The flag semaphore symbols for letters " N " ( green ) and " D " ( blue )

It is based on the semaphore symbols for " N " ( two flags held 45 degrees down on both sides, forming the triangle at the bottom ) and " D " ( two flags, one above the head and one at the feet, forming the vertical line ) ( for Nuclear Disarmament ) within a circle.

The symbol is a combination of the semaphore signals for the letters " N " and " D ," standing for " nuclear disarmament ".

Its symbol, a combination of the semaphore symbols for " N " ( nuclear ) and " D " ( disarmament ), entered modern popular culture as an icon of peace.

But the cross, for these people, had too many wrong associations-with the Crusaders, with military meals, with the public blessing by an American chaplain of the plane that flew to Hiroshima-and eventually the arms of the cross were allowed to drop, forming the composite basic semaphore signal for the letters N and D, and at the same time a gesture of human despair against the background of a round globe.

The design was a combination of the letters " N " ( two arms outstretched pointing down at 45 degrees ) and " D " ( one arm upraised above the head ) of the flag semaphore alphabet, standing for nuclear disarmament.

The first flags were used to assist military coordination on battlefields, and flags have since evolved into a general tool for rudimentary signalling and identification, especially in environments where communication is similarly challenging ( such as the maritime environment where semaphore is used ).

A binary semaphore is either locked or unlocked.

When it is locked, tasks must wait for the semaphore to unlock.

A binary semaphore is therefore equivalent to a mutex.

A typical solution is to have the task that owns a semaphore run at ( inherit ) the priority of the highest waiting task.

But this simplistic approach fails when there are multiple levels of waiting: task A waits for a binary semaphore locked by task B, which waits for a binary semaphore locked by task C. Handling multiple levels of inheritance without introducing instability in cycles is complex and problematic.

Deadlock is prevented by careful design or by having floored semaphores, which pass control of a semaphore to the higher priority task on defined conditions.

Although their real-time behavior is less crisp than semaphore systems, simple message-based systems avoid most protocol deadlock hazards, and are generally better-behaved than semaphore systems.

Thus semaphore is a method of telegraphy whereas pigeon post is not.

This is to be distinguished from semaphore which merely transmits messages.

* The semaphore line in France is superseded by the telegraph.

* A railway semaphore signal is designed so that should the cable controlling the signal break, the arm returns to the " danger " position, preventing any trains passing the inoperative signal.

Finally, with no one else left, Marcus himself is struck on the head by a railway semaphore signal over the tunnel, and drops lifeless into another wagon.

A semaphore telegraph, optical telegraph, shutter telegraph chain, Chappe telegraph, or Napoleonic semaphore is a system of conveying information by means of visual signals, using towers with pivoting shutters, also known as blades or paddles.

The semaphore tower at Chatley Heath, which replaced the Netley Heath station of the shutter telegraph, has been restored by Surrey County Council and is open to the public.

One of the principal hills in San Francisco, California is also named " Telegraph Hill ", after the semaphore telegraph which was established there in 1849 to signal the arrival of ships into San Francisco Bay.

* In chapter 10 of C. S. Forester's Hornblower and the Hotspur, the destruction of a French semaphore tower and a shore battery is a key plot point.

* In Alastair Reynolds ' Terminal World, the distant-future terrain is criss-crossed with semaphore lines relaying information between the one remaining city, Spearpoint, outlying communities and the airborne community Swarm.

In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment.

Stamped and formed metal straps riveted to the arched opening and the mailbox door served as a door latching mechanism, while a rotating red semaphore flag mounted on a shaft attached to the side of the mailbox served to notify the approaching mailman if outgoing mail was inside.

For example, semaphore, where the configuration of flags held by a signaller or the arms of a semaphore tower encodes parts of the message, typically individual letters and numbers.

The earliest examples of color codes in use are for long distance communication by use of flags, as in semaphore communication.

Resembling railway signals, they use semaphore arms and are illuminated at night by red and green gas lamps.

The electrical telegraph, or more commonly just ' telegraph ', superseded optical semaphore telegraph systems, such as those designed by Claude Chappe for the French military, and Friedrich Clemens Gerke for the Prussian military, thus becoming the first form of electrical telecommunications.

A semaphore network invented by Claude Chappe operated in France from 1792 through 1846.

French officers used the balloon to observe the movements of the Austrian Army, dropping notes to the ground for collection by the French Army, and also signalled messages using semaphore.

As part of this work four existing manual signal boxes were replaced by three new Power Signal Boxes ; and the semaphore signals and mechanical point linkages were replaced by colour light signals and point motors.

This was replaced by a semaphore station, which was part of a semaphore line that operated between 1822 and 1847.

The signal consisted of a semaphore arm ( Manufactured by Saxby and Farmer, who were railway signaling makers ), which was raised and lowered manually by a police constable who would rotate a handle on the side of the pole.

The semaphore arms were augmented by gas illuminated lights at the top ( green and red ) to increase visibility of the signal at night.

Lord George Murray, stimulated by reports of the Chappe semaphore, proposed a system of visual telegraphy to the British Admiralty in 1795.

In a deadlock, two or more tasks lock semaphores without timeouts and then wait forever for the other task's semaphore, creating a cyclic dependency.

A Popham semaphore was a single fixed vertical 30 foot pole, with two movable 8 foot arms attached to the pole by horizontal pivots at their ends, one arm at the top of the pole, and the other arm at the middle of the pole.

This approach does not work on multiprocessor systems where it is possible for two programs sharing a semaphore to run on different processors at the same time.

The semaphore solution to the Producer-consumer problem tracks the state of the queue with two semaphores:, the number of empty places in the queue, and, the number of elements in the queue.

The binary semaphore ensures that the integrity of the state of the queue itself is not compromised, for example by two producers attempting to add items to an empty queue simultaneously, thereby corrupting its internal state.

From Crawfordsville to near Lafayette, some semaphore signals were removed in 2009, however several are still in service, including two sets in Crawfordville, with their replacement signals adjacent for the day CSX makes the conversion ( as of November 10, 2010 ).

Flag semaphore signalling uses two flags, held in specific positions to signify letters.

The banning of semaphore led to the Borogravians cutting down the clacks towers along the Zlobenian border, which was one of the main causes of war between the two nations ( the war has been going on for so long that no-one can remember why it is actually happening ).

There are two signal boxes, Boat of Garten North and Boat of Garten South and signalling uses traditional British Railways mechanical semaphore signalling.

To the east of the station can be found the only two semaphore signals on the line guarding the manually operated road crossing ..

In preparation for electrification, Aston's two mechanical signal boxes, Aston No. 1 and No. 2, were closed, semaphore signalling was replaced by multiple-aspect colour light signals and control transferred to the power signal box at Birmingham New Street.

* two semaphore shunt signals at Downham Market station