Page 42 - DCAP608_REAL TIME SYSTEMS

Unit 3: Hard Versus Soft Real-time System

An important problem of this algorithm (and of all the algorithms of this kind) is that, to Notes
determine if one set of tasks is schedulable, we have to calculate the resulting processor
load. To do that we have to sum the worst case execution time of all the tasks in the set.
This leads to the processor being idle most of the time since usually the average execution
time of a task is significantly shorter than the worst case.
The valve 1 is an electrically operated logic element. When active, it connects the upper
and lower main cylinder chambers leading them to a similar pressure and disabling press
force capacity. When not, and if the lower chamber pressure exceeds the upper one, it
conducts an ascending flow. In electrical terms, this valve may be roughly defined as a
switch presenting a diode in parallel, whose cathode is connected to the upper chamber;
when the valve 1 is active the switch is closed.

Since the main cylinder exhibits a constant pressure in its upper chamber, decompressing
its lower chamber enables press force production. This action is achieved by opening the
valve 2 when valve 1 is off.
When a punching cycle is initiated, the male tool is some distance apart from the metal
sheet. Valve 1 and valve 2 are off. By this time, the main cylinder exhibits a significant
pressure on both chambers. The punching cycle involves the following sequential steps:
 The auxiliary cylinder initiates a fast descending movement and valve 1 conducts a
flow from the lower to the upper chamber.
 When the tool gets close to the metal sheet, the valve 2 is opened starting lower
chamber decompression. Press yields a growing force capacity and the tool
progressively penetrates the material.
 When the resistive force presents a quasi-fracture value, the press controller issues
a closing order to valve 2. This action is crucial to press work efficiency, since any
flow escaping the lower camber after the fracture is considered an energy waste.
 Soon after the fracture, the valve 1 is activated disabling the press force capacity and
the auxiliary cylinder initiates the press ascending movement.
Questions:
1. What do you infer from the case study?

2. Write down the case facts.

Source: http://paginas.fe.up.pt/saic/Membros/apmag/Ficheiros/drts.pdf
3.6 Summary

 A job is a unit of work that is scheduled by the system.
 A job executes on a processor and may depend on some resources.
 The basic component of any real-time application is jobs.
 The operating system treats jobs as a unit of work and allocates processors and resources.

 If all jobs are released when the system begins execution, then there is said to be no release
time.
 The release time r of a job is the instant of time at which the job becomes available for
i
execution.
 Deadline is defined as an instant of time a job’s execution is required to be completed.

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