Question

(10 marks) 4. a) With respect to the architecture of a computer system, there is a subsystem of components that moves coded data between external devices and the host system. Enumerate and explain five (5) constituents of the subsystem in question. b) The flow of execution of computer instructions are mostly interfered by signals commonly referred to as interrupts. When interrupts are triggered, the current instructions being executed are halted and placed on a stack for the CPU to attend to interrupt. Identify three (3) types of interrupts and explain its modus operandus.

Answer 1

(a)Architecture of the computer system

A computer system is basically a machine that simplifies complicated tasks. It should maximize performance and reduce costs as well as power consumption.

The interface between a computer’s hardware and its software is its architecture. The architecture is described by what the computer’s instructions do, and how they are specified. Understanding how it all works requires knowledge of the structure of a computer and its assembly language.

Five-Subcomponents of the computer system architecture

The different components in the Computer System Architecture are Input Unit, Output Unit, Storage Unit, Arithmetic Logic Unit, Control Unit.

Input Unit

The input unit provides data to the computer system from the outside. So, basically it links the external environment with the computer.

It takes data from the input devices, converts it into machine language and then loads it into the computer system. Keyboard, mouse etc. are the most commonly used input devices.

Output Unit

The output unit provides the results of computer process to the users i.e it links the computer with the external environment.

Most of the output data is the form of audio or video. The different output devices are monitors, printers, speakers, headphones etc.

Storage Unit

Storage unit contains many computer components that are used to store data. It is traditionally divided into primary storage and secondary storage.

Primary storage is also known as the main memory and is the memory directly accessible by the CPU. Secondary or external storage is not directly accessible by the CPU.

The data from secondary storage needs to be brought into the primary storage before the CPU can use it. Secondary storage contains a large amount of data permanently.

Arithmetic Logic Unit

All the calculations related to the computer system are performed by the arithmetic logic unit. It can perform operations like addition, subtraction, multiplication, division etc.

The control unit transfers data from storage unit to arithmetic logic unit when calculations need to be performed. The arithmetic logic unit and the control unit together form the central processing unit.

Control Unit

This unit controls all the other units of the computer system and so is known as its central nervous system.

It transfers data throughout the computer as required including from storage unit to central processing unit and vice versa.

The control unit also dictates how the memory, input output devices, arithmetic logic unit.

Diagram of flow of data:

STORAGE UNIT INPUT INPUT UNIT ARITHMETIC LOGIC UNIT OUTPUT UNIT OUTPUT CONTROL UNIT

The input data travels from input unit to ALU. Similarly, the computed data travels from ALU to output unit. The data constantly moves from storage unit to ALU and back again. This is because stored data is computed on before being stored again. The control unit controls all the other units as well as their data.

(b)Interrupts

The flow of execution of computer instructions are mostly interfered by signals commonly referred to as interrupts.

It sents signal to the CPU by external devices, normally I/O devices. They tell the CPU to stop its current activities and execute the appropriate part of the operating system.

Three types of Interrupts:

Hardware Interupts are generated by hardware devices to signal that they need some attention from the OS. They may have just received some data (e.g., keystrokes on the keyboard or an data on the ethernet card); or they have just completed a task which the operating system previous requested, such as transfering data between the hard drive and memory.

Software Interupts are generated by programs when they want to request a system call to be performed by the operating system.

Traps are generated by the CPU itself to indicate that some error or condition occured for which assistance from the operating system is needed.

Interrupts are important because they give the user better control over the computer.

Without interrupts, a user may have to wait for a given application to have a higher priority over the CPU to be ran.

Interrupts ensures that the CPU will deal with the process immediately.

CPU responds to the interrupt

A key point towards understanding how operating systems work is to understand what the CPU does when an interrupt occurs. The hardware of the CPU does the exact same thing for each interrupt, which is what allows operating systems to take control away from the current running user process.

CPUs rely on the data contained in a couple registers to correctly handle interrupts. One register holds a pointer to the process control block of the current running process. This register is set each time a process is loaded into memory. The other register holds a pointer to a table containing pointers to the instructions in the OS kernel for interrupt handlers and system calls.

The CPU performs the following actions in response to an interrupt:

1. Using the pointer to the current process control block, the state and all register values for the process are saved for use when the process is later restarted.
2. The CPU mode bit is switched to supervisory mode.
3. Using the pointer to the interrupt handler table and the interrupt vector, the location of the kernel code to execute is determined. The interrupt vector is the IRQ for hardware interrupts (read from an interrupt controller register) and an argument to the interrupt assembly language instruction for software interrupts.
4. Processing is switched to the appropriate portion of the kernel.