Real time Systems – Flashcards

System in which the correctness depends not only on the output values, but also on the time at which results are produced. RT means the system must be synchronized with the time in the environment.

System where it is crucial that a response occurs within required deadline. E.g. a.b.s

System where deadlines are important, but system will functions correctly if deadlines are occasionally missed. E.g. Washine Machine

System whose behavior is primarily caused by specific reactions to external events rather than being self generated.

System where actions are driven primarily by either the passage of time or the arrival of time periods.

Is encapsulated by hardware that it controls. It controls specialized hardware in which the computer system is installed.

has a microprocessor, complete controller on a single chip, performs a specific dedicated application

"An interrupt is the automatic transfer of software execution in response to hardware that is asynchronous with the current software execution. signal to the processor hardware or software application that an event needs immediate attention -e.g external devices, keyboard.

prioritizes interrupts and saves them in a queue, if more than one is waiting to be handled.

Environment or the physical system with which the computer interacts. Sensors; e.g. thermometer. Activators. e.g Heater, switch

Computer

Information about the environment - thermometers

Affect the environment - heater, switch

Each I/O device, has I/O, control and status registers mapped into memory locations. No need for special instructions for I/O. Need to know location of registers to perform/handle I/O functions.

Parts of a process that access shared variables must be executed indivisibly with respect to each other. "A sequence of statements that must appear to be executed indivisibly is called a critical section."

Program object that prevents simultaneous access to a shared resource. It is known as the synchronisation required to protect a critical section.

Two processes are deadlocked if each is holding a resource while waiting for a resource held by the other.

... is when a process wishes to perform an operation that can only be safely performed if another process has taken some action, or is in some defined state.

Variable or abstract data type that is used or controlling access by multiple process to a common resource in a concurrent system. Non negative integer

IF the value of S > 0; then decrease its value by one; otherwise we delay the process until s > 0; then we decrease it.

Increment the value of S by one.

Several computations are executing during overlapping time peri

The passing of information from one process to another.

Two processes are deadlocked, if each is holding a resource while waiting for a resource held by the other.

Two processes are executing but neither is able to make progress

occurring at the same time

Not occurring at the same time

Have a scheduled start time & explicit deadlines that must be met

Asynchronous arrival(not at same time); and specified response times; they are activated randomly and can arrive in bursts.

Aperiodic processes which have a minimum inter-arrival time.

One task in an infinite loop

Each process has a fixed, static, priority which is computed pre run time; the process is determined by their priority.

The runnable processes are executed in the order determined by the absolute deadlines of the processes. The order of process is determined by nearest deadline

If a system can be overloaded; then the use of simple static priorities or deadlines is not sufficient. Assign a value to each process; using a scheduling algorithm to decide which process runs next.

Each process is assigned(unique) priority based on its period. **The SHORTER the period = The HIGHER the priority**

Time elapsed between the time when task is ready to execute to the time when it finished its job.

When a process is waiting for a lower priority process; it is blocked.

Synchronous protocol for shared resources to avoid unbounded priority inversion and mutual deadlock due to wrong nesting of critical sections.

conceptual machine with a number of states

System components viewed as objects

Response time of a task or thread is defined as the time elapsed between the time when a task is ready to execute to the time when it finishes the job.

Conceptual machine with a number of states; in only one of the states at any specific time. Input events cause state transitions. Used for modelling behavioural aspects of the system

A graph where: -The nodes represent places -transitions are events that may occur (signified by bars) -Arc runs from a place to transition -Places from which an arc runs to a transition called the input places of the transition -Petri nets are used to specify concurrent systems as an alternative to finite state machines, which are strictly sequential and cannot be used to model parallelism.

Do not communicate or synchronise with each other.

Regularly communicate and synchronise their activities in order to perform some common operation.

communicate and synchronise in order to obtain resources

is either a point in time(time driven) or time interval(event driven) by which a system action must occur.