register, and shift register. In this lesson we will be focusing on shift registers.

What Are Shift Registers?

Shift registers are digital memory circuitry found in devices such as calculators, computers, and data processing systems. With the shift register, data or bits are entered into the system in a serial or parallel manner. They enter from one direction, and as more data is added, shift positions until they get to the output end. The two ends are referred to as the left and right ends. Movement of data can be from left to right, from right to left, or in both directions to make a bidirectional register.

Shift registers can be implemented such that each bit may be held in a latch. The output of one latch can be connected to be the input of another latch. Think of multiple connectors used to extend an electric cord. With registers, data can be fed one bit at a time (serially) or can be loaded in a batch all at the same time (in parallel).

Shift registers can serve as data storage spaces or data movement devices. They are therefore commonly implemented in devices such as calculators or computers. They serve as temporary storage units for binary data awaiting a mathematical operation such as addition or multiplication.

Shift Register Types & Operations

There are four different modes in which shift registers operate. These modes are determined by how the bits of data are shifted through the register. We'll now examine the different types of shift registers which can be implemented.

1. Serial In-Serial Out (SISO) Shift Register

In the serial in-serial out shift register, data is input serially until it reaches the output. At this time, it exits in a serial manner as well. The shifting (movement) of the data flows from left to right in the register. Each shift is initiated with a clock cycle. The figure below shows a 4-bit SISO register in operation:

Figure 1: SISO Shift Register

2. Serial In-Parallel Out (SIPO) Shift Register

In a serial in-parallel out shift register, the data is input serially one bit at a time and output in a parallel form. Each bit is shifted on its own clock pulse. The table below shows an illustration of a 4-bit SIPO shift register with each clock pulse:

1001 0 0 0 0
1 0 0 0
0 1 0 0
0 0 1 0
1 0 0 1

On the first clock pulse (CLK 0), digit 1 on the right is loaded. On the second clock pulse (CLK1), digit 1, which is already loaded, is shifted right and digit 0 is loaded behind it. This shifting and loading continues on subsequent clock pulses until all inputs are loaded and output.

3. Parallel In-Serial Out (PISO) Shift Register

The parallel in-serial out shift register receives the data input in parallel batches on every clock pulse and the data is shifted and output serially. This can be seen in the figure here:

Figure 3: PISO Shift Register

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