Computer System Architecture

Information Architecture

   1. which are the various Number System in use?

Various Number Systems in use are :
Decimal Numbers (having a base of 10)
Octal Number (having a base of 8)
Hexadecimal Number System (having a base of 16)
Binary Number System (having a base of 2)

   2. How are fractions represented and what are its parts?

Fractions are called floating point representation of the numbers. The floating point representation of a number has two parts.The first part represents a signed, fixed-point number called the mantissa. The second part designates the position of the decimal(or binary) point is called exponent. The fixed-point mantissa may be a fracion or an Integer. For example, the decimal number +4567.89 is represented in floating-point with a fraction and an exponent as follows:
Fraction         Exponent
+0.456789         +05
In this case the exponent value(+05) indicates that the actual position of the decimal point is five positions to the right of the indicated decimal point in the fraction.

   3. How negative number represented in the Computer?

In ordinary arithmetic, a negative number is indicated by minus sign and a poitive number by a plus sign . Because of hardware limitations, computers must represent everything with 1's and 0's, including the sign of a number. As a consequence, it is customary to represent the sign with a bit placed in the leftmost position of the number. The convention is to make the sign bit equal to 0 for positive and to 1 for negative. In addition to the sign, a number may have a binary (or decimal) point.The position of the binary point is needed to represent fractions, integers or mixed integer-fraction numbers.

   4. How is number represented as an integer in the commputer?

When an integer binary number is positive, the sign is represented by 0 and the magnitude by a positive binary number. When the number is negative, the sign is represented by 1 but rest of the number may be represented in one of three possible ways:
i. Signed-magnitude representation
ii. Signed- 1's Complement representation
iii. Signed 2's Complement representation

   5. How's arithmetic addition done?

the addition of two numbers in the signed-magnitude system follows the rules of ordinary arithmetic. If signs are same we add the two magnitude and give the sum of the common sign . If the signs are different , we subtract the smaller magnitude from the larger and give the sign of the larger magnitude in the result.

   6. How is arithmetic subtraction done ?

Subtraction of two signed binary numbers when negative numbers are in twos complement is very simple and can be treated as follows : Take the complement of the subtrahend( including including the sign bit) and add it to the minuend ( including the sign bit). A carry out of the sign bit position Is discarded.

   7. When an overflow occurs?

When two number of N digits each are added and the sum occupies N + 1 digits, we say that an overflow occurred. when the addition is performed with paper and pencil, an overflow is not a problem since there is no limits to the width of the page to write down the sum. An overflow is a problem in digital computers Because the width of the register is finite. A result that contains N + 1 bits cannot be accommodated in a register with a standard length of N bits. for this reason, many computers detect the occurrence of an overflow, and when it occurs,a correspondent flip flop is set which can then be checked by the user.

   8. What is Gray code?

Digital systems can process data in discrete form only. Many physical systems supply continuous output data. The data must be converted into digital form before they can be used by a digital computer. Continuous, or analog, information is converted into digital form by means of an analog to digital converter. The reflected binary or gray code is used for the converted digital data. the advantages of the gray code over straight binary number is that the Gray code changes by only one bit as it sequences from one number to the next.

   9. Which type of fields are found in the Instruction Format?

The most common field found in instruction formats are: and operation code field that specifies the operation to be performed; and An address field that designates the memory address or a processor registers; and a Mode field that specifies the way the operand or the effective address is determined.Other special fields are some time employees under certain circumstances, as for example a field that gives the number of shifts in shift-type instruction.The operation code field of an instruction is a group of bits that define various processor operations, such as add, subtract, complement and shift.

   10.what are Operands?

The bits that define the Mode field of an instruction code specify a variety of alternative for choosing the operands from the given address.Operations specified by computer instructions are executed on some data stored in the memory or processor registers,operands residing in memory are specified by the Memory address.Operands residing in processor registers are specified with a Register address.

   11. What are three-addressed instruction?

Computers with three address instruction formats can use each address field to specify either a processor register or a memory operand. the program in assembly language that evaluates X = (A+B) * (C+D ) is shown below ,together with comments that explain the register transfer operation of each instruction.

   12. What are two-address instructions?

Two address instruction are the most common in commercial computers, here again each address field can specify either a processor register or a memory word.

   13.What are one-address instruction?

One address instruction use an Implied accumulator(AC) register for all data manipulation, For multiplication and division there is a need for a second register. However,here we will neglect two registers and assume that the AC contains the result of all operations.

   14.What does zero-address instruction do?

Stack organised computer does not use an address field for the instructions ADD and MUL.The PUSH and POP instruction ,however need an address field to specify the operands that communicates with the stack.

   15. what are RISC instructions?

The instruction set of a typical RISC processor is restricted to the use of LOAD and STORE instructions when communicating between memory and CPU. All other instructions are executed within the registers of the CPU without referring to memory.A program for RISC-type CPU consists of LOAD and STORE instructions that have one memory and one register address and computational type instructions that have three addresses with all three specifying processor register.

   17.What do you understand by addressing mode?

The addressing mode specify the rule for interpreting and modifying the address field of the instruction before the operand is actually referenced.computers use addressing mode techniques for the purpose of accommodating one or both of the following provisions:
I. to give programming versatility to the user by providing such facilities as pointers to memory, Counters for loop control, indexing of data, and
Ii. program relocation and
iIi. to reduce the number of bits in the addressing field of the operation.

   18.describe the various phases of instruction execution.

Control unit of a computer is designed to go through an instruction cycle that is divided into three major phases:
I. Fetch the instruction from memory
Ii. decode instruction
Iii. execute the instruction

   19.what are implied modes?

In this mode the operands are specified implicitly in the definition of the instruction. for example, the instruction “ complement accumulator” is an implied mode instruction because the operand in the accumulator register is implied in the definition of the instruction. In fact, all register reference instructions that use an accumulator are implied mode instructions. zero address instruction in a stack organised computer are implied mode instructions since the operands are implied to be on top of the stack.

   20. what is immediate mode?

In this Mode the operand is specified in this instruction itself. in other words, an immediate mode instruction has an operand field rather than an address field. the operand field contains the actual operand to be used in conjunction with the operation specified in the instruction. Immediate Mode instructions are useful for initialising registers to a constant value.

   21. what are register modes?

In this mode the operands are in registers that resides within the CPU. the particular register is selected from a register field in the instructions. A k-bit field can specify any one of 2k registers.

   22.What is register indirect mode?

In this Mode instructions specify the register and the CPU whose contents give the address of the operand in memory.In other words, the selected register contains the address of the operand rather than the operand itself.Before using a register indirect Mode instruction, the programmer must ensure that the memory address of the operand is placed in the processor registers with a previous instruction.A reference to the register is then equivalent to specifying a memory address.The advantage of a register indirect mode instruction is that the address field of the instruction uses fewer bits to select a register than would have been required to specify a memory address directly.

   23. what is Direct Access mode?

In this mode the effective address is equal to the address part of instruction.The operand resides in memory and its address is given directly by the address field of the instruction.In a branch-type instructions address field specifies the actual branch address.

   24. what is indirect access mode?

In this mode the address field of the instruction give the address where the effective address is stored in memory Control fetches the instruction from memory and uses its address part to access memory again to read the effective address. A few addressing mode require that the address field of the instruction be added to the content of a specific register in the CPU.

   25. what is relative address mode?

In this mode the content of the program counter is added to the address part of the instruction in order to obtain the effective address. The address part of the instruction is usually a signed number( in 2’s complement representation) Which can be either positive or negative.When this number is added to the content of the program counter, the result produces an effective address whose position in memory is relative to the address of the next instruction.

   26. what is indexed addressing mode?

In this mode the content of an index register is added to the address part of instruction to obtain the effective address. The index register is a special CPU register that contains an index value. The address field of the instruction defines the beginning address of a data array in memory. Each operand in the array is stored in memory relative to the beginning address.

   27. What is register addressing mode?

In this mode the content of a base register is added to the address of the instruction to obtain the effective address.This is similar to the indexed addressing mode accept that the register is now called a bass register instead of an index register.The difference between the two modes is in the way they are used rather than in the way that they are computed.An index register is assumed to hold an index number that is relative to the address part of the instruction.

   28.What is timing cycle?

The time for all register in the basic computer is controlled by a master clock Generator.The clock pulses are applied to all flip-flops and registers in and the system,including the flip-flop and registers under control unit.The clock pulses do not change the state of register unless the register is enabled by a control signal.

Central Processing Unit

   1. What is central Processing Unit and what are its main parts?

It is actually the main brain of the computer system. it is here that the whole processing takes place.It consists of four main units:
I. Primary Storage
Ii. Arithmetic Logic Unit
Iii. Control Unit
IV. Output Unit

   2.Which purpose does the primary storage unit perform?

This is means stories area, which is also called the main memory area. Its main functions are :
  • holding data in its memory till it is required to be processed
  • holding the results of the processed data
  • holding program instructions which are required for processing.

   3. what does arithmetic and logic unit do?

This is used for the most of the logical processing, for example, for calculations or comparisons. The arithmetic operations like, +, - , *, and / are performed here. The logical operations like <, >, = , <=, >= and <> are also performed here.In most of the arithmetical operations the result is in numerical form while in the case of logical operations the result can be YES/NO or TRUE/FALSE.

   4. What is control unit?

This unit controls the flow and manipulation of data and information. It also controls the flow of data from input devices to memory and from memory to output devices.

   5.What's in an output unit?

The output unit consists of the output devices attached to the computer.These devices take machine-coded output results from the processor and convert them into a form that can be understood by the user.

   6.What is General register organisation?

A large number of registers are included in the CPU, it is most efficient to connect them through a common bus system. The register communicate with each other not only for direct data transfers but also while performing various micro operations.Hence, it is necessary to provide a common unit that can perform all the Arithmetic, Logic and shift micro operations in the processor.This is called the general register organisation.

   7.what is stack organisation?

A useful feature that is included in the CPU of most computers is a stack or last in first out (LIFO) list.A stack is a storage device that stores information in such a manner that the item stored last is the first item retrieved.The operation of a sack can be compared to a stack of trays.
The last tray placed on top of that is the first to be taken off. The stack in digital computers is essentially a memory unit with an address register that can count only ( after an initial value is loaded into it).

   8.What is Evolution of Arithmetic Expressions method?

Reverse polish notation, Combined with a stack arrangement of registers, is the most efficient way known for evaluating arithmetic expressions. This procedure is employed in some electronic calculators and also in some computers. The stack is particularly useful for handling long, complex problems involving chain calculations.

   9. What is CISC architecture?

One reason for the trend to provide a complex instruction set is the desire to simplify the compilation and improve the overall computer performance.The task of a compiler is to generate a sequence of machine instructions for each high-level language statement.The task is simplified if there ar machine instructions that implement the statements directly.The essential goal of a CISC architecture is to attempt to provide a single machine instructions for each statement that is written in a high-level language.

   10. Describe the various characteristics of CISC architecture.

The major characteristics of CISC architecture are: A large number of instructions- typically from 100 to 250 instructions; Some instructions that performs specialised task and are used infrequently; A large variety of addressing modes - typically from 5 to 20 different modes; Variable length instruction formats; and instructions that manipulate operands in memory.

   11.Describe the various characteristics of RISC processor.

The major characteristics of RISC processor are:
I. Relatively few instructions;
Ii. Relatively few addressing modes
Iii. Memory access limited to LOAD and STORE
Iv. All operations done within the registers of the CPU
V. Fixed-length, easily decoded instruction format
Vi. Single-cycle instruction execution
Vii. Hardwired rather than microprogrammed control

   12. What is microprogram sequencer?

The basic components of a microprogrammed control unit are the control memory and the circuits that select the next address. The address selection part is called a microprogram sequencer. A microprogram sequencer can be constructed with a digital functions to suit a particular application. However, just as there are large ROM units available in integrated circuit packages, so are general-purpose sequencers suited for the construction of microprogram control units.

   13. What is pipeline processing?

Pipelining is a technique of decomposing a sequential process in to sub-operations with each sub-process being executed in a specially dedicated segment that operates concurrently with all other segments.A pipeline can be visualised as a collection of processing segment through which binary information flows.

   14.What is array processor?

An Array processor is a processor that performs computation on large arrays of data.The term is used to refer to two different types of processors. An attached array processor is an auxiliary processor attached to a general purpose computer.It is intended to improve the performance of the host computer in specific numerical computation tasks.

   15. What is Attached array processor?

An Attached array processor is designed as a peripheral for a conventional host computer and its purpose is to enhance the performance of the computer by providing vector processing for complex scientific applications. It achieves high performance by means of parallel processing with multiple functional units. It includes an arithmetic unit containing one or more pipelined floating point adders and multipliers.

   16.What is SIMD Array Processor?

An SIMD Array Processor is a computer with multiple processing units operating in parallel. The processing units are synchronized to perform the same operation under the control of a common control unit, thus providing a single instruction stream, multiple data stream (SIMD) organisation. The master control unit controls the operation in the processor elements. The main memory is used for storage of the program.The function of the master control unit is to decode the instructions and determine how the instructions is to be executed.

Arithmetic Operations

   1. How computers perform arithmetic operations using integers?

Most computers use the signed two's complement representation when performing arithmetic operations with integers.

   2. what logic is applied when using arithmetic or subtraction arithmetic?

When the signs of A and B are identical(different), add the two magnitudes and attach the sign of A to the result. When the signs of A and B are different (identical),compare the magnitudes and subtract the smaller number from the larger. Choose the sign of the result to be the same as A if A > B or the complement of the sign of A if A < B. If the two magnitudes are equal, subtract B from A and make the sign of the result positive.

   3. How is addition or subtraction done using signed two's complement?

The leftmost bit of a binary number represents the sign bit: 0 for positive and 1 for negative. If the sign bit is 1, the entire number is represented in 2’s complement form.
Thus +33 is represented as 00100001 and -33 is as 11011111. Note that 11011111 is the 2’s complement of 11011111, and vice versa. The addition of two numbers in signed 2’s complement form consists of adding the numbers with the sign bits treated the same as the other bits of the number. A carry-out of the sign-bit position is discarded.

   4. how does hardware handle this signed magnitude data?

When multiplication is implemented in a digital computer, it is convenient to change the process slightly.
First,instead of providing registers to store and add simultaneously as many binary numbers as there are bits in the multiplier,it is convenient provided an adder for the summation of only two binary numbers and successfully accumulate the partial products in a register.Second,instead of listing the multiplicand to the left, the partial product is shifted to the right, which results in leaving the partial product and the multiplicand in the required relative positions.

Third, when the corresponding bit of the multiplier is 0, there is no need to add all 0’s to the partial product since it will not alter its value.

   5.What is Booth algorithm multiplication method?

Booth algorithm gives a procedure for multiplying binary integers to signed-2’s complement representation.It operates on the fact that string of 0s in multiplier require no addition but just shifting, and string of 1’s in the multiplier from bit weight 2k to weight 2m can be treated as 2k + 1 - 2m.

   6. what is array multiplier?

Checking the bit of the multiplier one at a time and forming partial product is a sequential operations that require a sequence of add and shift micro operations. The multiplication of binary numbers can be done with one micro operation by means of a combinational circuit that forms the product bits all at once.

   7. how is division handled in processing?

Division of two fixed point binary numbers in signed- magnitude representation is done with paper and pencil by a process of successive compare, shift and subtract operations.Binary division is simpler than decimal division because the quotient digits are either 0 or 1 and there is no need to estimate how many times the dividend or partial reminder fits into the divisor.

   8. how is divide overflow handled?

The division operation may result in a quotient with an overflow. This is not a problem when working with paper and pencil but it is critical when the operation is implemented with hardware.This is because the length of a register is finite and will not hold a number that exceeds the standard length.To see this consider a system that has 5 registers.

   9. what is the principle of BCD adder?

Consider the Arithmetic addition of two decimal digits in BCD, together with a possible carry from a previous stage.Since each input digit does not exceed 9, the output sum cannot be greater than 9 + 9 + 1 = 19, the 1 in the sum being an input-carry.Suppose that we apply two BCD digits to a 4-bit binary adder. The adder will form the sum in binary and produce a result that may range from 0 to 19.

   10.What is the principle of BCD subtraction?

A straight subtraction of two decimal numbers will require a subtractor circuit that will be somewhat different from a BCD adder. It is more economical to perform the subtraction by taking the 9’s or 10’s complement of the subtrahend and adding it to the minuend. Since, the BCD is not a self-complementing code, the 9’s complement can’t be obtained by complementing each bit in the code. It must be formed by a circuit that subtracts each BCD digit from 9.

Memory Organisation

   1.Describe memory hierarchy.

The memory hierarchy system consist of all storage devices employed in a computer system from the slow but high capacity auxiliary memory to a relatively faster main memory, to an even smaller and faster cache memory accessible to the high speed processing logic.

   2. what is cache?

The cache memory is employed in computer system to compensate for the speed differential between main memory access time and processes logic. CPU logic is usually faster than main memory access time with the result that processing speed is limited, primarily by the speed of main memory.

   3. what is the main memory of computer system?

It is the central Processing Unit in a computer system. It is a relatively large and fast memory used to store programs and data during the computer operation. The principal technology used for the main memory is based on Semiconductor Integrated circuits. Integrated circuits RAM chips are available in two possible operating modes Static and Dynamic.

   4. what is a bootstrap loader?

Bootstrap loader is a program function is to start the computer software operating when power is turned on.

   5. what is memory address map?

The designer of a computer system must calculate the amount of memory required for the particular application and assign it to either RAM or ROM. The interconnection between memory and processor is then established from knowledge of the size of memory needed and the type of RAM and ROM chips available.The addressing of memory can be established by means of a table that specifies the memory address assigned to each chip. The table called a memory address map, is a pictorial representation of assigned address space for each chip in the system.

   6. which memory is connected to CPU?

RAM and ROM chips are connected to a CPU through the data and address buses. The low-order lines in the address bus select the byte within the chips and other lines in the address bus select a particular chip through the chip select input.

   7. what kind of auxiliary memory used?

The most common auxiliary memory devices used in computer systems are magnetic disks and tapes. Other components used but not as frequently , are magnetic drums, magnetic bubble memory and optical disks. To understand fully the physical mechanism of auxiliary memory devices One must have a knowledge of magnetics, electronics and electromechanical Systems. all the physical properties of these storage devices can be quite complex, their logical properties can be characterized and compared by a few parameters.

   8. what is a magnetic disk?

Magnetic disk is a secured constructed of metal or plastic coated with magnetized material. Often both sides of the disks are used and several disks may be stacked on one spindle with read/write heads available on each surface. All disks rotate together at high speed and are not stopped for access purposes. Bits are stored in the magnetized surface in spots along concentric circles called Tracks. The tracks are commonly divided into sections called Sectors. In most systems, the minimum quantity of information which can be transferred is a sector. Some units use a single read/write head for each disk surface. In this type of unit, the track address bits are used by a mechanical assembly to move the head into the specified track position before reading or writing.

   9. what is floppy disk drive?

A disk drive with removable disks is called a floppy disk. The disk used with a floppy disk drive are small removable disks made of plastic coated with magnetic recording material. There were two commonly used sizes are with diameter of 5.25 and 3.5 inches.The 3.5 inch disks are smaller and can store more data than can the 5.25 inch disks.

   10. what is a Magnetic tape?

A magnetic tape transport consists of the electrical, mechanical and electronic components to provide the bars and control mechanism for a Magnetic tape units. The tape itself is a strip of plastic coated with a magnetic recording medium. Bits are recorded as magnetic spots on the tape along severed tracks.Usually, 7 or 9 bit are recorded simultaneously to form a character together with a parity bit.

   11. what is associative memory?

Memory unit accessed by content is called and associative memory or content addressable memory (CAM). This type of memory is accessed simultaneously and in parallel on the basis of data content rather than by specific address or location. When a word is written in an associative memory, no address is given. The memory is capable of finding an Empty unused location to store the word.

   12. how is writing operation done?

An associative memory must have a write capability for storing the information to be searched. Writing in an associative memory can take different forms depending on the application.If the entire memory is located with new information at once prior to the search operation then the writing can be done by addressing each location in sequence.

   13. what is mapping?

The basic characteristic of cache memory is its fast access time. Therefore very little or no time must be wasted on searching for words in the cache. The transformation of data from main memory to cache memory is referred to as a mapping process. Three types of procedures are of practical interest when considering the organisation of cache memory,
I. Associative Mapping
ii .Direct Mapping and
Iii. Set -associative Mapping

   14. what is virtual memory?

In memory system, programs and data are first stored in auxiliary memory. Portions of a program or data are brought into main memory as they are needed by the CPU. Virtual memory is a concept used in some large computer systems that permit the users to construct programs as though a large memory space were available, equal to the totality of auxiliary memory.

   15. what is inter-leaved memory?

Pipeline and Vector processors often require simultaneous access to memory from two or more sources. An instruction pipeline may require the fetching of an instruction and an operand at the same time from two different segments. Similarly the arithmetic pipeline usually requires two or more operands to enter the pipeline at the same time. Instead of using two different memory buses for simultaneous access , the memory can be partitioned into a number of modules connected to a common memory address.

Input-Output Organisation

   1. What are input and output devices are used for?

The input-output subsystem of a computer, referred to as I/O, provides an efficient mode of communication between the central system and the outside environment. Programs and data must be entered into computer memory for processing and results obtained from computations must be recorded or displayed for the user.

   2. what are ASCII alphanumeric characters?

Input and output devices that communicates with people and the computer usually involved in the transfer of alphanumeric information to and from the device and the computer. The standard binary code for the alphanumeric characters is ASCII (American Standard Code for Information Interchange).

   3. Describe the various forms of differences between computer and peripheral communications.

The major differences are: Peripherals are electromechanical and electromagnetic devices and their manner of operation is different from the operation of the CPU and memory., which are electronic devices.Therefore, a conversion of signal values may be required.The data transfer rate of peripheral is usually slower than the transfer rate of the CPU, and consequently, a synchronization mechanism may be needed. Data codes and formats in peripherals differ from the word format in the CPU and memory; and The operating modes of peripherals are different from each other and each must be controlled so as not to disturb the operation of other peripherals connected to the CPU.

   4. which three methods are used by computer buses to communicate with memory and I/O.

There are three ways that computer buses can be used to communicate with memory and I/O:
i. Use two separate buses, one for memory and the other for I/O; and
ii. Use one common bus for both memory and I/O but have separate control lines for each ; and
iii. Use one Common bus for memory and I/O with common control lines.

   5. how is data transfer handled?

Data transfer to and from peripherals may be handled in one of three possible modes:
i. Programmed I/O
ii. Interrupt-Initiated I/O
iii. Direct Memory Access

   6. what is programmed I/O?

Programmed I/O operations are the result of I/O instructions written in the computer program. Each data item transfer is initiated by an instruction in the program. Usually, the transfer is to and from a CPU register and peripheral. Other instructions are needed to transfer the data to and from CPU and memory. Transferring data under program control requires constant monitoring of the peripheral by the CPU.

   7. what is direct memory access?

The transfer of data between a fast storage device such as magnetic disk and memory is often limited by the speed of CPU. Removing the CPU from the path letting the peripheral device manage the memory buses directly would improve the speed of transfer. This transfer technique is called a Direct Memory Access(DMA).

   8. what is Daisy Chain priority?

The daisy-chaining method of establishing priority consists of a serial connection of all devices that request an interrupt. The device with the highest priority is placed in the first position, followed by lower priority devices upto the device with the lowest priority, which is placed last in the chain. The interrupt request line is common to all devices and forms a wired logic connection. If any device has its interrupt signal in the low-level state, the interrupt line goes to the low-level state and enables the interrupt input in the CPU.

   9. how does CPU initialise the DMA?

The CPU initialises the DMA by sending the following information through the data bus. The starting address of the memory block where data are available( for read) or where data are to be stored (for write); The word count, which is the number of words in the memory block;Control to specify the mode of transfer such as read or write; and a control to start the DMA transfer.

   10. what is parallel priority interrupt?

The parallel priority interrupt method uses a register whose bits are set separately by the interrupt signal from each device.Priority is established according to the position of the bits in the register. In addition to the interrupt register , the circuit may include the mask register whose purpose is to control the status of each interrupt request, The mask register can be programmed to disable lower-priority interrupts while a higher-priority device is being serviced.

   11. what is priority encoder?

The priority encoder is a circuit that implements the priority function.The logic of the priority encoder is such that if two or more inputs arrive at the same time , the input having the highest priority will take precedence.

   12. what is input output processor?

Instead of having each interface communicate with the CPU , a computer ma incorporate one or more external processors and assign them the task of communicating directly with all I/O devices.An input-output processor(IOP) may be classified as processor with direct memory access capability that communicates with I/O devices. In this configuration, the computer system can be divided into a memory unit, and a number of processor comprised of the CPU and one or more IOPs.