COMPUTER DATA CONVERSION

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 determines how much data it can compute with at a time. This is sometimes used to label the processor’s size.

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.

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 bytes of memory or 4 gigabyte.

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.

CHARACTERISTICS OF BUS

A bus is characterised 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 of width and frequency.

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 synchronisation signals coming from the control unit and travelling to all other hardware components. It is bidirectional bus, as it also transmits response signals from the hardware.

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.

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.

    (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.

   (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.

   (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).

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.

 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 realised 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.

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.