Registers are the fastest type of memory in a computer and are made up of flip-flops. There are two types of registers: special purpose registers that are used internally by the processor, and general purpose registers that can be accessed by programming to store intermediate results. Important special purpose registers include the program counter, memory address register, and instruction register. Register transfer language describes the transfer of data between registers using microoperations, and memory transfer operations describe transferring data between registers and memory.

1. Computer Organization
and Architecture
Topic: Registers
by
Upendra Mishra
(M. Tech., Ph.D.*)
KIET Group of Institutions

2. Quick recap to Memory Hierarchy
Secondary Memory
Main Memory
Cache
Register
Smaller in size,
Faster in operation
and
costlier
Larger in size,
Slower in operation
and
cheaper
Flip
flops

3. Introduction to Register
 Register is a digital device that is very fast memory unit that
may accept, store and transfer data or instruction.
 Registers is a group of flip-flops.
 Question: What is flip-flop?:
 Flip flop is able to store 1 bit of
data/information.
 This means n-bit register have n-flip flops

4. Two broad categories of Register
Special Purpose Registers
 Users do not access Special Purpose
registers. These registers are for
processor internal processing.
 Examples: Program Counter,
Memory Address Register,
Memory Data Register ,
Instruction Register, Flag register
General purpose register
 General purpose register is used to
store data intermediate results
during program execution. It can be
accessed via assembly
programming.
 Examples: Accumulator, Data Register

6. Computer components:
Top-Level View

7. Brief Description of Important Register
 PC(Program Counter): Contains the address of the next instruction to be
fetched from memory.
 MBR(Memory Buffer Register): Contains a word to be stored in memory or sent to
the I/O unit, or is used to receive a word from memory or from the I/O unit.
 MAR(Memory Address Register): Specifies the address in memory of the
word to be written from or read into the MBR.
 MDR(Memory Data Register ): It contains the data to be stored in the computer
storage (e.g. RAM), or the data after a fetch from the
computer storage.

8. Brief Description(contd.)
 IR(Instruction Register): Contains the instruction most recently fetched.
 Flag register: Depending upon the value of result after any arithmetic and logical
operation the flag bits become set (1) or reset (0).
 AC(Accumulator): Employed to hold temporarily operands and results of ALU operations.
 Data Register: A register used in microcomputers to temporarily store data being
transmitted to or from a peripheral device.
 Index Register: These are used for indexed addressing and may be
auto indexed

9. Topic: Register Transfer and
Memory Transfer

10. Register Recap
Register is a digital device that is very fast memory unit that may
accept, store and transfer data or instruction.
It is made of Flip flops.

11. Microoperations
The operations executed on data stored in registers are called
MICROOPERATIONS.
 A Microoperations is an elementary operation performed on the information
stored in one or more registers.
 The result of the operation may replace the previous binary information of a
register or may be transferred to another register.
Example, Counter, increment and load, bidirectional shift

12. Register Transfer

13. Register Transfer Language
Register Transfer Language
Register Transfer Language
implies the availability of hardware logic
circuits that can perform a stated
microoperation and transfer the result of
the operation to the same or another
register.
is borrowed from programmers, who
apply this term to programming
languages.
Expresses the symbolic form the
microoperation sequences among the
registers of a digital module.

14. Block representation of Register
Computer registers are designated by capital letters.
PC: Program Counter IR: Instruction Register
R1: Processor Register MAR: Memory Address Register
N – bit Register -> n number of flip flops
For Example: 8 - bit Register
For Example: 16 - bit Register
7 6 5 4 3 2 1 0
R1
PC PC(H) PC(L)
15 0 15 8 7 0

15. Information Transfer
• R2  R1 : denotes a transfer of the content of register R1 into register R2.
• Normally, we want the transfer to occur only under a predetermined control condition. This
can be shown by means of an if-then statement.
If (P = 1) then (R2  R1)
where, P is a control signal which results in Boolean value 0 or 1.
P: R2  R1
• It symbolizes the requirement that the transfer operation be
executed by the hardware only if P = 1 .

16. Block Diagram
Timing Diagram
P: R2  R1
Clock pulse is
a time varying
voltage signal
applied to
control
operation of a
flip flop.

17. Memory Transfer
The transfer of information from a memory word to the outside environment is called a
READ
operation.
Read: DR  M[AR]
The transfer of new information to be stored into the memory is called a WRITE operation.
Write: M[AR]  DR
A memory word will be symbolized by the letter M.
M[AR]: to specify the address of M when writing memory transfer
operations, address is enclosed in square brackets.(here, AR is
address register)

18. Arithmetic Microoperations
Symbolic Representation Description
R3 ← R1 + R2 The contents of R1 plus R2 are transferred to R3.
R3 ← R1 - R2 The contents of R1 minus R2 are transferred to R3.
R2 ← R2' Complement the contents of R2 (1's complement)
R2 ← R2' + 1 2's complement the contents of R2 (negate)
R3 ← R1 + R2' + 1 R1 plus the 2's complement of R2 (subtraction)
R1 ← R1 + 1 Increment the contents of R1 by one
R1 ← R1 - 1 Decrement the contents of R1 by one

19. Thank
You

20. MCQ
 A register is defined as ___________
A. The group of latches for storing one bit of information
B. The group of latches for storing n-bit of information
C. The group of flip-flops suitable for storing one bit of information
D. The group of flip-flops suitable for storing binary information
 Answer: D

21. Thank
You