Control Unit (CU)
responsible for the operation of the CPU. It controls the retrieval of instructions from the primary memory as well as the sequence of their execution. It does the fetching of data and instructions in the fetch-decode-execute cycle
Arithmetic Logic Unit (ALU)
performs all the basic arithmetic, logical or input/output operations. sometimes referred to as the ‘core’
Memory Address Register (MAR)
holds the address of the data (address - the specific location in the RAM) to be used by the ALU, so that the ALU can fetch the corresponding content from the memory and process it accordingly.
Memory Data Register (MDR)
holds the data that is to be used by the ALU and then saved to the RAM. The MAR tells us the location and the MDR finds/fetches the data. That data/connection is done using the Data Bus.
primary memory
The primary memory is the only storage directly accessible by the CPU. The primary may hold both data and instructions that are currently running on the computer system. These are stored as binary/machine code.
Random Access Memory (RAM)
- type of memory that can be accessed randomly (any byte of memory can be accessed without touching the preceding bytes).
- Found in servers, PCs and other devices.
- Volatile : all data is lost when device is turned off.
Read-Only Memory (ROM)
- Computer memory on which data has been pre-recorded.
- Once data has been written onto a ROM chip, it cannot be removed and can only be read.
- Non-volatile : data is kept even when the device is turned off.
- Used extensively in calculators and peripheral devices such as laser printers, whose fonts are often stored in ROMs.
Cache
- A small amount of memory which is a part of the CPU used to temporarily hold instructions and data that the CPU is likely to reuse.
- Cache saves fetching the instructions and data repeatedly from the RAM (which is a relatively slow process).
- Effectiveness measured in hit rate
Explain the machine instruction cycle
- Fetch instruction from primary memory to control unit: Sends appropriate address through the memory (address) bus to the primary memory. The instructions in the address is then copied to the data bus and sent to the control unit (CU).
- Decode instruction in control unit: Decoding allows the CPU to be aware of any additional data that are necessary for the execution of the instruction. Required data that needs to be loaded from the primary memory is fetched (back to step 1).
- Execute instruction: The CPU executes the instruction using the necessary data that have been loaded and calculates a result. (Once again any necessary additional data is fetched and loaded from the primary memory, repeat back to step one for that new data).
- Store result of execution and check for next instruction
secondary memory / persistent storage
Secondary memory is a relatively slow memory that may be written to but is also non-volatile
∴ contents are persistent as they are not wiped if power is lost. This is why secondary memory is often referred to as persistent storage. (Higher capacity than primary memory).
Virtual memory
- the use of secondary memory as if it was primary memory
- allows for the utilisation of more primary storage capacity than is physically available
examples of secondary memory
- Hard drive (a.k.a. Hard disk)
- CD-RW, DVD-RW
- USB flash drive
- Zip disk / floppy disk
- Magnetic tape
primary vs secondary memory
- Most computers are equipped with a smaller amount of primary memory and a larger amount of secondary memory
- Primary memory is volatile, which means it does not retain data when the power is turned off.
- Primary memory is more expensive compared to secondary memory.
- Primary memory is much faster than secondary memory.
- Primary memory is directly accessed by the CPU
- Secondary memory is not directly accessible by the CPU
- Secondary memory is non-volatile, which means that it retains data when the power is turned off
operating system (OS)
An Operating System (OS) is a set of software that controls the computer’s hardware resources and provides services for computer programs. It is a very important part of a computer system since it acts as an intermediary between software applications (i.e. programs) and the computer hardware
functions of an os
- Peripheral communication
- Memory management
- Resource monitoring and multitasking
- Networking
- Disk access and data management
- Security
peripheral communication (OS)
The OS is responsible for communicating directly with the hardware and providing an interface between hardware devices and applications. This allows for applications to use hardware devices in a trouble-free manner.
memory management (OS)
An OS is responsible for all the memory that is available in a computer system. An OS manages how the memory is used by applications and ensures that one application does not interfere with memory that is being used by some other application.
resource management and multitasking (OS)
An application running takes up resources. These include the amount of memory the application is occupying, or how much processor time it needs in order to function properly. An OS is responsible for the efficient allocation of resources so that an application can run as effectively as possible. (Multitasking notes).
networking (OS)
An OS manages connections to and interactions with networks of other computer systems so as to allow the sharing of resources. An OS acts as an intermediary between applications and networks, allowing applications to interact with networks in a straightforward manner.
disk access and data management (OS)
The OS is responsible for keeping track of files, as well as which files are being used by which applications so that an application does not overwrite another applications’ files. The OS is also responsible for coordinating the transfer of data from the disk files into the primary memory.
security
The most common method of protection is to provide some form of identity to the user that will allow his/her authentication (most often a username and a passwords). This is a form of authentication. Another security measure is the use of log files that keep track of the activity of any user in the computer system.
application software
Application softwares are programs designed for the end-user that enable the completion of various tasks in order to increase productivity.
application software types
- Word processors
- Spreadsheets
- Database management systems (DBMS)
- Web browsers
- Computer Aided Design (CAD)
- Graphic processing software
word processors (application software)
A software application that is used for the production of any sort of document. It includes tools for the composition, editing, formatting and possibly printing of documents. Word processing is the action of creating documents using a word processor.
