ITGS Software Reliability ITGS All IT systems are
ITGS Software Reliability
ITGS • All IT systems are a combination of: – – Hardware Software People Data • Problems with any of these parts, or a combination of them, can cause an IT system to fail.
ITGS • All software contains bugs – Errors and mistakes made by the programmers • Safety critical systems – Systems where failure could cause injury or death • Special measures must be taken to keep the number of failures as low as possible
ITGS • Some infamous software bugs: – The Millennium Bug (Y 2 K or Year 2000) • Computer software created before 1990 often stored dates using 2 digits instead of 4 • Purpose of this was to save money - memory was expensive, and every digit saved helped • As the year 2000 approached, people realized that these systems were representing the year 2000 as ‘ 00’ - which meant that it preceded ‘ 99, and was indistinguishable from the years 1900, 1800, etc. • Thanks to advance publicity, most problems were fixed, although some credit cards were refused because of expiration date issues • Reveals a lot about the assumptions humans make when using computers
ITGS • Infamous software bugs, cont’d – Denver Airport Baggage System • System designed to automatically transfer passengers’ bags from the check-in desk to the airplane - Feb. 1995 (cost $234 million to develop) • Problems: bags falling out of carts, getting stuck on conveyor belts, labels getting dirty and not being able to be read • System abandoned in 2005 • Illustrates how had it is for computers to work in the real world
ITGS
ITGS • Infamous software bugs, continued – Mars Climate Orbiter - 1998 • Designed to land on Mars and study its surface (cost - $327 million) – Destroyed as it attempted to land because it entered the atmosphere at a higher speed than intended » Some of the project’s software developers had used metric units for their calculations, while others had used Imperial units (pounds) - this led to the simple miscalculation
ITGS Case study: Therac-25 (review) – A computer-controlled machine for administering radiation treatment to cancer patients in 1982 • 2 modes: – Low energy electron mode, designed to focus on a specific part of the body – High energy x-ray mode, designed to distribute energy over a wider area of the body » A tungsten shield would move into place during x-ray mode to protect the patient from harm
• ITGS • Therac-25 reused software from two previous models in which there had been no problems • The designers also removed hardware safety locks which physically prevented certain erroneous conditions • Accident 1 (June 1985) – Large overdose given to breast cancer patient – Nurse noticed she was warm after treatment, but hospital denied any mistake and she received future treatments – Patient lost use of one arm and had to have both breasts removed
ITGS • Accident 2, July 1985 – Machine gave an error message during treatment: the message ‘No Dose’ prompted the hospital tech to start it again • He ended up starting again 5 times, not realizing that each time the patient had been given a radiation dose • Overall, the patient received 13, 000 - 17, 000 rads – 200 rads is a typical dose, and 1000 rads can be fatal) • Accident 4, March 1986 – Malfunction ‘ 54” error • Treatment stopped immediately, but the patient had received an overdose – She suffered severe neurological damage and died 3 wks later – The nurse was able to recreate the sequence of the error: » It turns out she entered info too quickly, then altered it • Finally removed from sserice after 3 people were killed and 3 more seriously injured
ITGS • User interfaces – Command Line Interface (CLI) • The user interacts with the computer solely by typing commands – Some of the earliest interfaces – Most modern operating systems have a command prompt (also called a terminal) built in » PC: type ‘cmd’, press enter » Mac: Applications>Utilities
ITGS • User interfaces – Menu driven Interface (MDI) • Let the user make decisions from a series of predetermined options that may be spread across multiple screens or menus – ATMs, restaurants, etc.
ITGS • Ways to get help – Read-me files • Included with software files, these detail last minute changes or known problems which have not been included in the printed documentation. – Web: • FAQ’s (frequently asked questions) • Tutorials – Wizards: • Can guide users through a complex task by breaking it into steps and asking a series of questions
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