MOTHERBOARD

The Motherboard

Understanding the Motherboard

The motherboard is all those components that make up the actual computer important units such as hard disk and interfaces (collectively called peripherals) are attached to the motherboard.

A motherboard has several slots onto which the circuit boards of other devices like graphics adapters and the interfaces are located. These slots are called bus slots. If a motherboard has such slots and no electronic components are connected to it, then the motherboard is said to be modular. The motherboard in a modular PC is divided into bus board and processor board. The bus board has the slots for the adapters and the interfaces and the processor board is inserted onto it as any other board.

The central component of the motherboard is the main processor (in the figure, It is the CPU 80x86). The CPU is associated with a coprocessor for mathematical application. The motherboard, in addition to this, houses a cache controller and a cache Ram to enhance the overall performance of the computer. Due to technological advances, these components, now-a-days are clubbed together in a single chip. The motherboard also houses other devices like main memory or ram, the ROM BIOS, the DMA controller (8237)², timer (8254), keyboard interface and bus slots.

3.2 Components of the Motherboard

Processor and Coprocessor

we can see that the 80x86 processor is the central unit of the board. The processor does the actual execution of the program. The main processor is generally supported by a coprocessor or processor extension. These are processors, which performs extensive mathematical calculation with high accuracy. They thus assist the main processor and increase the overall efficiency. The coprocessor of 80x86 as 80x87. There are coprocessors exiting. Which are manufactured by other companies such as weitek and Cyrix.

During Purchase, a co-processor will not be present in the motherboard. It has to be bought externally and inserted into the slot meant for the purpose. The processor will automatically recognize the co-processor and will transfer controls to it as and when required.

RAM

The RAM (Random Access Memory) is a vital component of the motherboard. It is used to temporarily store data because everything that is stored in the RAM will be wiped out as soon as electricity is switched off. RAM is available in the market in the form of memory chips.

RAMs are usually divided into a set of banks. Each bank has to be equipped with memory chips. The computer does not recognize banks that are partially filled.

The data required by the processor or the coprocessor resides in the memory. When data is needed for computation, the processor sends a request for the data to the memory. This is done by sending the address of the data via the address bus. The memory, after receiving the address, processes the request and puts the requested data in the data bus from where the processor reads the data.

The performance of RAM is generally measured in term of access time. This is the time taken for the data to be available to the processor, after the request for the data is sent to the memory. Modem chips typically have an access time of 60-100 Ns. For very fast data access, there exist special chips which ahs an access time of 15-25 Ns. These special chips are called cache and are explained in section 3.2.3

3.2.3 Cache

            A different type of memory exist in the motherboard. Compared to the conventional RAM chips, the cache is smaller and faster. Its access time is much lower than conventional RAM.The cache holds the data that are frequently required by the processor. The processor, therefore, has to wait a considerably lesser time for data.

When the processor requests some data, the cache controller first checks whether the data is present in the cache. If it is present, then it is immediately made available to the processor. If the data is not held in the cache memory, then the cache reads main memory and simultaneously transfers the read memory, then the cache reads the main memory and simultaneously transfers the read memory, then the cache reads the main memory and simultaneously transfers the read data to the processor. Similarly, when processor wants to write to the main memory, it Write onto the cache memory at high speed. The cache controller then writes the data back to the main memory later.

The cache is transparent to the processor. This means that the processor may not necessarily be aware of the presence of the cache. The newer major powerful 80 x 83 family processor has the processor, coprocessor, an 8-KB cache memory and a cache controller incorporated in it.

 ROM :- The motherboard also includes a ROM (Read Only Memory). The difference between RAM and ROM is that, the contents of ROM are not lost when the computer is powered down. The RAM, On the other hand, looses all its contents. There are various types of ROM available in the market today. They are PROM (Programmable ROM), EPROM (Erasable PROM), and EEPROM (Electronically EPROM). These ROM have additional features than their ancestors. For example, a PROM chip is programmable. The contents of an EPROM chip can be erased and rewritten.

The ROM contains some programs and data that are required by the computer during Power up. The ROM also houses a set of various support routines that are essential for accessing keyboard, graphics adapter etc. They are collectively called ROM-BIOS.

BUS SLOTS

The bus slots refer to the into which the various buses in the system are connected. As mentioned above, data is exchanged between the devices by the means of the data bus. The address of the data is specified by the address bus. To control the data transfer, the processor must inform the memory whether it wants to read data or write data. This is done by the means of a signal that is carried by an electrical line in the bus. This signal is often called write enable. There are various signals like the one that is mentioned above. Due to gather varying number of singles, the bus slots often have varying number of contact points. For instance, the address bus along with all the control signals is called system bus. They inform all the adapter cards inserted into the bus slots about all the operations taking place in the PC. Theoretically, it does not matter into which bus slot an adapter card is inserted. However, the cards may not function, as they should in some slots.

Bus divided into three types:-

1. Data Bus: - This type of Bus that is carries of data and file one place to another place. In real sense Bus is a group of parallel lines. Bus affects the speed at which data can travel between hardware components since wire can wire transfer one bit at a time. 16 wires Bus can move 16 bits at a time.

2. Address Bus: - In types of Bus to kept of the full location to relate to data bus. It connects only the CPU and memory and helps in locating memory address faster today most CPU has 32 bit address Buses that can address 4 GB of Memory.

3. Control Bus: - It controls the direction flow origin and signals for every operation it performs.

DMA

Sometimes it is required to transfer extensive amount of data between the secondary memory and the primary memory or vice versa. These jobs tend to be long and time consuming. Thus, if the processor is performing these tasks, a lot of time is spent on transferring data rather than executing programs. Thus, to avoid loosing precious processor time and improve the overall performance of the processor, a separate unit is placed in the motherboard. Which exclusively caters to these kinds of tasks? Such as unit is called DMA (Direct Memory Access) Unit.

TIMER

Since the computer is a digital device, its operations are based on a clock. This clock is used for several purposes. The most important of the uses of the clock is to supply a clock single based on which the CPU performs its operations. A timer chip based on the single based on which the CPU performs its operations. A timer chip based on the motherboard controls the functions of the clock. In addition to the function mentioned motherboard controls the functions for the clock. In addition to the function mentioned above, the timer chip also issues timer interrupts. These are used to update to internal clock and to refresh the contents of the RAM.

Registers

i. Introduction

Registers, in simple terms, can be defined as small storage components that are used for a very fast access of data. They can also be defined as interfacing devices between the software and hardware. Command that directly access the hardware access them through controller chip and other device controllers are all accessed via the registers. The address, size and the meanings assigned to the registers depend on the hardware. Registers function both as memory and as workbench. A microprocessor typically has as number of registers. Some registers are decicated to specific function while some are general purpose.

            Registers have limited effect to the microprocessor. For example, let us consider that the ALU has to do a subtraction of two numbers. If the two numbers are fetched from the memory and stored in two registers, then the operation becomes faster because the access of data from the registers are much faster. Howerer, adding more registers to gteh microprocessor does not benefit the processor because the processor is designed to operate on two operands and thus, at the most requires two registers has quite and impact on the performance of the processor. The more the number of bits the register can hold, much faster is the operation.

Let us now see how registers work. Consider a case where a user is using an application program. He instructs the application program to open a file. This commandis converted into a system call to the operating system. The operating system in turn converts the command and accesses the BIOS routines to execute the user command. The BION then accesses the corresponding hardware with the help of the corresponding register. The BIOS converts the defined function call into a command sequence for the register concerned.

Various registers

As said earlier, there are a number of register performing various function. Some important register and their associated functions are listed below.

·         Instruction Pointer

This register is used to print to the next instruction that is to be fetched by the processor for execution.

·         Flag Register

The flag register is the collection of a number of flags, as shown in figure 3.2. These are required by the processor for performing various operations.

·         Segment Registers

This register is used by a processor in a segmented memory environment to point to a particular memory segment. In the other memory management schemes, various register are present which are used to point to the various memory portions as per the scheme.

·         Address Register

This register is used to store the address of the data that has to be either stored in the main memory or fetched from the main memory. It is also used when the data transfer concerns any other device.

·         Data Register

This register is used to hold the data has to be transferred between two devices.

·         Pointer and Index Register

These registers are used by the ALU to perform arithmetic and logical function. They may also be used by some instructions for some special purpose.

Scratch Pad Registers

These registers are used by the ALU for storing temporary value and intermediate results. These registers are not accessible by any instruction.

Accumulator

The accumulator is a register that is used by the ALU to temporarily hold the result of a previous operation.

Various other register are used for a number of other purpose. These registers are explained as and when they are required.

BUS

Introduction

            Buses are a set of parallel electrical conducting lines that connect various components on the motherboard. The electrical lines are combined in different ways to suit different functions and each combination is given a different name. The internal buses also provided electrical interconnection between the processor components and the interface device used with peripheral equipmet.

Data Handling and Addressing Limits

The width of the bus is a limiting factor in data and address handing . so , what dddod we mean by the width of a bus.? The width of a bus can be defined as  the number of electrical lines that make up the bus. in simple terms, the width of a bus can be defined as the number of bits it can carry at one Instance of time . For example , if we say , “this is a 16-bit bus”, we mean that there are 16 electronic lines that combine together to from

The bus.

The most important function of the bus is the transfer of data between the processor and Peripherals or between different peripherals. Ideally, the bus width should match the data path width of the microprocessor. The advantage of using this architecture is that during data transfer, the entire word may be transferred in one exchange.          

This will improve the overall performance of the processor. If the bus width is narrower, it may take several exchanges to read the full word. From this, we can conclude that the more the number of data lines on to the bus, the better is the performance. However, adding more number of data lines on to the bus, adds complexity to the circuits in the memory as well as the processor.

We know that the number of microprocessor address lines and the number of bus address lines decide the maximum memory range that the processor can address. Usually the bus provides the full range of address lines. However, if the addresses that are not included are in the upper range of the total address space, the address lines in the bus are something shortchanged.

Arbitration and bus-matering

The buses of today connect more than just the components of the microprocessor. A bus that is connected directly to the processor data and address lines are called local bus. This bus is meant to service the components near the processor. They are connected via the PC Bus. The desilgn of the PC bus is such that the components connected to the bus are in direct control and control the processor. The processor, however, need not always control the operation of the bus. it may transfer   control circuits like the DMA  controller.

Another technology that is used with the buses is the arbitrated expansion bus. in this design  , the processor is not in full control of the bus but it is connected to the bus just as any other accompaniment. The control of the bus is handed over to specialized circuits called the bus controller. However, the processor is still in control of bus transfers. If the processor wants , ilt may alienate itself from the transfers between other devices in the   system.

Dual   Bus  Architecture

Single bus architecture something has devastating effects on the performance of the computer. For example, a 50 MHz. computer may slow down   to 8.25 MHz every time memory is accessed   . This is because the processor and bus speed may not tally.

One method to avoid such a situation is to use two buses-one for the 1/0¹⁰ expansion bus. Compaq computer corporation was the first company to introduced dual-bus PC. This PC was called desktop and had two separate buses for memory, which operated at the microprocessor speed and a bus for 1/0 operations, which operated at a much slower speed.

Microprocessor Bus Control

To read or write data on to the memory, the microprocessor first sends the address of the data then sends a special signal called Address Latch Enable to indicate the devices attached to the bus that a valid address has been sent. The device then locks the circuit for the address. This is called latching. The Processor then sends a memory read command signal or a memory write command signal as desired. These tow commands specify whether data has to be read from the memory or data has to be written onto the memory. The data is consecutively put onto the bus by the device or the microprocessor.

The I/O Read Command signal instructs the device to move data placed at the input port to the data bus from where it will be accessed by the microprocessor. The I/O write Command signal, on the other hand, instructs the device to move the data placed in the data lines by the microprocessor to its output port. A fact that has to be noted here is that the I/O devices are much slower than the microprocessor. Therefore, a provision exists in the bus to make the microprocessor wait for the I/O device to complete its operation. This is done with the help of the I/O Channel Ready signal. If the signal is inactive, it means that the microprocessor has to wait for the I/O device to complete its operation.

ALU

Introduction

The ALU (Arithmetic and Logic Unit) is a section where all the arithmetic and logical functions are carried out. We know that every arithmetic operation requires two operands. The operation then produces a result. This ALU, therefore, must be able to handle two data words and the result. This concept is dependent on the storage and processing techniques, which may vary, from model to model. Logical operation usually involves comparisons. Circuits in the ALU compare two values by subtracting on from another. For example, to determine whether number A is greater than number B, B is subtracted from A and the sign of the result is determined. Depending on the sign of the result, the ALU does further operations. Alphabetic data can also compare according to their assigned order sequence.