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The material on this page is reprinted from N.G. Leveson, & C.S. Turner. "An Investigation of the Therac-25 Accidents." Computer, Vol. 26, No. 7, July 1993, pp. 18-41. Copyright © 1993 Institute of Electrical and Electronics Engineers. This material is posted here with permission of IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of St. Olaf College's products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by sending a blank email message to By choosing to view this document, you agree to all provisions of the copyright laws protecting it.


Therac-25 Software Design

We know that the software for the Therac-25 was developed by a single person, using PDP-11 assembly language, over a period of several years. The software "evolved" from the Therac-6 software, which was started in 1972. According to a letter from CMC to the FDA, the "program structure and certain subroutines were carried over to the Therac-25 around 1976." Apparently, very little software documentation was produced during development. In a 1986 internal FDA memo, a reviewer lamented, "Unfortunately, the CMC response also seems to point out an apparent lack of documentation on software specifications and a software test plan."

The manufacturer said that the hardware and software were "tested and exercised separately or together over many years." In his deposition for one of the lawsuits, the quality assurance manager explained that testing was done in two parts. A "small amount" of software testing was done on a simulator, but most testing was done as a system. It appears that unit and software testing was minimal, with most effort directed at the integrated system test. At a Therac-25 user group meeting, the same quality assurance manager said that the Therac-25 software was tested for 2,700 hours. Under questioning by the users, he clarified this as meaning "2,700 hours of use."

The programmer left CMC in 1986. In a lawsuit connected with one of the accidents, the lawyers were unable to obtain information about the programmer from CMC. In the depositions connected with that case, none of the CMC employees questioned could provide any information about his educational background or experience. Although an attempt was made to obtain a deposition from the programmer, the lawsuit was settled before this was accomplished. We have been unable to learn anything about his background.

CMC claims proprietary rights to its software design. However, from voluminous documentation regarding the accidents, the repairs, and the eventual design changes, we can build a rough picture of it.

The software is responsible for monitoring the machine status, accepting input about the treatment desired, and setting the machine up for this treatment. It turns the beam on in response to an operator command (assuming that certain operational checks on the status of the physical machine are satisfied) and also turns the beam off when treatment is completed, when an operator commands it, or when a malfunction is detected. The operator can print out hard-copy versions of the CRT display or machine setup parameters.

The treatment unit has an interlock system designed to remove power to the unit when there is a hardware malfunction. The computer monitors this interlock system and provides diagnostic messages. Depending on the fault, the computer either prevents a treatment from being started or, if the treatment is in progress, creates a pause or a suspension of the treatment.

The manufacturer describes the Therac-25 software as having a stand-alone, real-time treatment operating system. The system is not built using a standard operating system or executive. Rather, the real-time executive was written especially for the Therac-25 and runs on a 32K PDP 11/23. A preemptive scheduler allocates cycles to the critical and noncritical tasks.

The software, written in PDP 11 assembly language, has four major components; stored data, a scheduler, a set of critical and noncritical tasks, and interrupt services. The stored data includes calibration parameters for the accelerator setup as well as patient-treatment data. The interrupt routines include:

  • a clock interrupt service routing,
  • a scanning interrupt service routing,
  • traps (for software overflow and computer-hardware-generated interrupts),
  • power up (initiated at power up to initialize the system and pass control to the scheduler),
  • treatment console screen interrupt handler,
  • treatment console keyboard interrupt handler,
  • service printer interrupt handler,
  • service keyboard interrupt handler.

The scheduler controls the sequences of all noninterrupt events and coordinates all concurrent processes. Tasks are initiated every 0.1 second, with the critical tasks executed first and the noncritical tasks executed in any remaining cycle time. Critical tasks include the following:

  • The treatment monitor (Treat) directs and monitors patient setup and treatment via eight operating phases. These are called subroutines, depending on the value of the Tphase control variable. Following the execution of a particular subroutine, Treat reschedules itself. Treat interacts with the keyboard processing task, which handles operator console communication. The prescription data is cross-checked and verified by other tasks (for example, the keyboard processor and the parameter setup sensor) that inform the treatment task of the verification status via shared variables.
  • The servo task controls gun emission, dose rate (pulse-repetition frequency), symmetry (beam steering), and machine motions. The servo task also sets up the machine parameters and monitors the beam-tilt-error and the flatness-error interlocks.
  • The housekeeper task takes care of system-status interlocks and limit checks, and puts appropriate messages on the CRT display. It decodes some information and checks the setup verification.

Noncritical tasks include

  • Check sum processor (scheduled to run periodically).
  • Treatment console keyboard processor (scheduled to run only if it is called by other tasks or by keyboard interrupts). This task acts as the interface between the software and the operator.
  • Treatment console screen processor (run periodically). This task lays out appropriate record formats for either displays or hard copies.
  • Service keyboard processor (run on demand). This task arbitrates non-treatment-related communication between the therapy system and the operator.
  • Snapshot (run periodically by the scheduler). Snapshot captures preselected parameter values and is called by the treatment task at the end of a treatment.
  • Hard-control processor (run periodically).
  • Calibration processor. This task is responsible for a package of tasks that let the operator examine and change system setup parameters and interlock limits.

It is clear from the CMC documentation on the modifications that the software allows concurrent access to shared memory, that there is no real synchronization aside from data stored in shared variables, and that the "test" and "set" for such variables are not indivisible operations. Race conditions resulting from this implementation of multitasking played an important part in the accidents.