Info-Grade: CA Notes

Pipelining Hazards

Define arithmetic pipelining. Explain pipelining hazards with examples.

Pipelining is a technique where multiple instructions are overlapped during execution. Pipeline is divided into stages and these stages are connected with one another to form a pipe like structure. Instructions enter from one end and exit from another end. There are two types of pipeline.

Arithmetic Pipeline
Instruction Pipeline

Arithmetic pipelines are usually found in most of the computers. They are used for floating point operations, multiplication of fixed point numbers etc.

Pipeline hazards are situations that prevent the next instruction in the instruction stream from

Pipelining Hazards

Pipeline hazards are situations that prevent the next instruction in the instruction stream from executing during its designated clock cycles. In another word, any condition that causes a stall in the pipeline operations can be called a hazard. There are mainly three types of hazards, They are :

Data Hazards
Control Hazards or instruction Hazards
Structural Hazards.

Example:

A=3+A

B=A*4

For the above sequence, the second instruction needs the value of ‘A’ computed in the first instruction. Thus the second instruction is said to depend on the first. In this situation data hazards is arises. A data hazard is any condition in which either the source or the destination operands of an instruction are not available at the time expected in the pipeline.

Memory Hierarchy and Cache Memory Mapping Functions

Define memory hierarchy. Explain cache memory mapping functions with example.

Memory hierarchy is a concept that is necessary for the CPU to be able to manipulate data. In computer architecture, the memory hierarchy separates computer storage into a hierarchy based on response time. Since response time, complexity, and capacity are related, the levels may also be distinguished by their performance and controlling technologies. The following figure shows the hierarchy of memory in computer.

Cache is used by the CPU for memory which is being accessed over and over again. Instead of pulling it every time from the main memory, it is put in cache for fast access. It is also a smaller memory, however, larger than internal register.

Cache memory is used to reduce the average time to access data from the Main memory. The cache is a smaller and faster memory which stores copies of the data from frequently used main memory locations.

There are different levels of catch memory. Level 1, Level 2, Level 3 etc. Levels of catch is based on the architecture of computer.

Cache Mapping
There are three different types of mapping used for the purpose of cache memory which are as follows:

Direct mapping
Associative mapping and
Set-Associative mapping.

Direct Mapping

Direct catch mapping

The simplest way to determine cache locations in which store Memory blocks is direct Mapping technique.
In this block J of the main memory maps on to block J modulo 128 of the cache. Thus main memory blocks 0,128,256,….is loaded into cache is stored at block 0. Block 1,129,257,….are stored at block 1 and so on.
Placement of a block in the cache is determined from memory address. Memory address is divided into 3 fields, the lower 4-bits selects one of the 16 words in a block.

When new block enters the cache, the 7-bit cache block field determines the cache positions in which this block must be stored.
The higher order 5-bits of the memory address of the block are stored in 5 tag bits associated with its location in cache. They identify which of the 32 blocks that are mapped into this cache position are currently resident in the cache.
It is easy to implement, but not Flexible

Divide 21 by 6 using restoring Two's complement method

Organization of Micro-Programmed Control design

Explain the Organization of micro-programmed control design.

Micr-program is a process of writing microcode for a microprocessor. Microcode is low-level code that defines how a microprocessor should function when it executes machine-language instructions. Typically, one machine language instruction translates into several microcode instruction, on some computers, the microcode is stored in ROM and can not be modified.

Micro programmed Control Unit:

A control unit with its binary control values stored as words in memory is called as micro programmed control. Each word in the control memory contains micro instruction that specifies one or more micro operations for the system. A sequence of micro instructions constitutes a micro program.
Micro programmed implementation is a software approach in contrast to the hardwired approach.
It deals with various units of software but at the micro level i.e. micro-operation, micro-instruction, micro-program etc.
Different key elements used for implementation of a control unit using micro programmed approach is shown in fig. below:

Control Address Register (CAR)

It contains the address of next micro instruction to be read. This is similar to the program counter(PC) which stores the address of the next instruction.

Control Memory

The set of micro instruction is stored in control Memory (CM) also called as control store.

Control Buffer Register(CBR)

When microinstruction is read from the control memory, it is transferred to a control Buffer Register (CBR), which is similar to the instruction Register (IR) that stores the opcode of the instruction read from the memory.

Sequencing

It loads the control Address register with the address of the next instruction to be read abd issues a read command to control memory.

Instruction Execution Cycle

Explain the instruction execution cycle with state diagram.

The instruction cycle is the cycle which the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instructions. It is also known as the fetch–decode–execute cycle or simply the fetch-execute cycle. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage. In an improved instruction execution cycle, we can introduce a next cycle known as the interrupt cycle.

Fetch

It is the process of obtaining instructions from the memory. The next instruction is fetched from the memory address that is currently stored in the PC (program counter) and stored into the IR (instruction register). At the end of the fetch operation, the PC points to the next instruction that will be read at the next cycle.

Decode

It is the stage of understanding the instructions. During this stage the encoded instruction present in the instruction register is interpreted by the decoder.

Execute

In this stage control unit sends the instructions and data to ALU for taking suitable action on the instruction and writing the result back to a register. If the ALU is involved, it sends a condition signal back to the CU. The result generated by the operation is stored in the main memory or sent to an output device.

Repeats Cycle

Once the execution cycle is complete, It repeats the same process/cycle for the next instruction.

Computer System and John Von Neumann's architecture of computer system

What is computer system? Discuss its components. Explain John Von Neumann's architecture of computer system.

A computer system is a set of integrated devices that input, output, process, and store data and information. Computer systems are currently built around at least one digital processing device. There are five main hardware components in a computer system: Input, Processing, Storage, Output and Communication devices.

Input

Input means data and instruction given to the computer which is most essential for producing meaningful and useful output. The unit which is used to give input to the computer system is called input unit and it is formed by various input devices attached to the computer such as keyboard, mouse, joystick, trackball touch screen, MICR (Magnetic Ink Character REader) etc into computer understandable form. The input unit establishes the communication link between the user and the computer system.

Processing

Processing unit is also called 'Central Processing Unit' and it is the control center for a computer. It guides, directs and governs all operations and components inside the computer. It is considered as brain of computer. It is linked with various peripheral devices including I/O devices, secondary storage and memory unit. it performs arithmetic operations, logical comparison, transfer information between all parts of computer and executes instructions. CPU consists of ALU, CU and Registers.

Output

Output is the processed data which is very useful and meaningful to us and we can get it from the computer in the form that we want. The unit which is used to provide output is called output unit. It is always in the form of human readable or understandable. There are various types of output devices such as Monitor, Printer, Plotter, Speakers etc.

Memory/Storage Unit

A storage device is any computing hardware that is used for storing, porting and extracting data files and objects. It can hold and store information both temporarily and permanently, and can be internal or external to a computer, server or any similar computing device. Data and instruction are stored in memory in the binary form.

Von Neumann Architecture

Von Neumann architecture is based on the stored-program computer concept, where instruction data and program data are stored in the same memory. This design is still used in most computers produced today. This architecture was first published by John von Neumann in 1945.

Von Neumann Architecture

His computer architecture design consists of a Control Unit, Arithmetic and Logic Unit (ALU), Memory Unit, Registers and Inputs/Outputs.

CPU (Central Processing Unit)

The CPU is an electronic circuit which is responsible for executing the instructions of a computer program. Sometimes it is also referred to as the microprocessor or processor. It contains ALU, CU and Registers.

Registers

Registers are high speed storage areas in the CPU. All data must be stored in a register before it can be processed. Some registers are

MAR (Memory Address Register)
MDR (Memory Data Register)
AC (Accumulator)
PC (Program Counter)
CIR (Current Instruction Register)

ALU (Arithmetic and Logic Unit)

The ALU allows arithmetic (add, subtract etc) and logic (AND, OR, NOT etc) operations to be carried out.

CU(Control Unit)

The control unit controls the operation of the computer’s ALU, memory and input/output devices, telling them how to respond to the program instructions it has just read and interpreted from the memory unit.

MU (Memory Unit)

The memory unit consists of RAM, sometimes referred to as primary or main memory. Unlike a hard drive (secondary memory), this memory is fast and also directly accessible by the CPU.

Buses

Buses are the means by which data is transmitted from one part of a computer to another, connecting all major internal components to the CPU and memory. There are three types of buses

Address Bus (Carries the addresses of data)
Data Bus (Carries data between different units)
Control Bus (Carries control signals from CPU to control the activities)

Concept of System Buses (Computer Architecture)

As we need a highway to travel from one place to another place. Similarly, the input and output of computer system also need a path to transmit data, information and control from one device to another device. The data transmit from input device to CPU, CPU to memory are common examples of system bus.

The path through which data and instruction flow is called bus of computer. It is a collection of wires, chips and slots inside the computer through which data and information are transmit from one part of computer to another.

Normally, Bus is available from 1 bit and above. The width of the bus determines how much data can moved at a time. Nowadays, PATA cables are replaced by SATA cables because of faster and small in size. These are also a way of transferring data and information from HDD to memory. Beside these cables there are numbers of buses inside the computer system (motherboard). There are three types of System bus they are :

1) Address Bus

The pathway through which transmit of address of memory location is called address bus. All types of memory devices have microscopic memory cells which are identified with unique humans known as memory address or memory locations. When the CPU reads data or instruction from memory or writes data to memory, it must specify the address of the memory location it is going to access. Unlike the other buses, the address bus always receive memory location from the CPU. There is one way flow in address bus.

2) Data Bus

The pathway (circuit / chips / wires / slots) through which transmit of data from one memory location to other is called Data Bus. When the CPU fetches data from memory, it first output the memory address on its address bus. Then the data bus. When writing data to memory, the CPU first outputs the address onto the address bus, then output the data onto the data bus. Memory the reads and stores the data to the proper location. The data flows in bidirectional way.

3) Control Bus

The pathway (circuit / chips / wires / slots) through which transmit of control signal to operate and control devices and software is called control bus. It is different from other two buses. The control bus is the collection of individual control signals for timing and controlling function sent by the control unit to other unit of the system. These signals indicate where data is to read or written, whether the CPU is accessing memory or an input/output device, and whether the I/O device or memory is ready to transfer data.