Shift registers are basically storage units that are used to store, transfer, or manipulate binary bits (0’s and 1’s) in CPUs of computers to keep the frequently used data at hand. Another important device that implements shift registers is a calculator where it is used to store two binary numbers to be added and also the result. They are also used for binary division and multiplication. Earlier, computer processors also comprised of shift registers. Shift registers are also widely used to make various types of counters.
A shift register constitutes a number of D-type data latches connected in a chain such that the output of the first register serves as the input for the next one, and so on. While used for data storage purposes, the clock inputs for each latch are connected together to the clock pulses. New data is input at the first register with each clock pulse and the data is subsequently carried to the other registers in series. These registers can be positive-edge triggered or negative edge triggered.
Each latch is capable of storing one binary bit. So, the number of D flip-flops that a shift register uses depends on the number of binary bits that are to be stored in it. The common 8-bit shift register, for example, comprises of eight D-type data latches.
The movement of data in a register can be from left to right (right-shifting), right to left (left-shifting), or both right and left shifting, called bidirectional shift register. Multiplication operation can be implemented by performing a left shift of the binary digits while division can be implemented by performing a right shift in the bidirectional shift registers.
Based on how data is fed into the registers and how the output is derived, bidirectional shift registers are classified into the following four types:
1. Serial in-Serial out (SISO) - In this type of shift register, data is loaded into the resister one after another with each clock pulse. The output is also derived serially.
2. Serial in-parallel out (SIPO) - In this type of shift register, the data is loaded into the register one after another with each clock pulse. The stored data is available to be read parallel i.e. all bits at once. The 74HC164 IC is a good example of an 8-bit Serial in-parallel out shift register.
3. Parallel in- Serial out (PISO) - Data is loaded into the register at once in a single clock pulse. The stored bits are read one by one in a serial manner with clock pulses. The 74HC165, for example, is an 8-bit Parallel in-serial out shift register.
. Parallel in- parallel out (PIPO)- In this type of shift register, data is entered into all the individual d-latches of the register at once, which are available to be read at once as well.
