Registers are temporary storage area for instructions or data. They are not a part of memory; rather they are special additional storage locations that offer the advantage of speed.

1. DATA CONVERSION
REGISTERS, ADDRESSES AND BUSES
Monday, November
21, 2016

2. LEARNING OUTCOMES
At the end of the lesson, I should be able to:
a) -Definition of terms: Register, Address & Bus
b) -Types of registers
c) -Function of each register and main memory
d) -Differences between register and main
memory
e) -Outline operating procedure for data
processing
f) -Factors affecting speed data transfer

3. REGISTERS
Registers are temporary storage area for instructions
or data. They are not a part of memory; rather they are
special additional storage locations that offer the
advantage of speed. It works under the direction of the
control unit to accept, hold, and transfer instructions
or data and perform arithmetic or logical comparisons
at high speed. Most operations are done on the
register; the processor can’t directly perform
arithmetic in memory. For example, if you want to add
1 to a memory address, the processor will normally do
this by loading the initial value from memory into a
register, adding 1 to the register, and then saving the
value back to memory.
The width (in bits) of the processor’s register

4. ADDRESS
A memory address is an identifier for a memory
location, at which a computer program or a
hardware device can store data and later retrieve
it. In modern byte – addressable computers,
each address identifies a single byte of storage;
data too large to be stored in a single byte may
reside in multiple bytes occupying a sequence of
consecutive addresses. Some microprocessors
were designed to be word – addressable, so that
the addressable storage unit was larger than a
byte. The efficiency of addressing of memory
depends on the size of the address bus.

5. In a computer program, an absolute address,
(sometimes called an explicit address or specific
address), is a memory address that uniquely
identifies a location in memory. This is different
from a relative address, which is not unique and
specifies a location only in relation to
somewhere else (the base address). Virtual
memory also adds a level of indirection. Very
often, when referring to the word size of a
modern computer, one is also describing the
size of virtual memory addresses of that
computer. For example, a computer said to be
“32 bits” usually treats memory addresses as 32
– bit integers; a byte addressable 32 – bit
computer can address 232 = 4,294,967,296

6. BUS
A bus, in computing is a set of physical
connections (cables, printed circuits etc.) which
can be shared by multiple hardware components
in order to communicate with one another. The
purpose of bus is to reduce the number of
pathways needed for communication between
the components, by carrying out all
communications over a single data channel.

7. CHARACTERISTICS OF BUS
A bus is characterized by the amount of
information that can be transmitted at once.
Width is used to refer to the number of bits that
a bus can transmit at once.
Frequency is the speed of the bus, which is the
number of data packets sent or received per
second. It is expressed in Hertz (Hz).
Cycle is the each time that data is sent or
received.
Transfer speed is the amount of data which it
can transport per unit of time. It is the product

8. BUS SUBASSEMBLY
Each bus is generally constituted of 50 to 100
physical lines, divided into three subassemblies
which are:
(i) Address bus (sometimes called memory bus)
transports memory addresses which the
processor wants to access in order to read or
write data. It is unidirectional bus.
(ii) Data bus transfers instructions coming from
or going to the processor. It is bidirectional bus.
(iii) Control bus (or command bus) transports
orders and synchronization signals coming from
the control unit and travelling to all other
hardware components. It is bidirectional bus, as

9. PRIMARY BUS
There are two buses within a computer;
Internal bus (also known as front – side bus
(FSB)) allows the processor to communicate with
the system’s central memory (RAM).
Expansion bus (also known as input/output bus)
allows various motherboard components to
communicate with one another. However, it is
mainly used to add new devices using what are
called expansion slots connected to the
input/output.

10. TYPES OF REGISTER
(A) MDR (MEMORY DATA REGISTER)
This is the register of a computer’s control unit
that contains the data to be stored in the
computer storage (e.g RAM), or the data after a
fetch from the computer storage. It acts like a
buffer and holds anything that is copied from
the memory ready for the processor to use it.
The MDR is a two – way register because when
data is fetched from memory and placed into
the MDR, it is written to in one direction. When
there is a write instruction, the data to be
written is placed into the MDR from another CPU
register, which then puts the data into memory.

11. (B) CIR (CURRENT INTERRUPT
REGISTER)
It captures the value that is winning the
interrupt arbitration. The CIR is updated at the
beginning of an interrupt acknowledge bus cycle
or in response to an update CIR command. The
contents remain in the CIR until another
interrupt acknowledge cycle or update CIR
Command occurs.

12. (c) User – Accessible Register: The most
common division of user – accessible registers is
into data registers and address registers.
(d) Data Registers: They are used to hold
numeric values such as integer and floating–
point values.
(e) GPRs (General Purpose Registers): They
can store both data and addresses.
(f) FPRs (Floating Point Registers): They
store floating point numbers in many
architectures.

13. (g) Constant Registers holds read – only values
such as one, or pi.
(h) Special Purpose Registers: They hold
program state
(i) Instruction Registers store the instruction
currently being executed.
(j) Model – Specific Register (also known as
machine – specific register) store data and
settings related to the processor itself.
(k) Control and status register: It has three
types which are program counter, instruction
register and status word (PSW).

14. DIFFERENCES BETWEEN REGISTERS AND MAIN
MEMORY
REGISTER MAIN MEMORY
1 Registers are very
small but are extremely
fast.
RAM is much larger
and smaller memory
that applications use
as a scratch space.
2 It holds data temporary It holds
information/data
permanently
3 It serves as an
assistance to the main
memory
It does not assist
4 It is an extremely fast
and expensive form of
computer memory.
It is much cheaper.

15. DATA – FETCH – EXECUTE CYCLE
John Von Neumann introduced the idea of the
stored program. Previously data and programs
were stored in separate memories. Von
Neumann realized that data and programs are
indistinguishable and can therefore use the
same memory. This led to the introduction of
compliers which accepted text as input and
produced binary code as output.
The Von Neumann architecture uses a single
processor which follows a linear sequence of
fetch – decode – execute. In order to do this, the
processor has to use some special registers.

16. DATA TRANSFER SPEED
Computer data bus width indicates how much
data the chip can move through at once and the
sizes of address bus indicates how much the
memory chip can handle.
Increasing the data bus will increase the
quantity of data that the bus carry at one time
and so speed up the performance/processing of
the computer. A computer with a data bus of 32
lines is called 32 bits computer and the word
length is 32 bits.
Word length is the number of bits the CPU can
process in a single operation.

17. ASSIGNMENT
Briefly outline the factors that affect data speed.

18. TERMINOLOGY
• Registers
• Bus
• Fetch
• Execute
• Interrupt