Page 133 - Handout Digital Electronics

LECTURE 18 COMPUTER REGISTERS
18.1 Introduction
A computer register is a group of flip flops used to temporarily store binary data. A computer register is
used to quickly accept, store and transfer data and instructions that are being used immediately by the
Central Processing Unit (CPU). In the computer there are various types of registers used for various
purposes. Some of the most used registers in the digital computers are:

• Accumulator (AC) – used to store intermediate results of processing
• Data registers are commonly known as memory data register (MDR)- this register is found in the
control unit and is used as a gateway for data from memory to CPU registers and for
results(information) from the CPU registers to main memory
• Address register also known as the memory address register (MAR) - this register has its outputs
connected to the address bus. During instruction execution, the contents of the program counter
(address of the next instruction to be executed) are transferred to this register. In this register the
binary bits are put in their corresponding order and will point to a main memory location where
the instruction or data is to be read from or stored into.
• Program counter (PC) – is used to hold the address of the next instruction to be executed. After
an instruction has been fetched from memory, the PC is updated to contain the address of the
next instruction to be executed. Depending on the address and memory organization, the PC can
be updated by adding one, a two or a four, for example:
PC← [PC] 1 or PC← [PC]+ 4

The size of a register (the number of binary bits which it can store) is determined by the number of flip
flops used to implement that register. For an example a 16-bit register will be implemented on 16 flip
flops where each flip flop can only store 1 bit.

The most popular types of registers used for temporary data storage are implemented using the D flip
flop and the SR type of flip flops. Computer registers can be organized in serial or in parallel.

18.2 Computer register operations
The register operations include data transfer, shift operations and register modifications.

Data transfer operation

This is the loading of data from the external sources into the computer registers or the transferring of
data or information from one register to another. The data transfer operation can be serial or parallel.
Below is a parallel load register implemented on D flip flop

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