4.7 - Fundamentals of computer organisation and architecture

Processor Main memory Address bus Data bus Control bus I/O controllers

Sometimes referred to as the CPU - central processing unit Processes data by executing program instructions - machine code instructions based on a specific instruction set

Instructions need to be transferred from secondary storage into main memory from where they can be fetched, decoded and executed The data that needs to be processed is also loaded into main memory or provided by input/output devices via I/O controllers

Two types of main memory: RAM - working memory used by processor during the fetch-decode-execute cycle ROM - used in boot process for computer system Instructions or data are stored as binary sequences in memory locations Each one has a physical address - a number used to locate the location and access its contents Distinct from secondary storage - can be accessed directly by the processor; secondary storage must be accessed through I/O controllers

Components each device has that act as an interface between core computer system and peripherals They connect all external and secondary storage devices to the processor Not favourable for peripherals to be directly attached to processor - each device operates with different speeds and electronic signals They provide the mechanism for: - Input data to be received for processing from input devices like keyboards - Computation results to be outputted from the system to output devices like display screens Provide a set of addressable registers that the processor can access to communicate with I/O devices

Physical port on the controller provides connection for the peripheral Device has a cable/connector to connect it to the port Large desktop computers usually have slots where additional specialist controllers can be added

Controller circuitry facilitates exchange of data in the form of electrical signals between the processor and peripheral Controller then translates signals into correct form Peripheral device almost certainly operates slower than processor Interface provides buffer - disparate timing requirements can be satisfied

A piece of software that provides an interface for the peripheral device Enables operating systems to access functions of hardware without having to be specifically configured for every device used Many devices are now ‘plug and play’ - driver is either embedded or automatically downloaded when the hardware is connected

Bus - a communication system that is used to transfer data between components System bus - a set of parallel connectionsthat allow internal components to communicate with each other and exchange data

Data bus - a bidirectional bus that transfers data and instructions Address bus - a unidirectional bus that specifies the address of a memory location to read/write data Width (number of parallel lines) = number of bits that can be used to form an address of a memory location

Sends control signals to manage and orchestrate operations that take place inside a computer system Exchangingstatus signals between the components of the computer system Transmittingclock signals required for the coordination of operations Specifying the type of data that is being transferred via the other buses (i.e. data values, instructions, or addresses)

A processor A memory unit that can communicate with the processor directly Connections for input and output devices Secondary storage for saving/backing up data

Uses the idea of storing program instructions and data in main memory and moving them between memory and the processor Processor can access the instructions and data in the main memory as required to execute a program using buses Same address and data buses used in transferringinstructions and databetween main memory and the processor Commonly used in general-purpose computers that are expected to accommodate the varying needs of the end-users

Keeps instructions and data in separate memories Processor accesses the ‘instructions memory’ and ‘data memory’ using separate data and address buses Used extensively in embedded systems (eg. DSP systems) and also in microcontroller devices and some types of smartphones

Enables more flexible use of main memory - allows processor to run a variety of programs that are not known in advance

Can be exploited by hackers who could disguise malware instructions as data that the processor may unknowingly execute

Allowing both memories to be accessed simultaneously minimises issue of keeping processor waiting while loading/saving data into memory - increases processor performance Each memory can be adapted to meet the needs of a particular system - different sizes, word lengths or implemented using different technologies E.g. the instruction memory can be implemented as ROM in systems with predetermined uses - protects programs from accidental or deliberate changes by hacking

Machine code instructions are stored in main memory They are fetched and executed serially by a processor that performs arithmetic and logical operations

Responsible for performing arithmetic calculations and logical operations that include: - Addition, subtraction, multiplication, division - Logical bitwise operations, such as AND, OR, NOT, and XOR - Comparisons between values, such as greater than, less than, equal to - Shifting binary patterns to the left or right

Responsible for directing the operations of all the other components of the processor Uses control signals to: - Enable data to be read (loaded) from and written (stored) to the main memory - Determine the operation the ALU will carry out at each instance Decodes every instruction (analyses an opcode and operand) that the processor will execute to determine what needs to be done Organises the sequence of micro-operations that need to be performed for an instruction to be carried out

Used to synchronise the operations of the processor components Generates regular clock pulses by emitting a signal that continuously oscillates between a low (0) and high (1) state Rising/falling edge - a change from a low/high to a high/low state Clock period - the time taken between two sequential rising edges (one clock cycle) Every operation of the processor typically requires a number of clock cycles to complete - specific to each processor

Locations of computer memory within the processor that provide extremely fast access Functions: - Used to temporarily store and access the results of operations - Prevents having to access main memory and instruction execution slowing down - Used to keep results of intermediate calculations produced as a part of a larger computation (connected to ALU)

Processor-based registers that are used for a specific purpose

Program Counter (PC) - holds the address of the next instruction to be executed by the processor Current Instruction Register (CIR) - holds the current instruction that the processor is executing Memory Address Register (MAR) - temporarily holds the address of the memory location (in main memory) that the processor needs to access, either to read from or write to Memory Buffer Register (MBR) - temporarily holds the data (data values or instructions) that are read from or written to the main memory Status Register - stores information about the result of the last instruction that the ALU executed