Directed Reading 1 Girish Ramesh – Andres Martin-Lopez – Bamdad Dashtban –

Background CEG & AECL Therac 6 Therac 20 AECL Therac 25 -Dual Mode -Same PDP 11 [1] -One machine for all treatments -Software more responsible for safety -Less Hardware safety mechanisms and interlocks -Software is the key element Scanning Magnets Ion Chamber Flattening Filter Electron Beam 5-25 MeV Light Mirror

Therac 25 – The Story -Kennestone Regional Oncology Center, June Yakima Valley Memorial Hospital, December 1985 No specific bug found. - Ontario Cancer Foundation (Hamilton), July Yakima Valley Memorial Hospital, January East Texas Cancer Center (Tyler), March-April Massive Overdose. -Display error : No dose or low dose -Concurrent access to shared memory. -System not aware of changes in the mode/energy variable, if the cursor position changed after entering data. -Overdose due to exposure in field light mode. -Software error due to variable overflow. -Race condition[2] in system.

Conclusions Parallel computing has dependency on  software and hardware used. No techniques applied to avoid: - race conditions. - access to shared memory. In Therac 25: overvalued software over hardware. No proper Software Engineering[3] process applied: RequirementsDesignImplementationVerificationMaintenance Reuse of software. Not well documented. No Integration testing. No deep error investigation. Concurrency implementation and verification is not easy.

Citations [1] [2] [3]