STXIT - Background

Background

The macro was an abbreviation of the term "Set Exit," and was used by programs written in assembly language. It is used to create a recovery routine in the event of program errors, similar to signal handlers in C and Try/Finally blocks in C++, Java and other object oriented languages.

The STXIT macro would provide an error recovery address for several major types of errors and certain operator signals. The errors covered depended upon the operating system. In general, the errors which it provided recovery included

  • address protection violate (accessing memory not owned by the application)
  • privilege violation (attempts to execute privileged instructions)
  • divide check (division by zero)
  • floating point check (floating point underflow or overflow)
  • timer runout (where a timer set by the program has expired) this particular interrupt is not actually an error, it is how a program responds to a watchdog timer
  • Interrupt resume, a function on the Univac 90/60 where an interactive user has issued a break to a program to go to command mode, and issued an INTR command to cause the program to resume at the INTR point. If the program has not specified an INTR resume address, the INTR command is rejected. This is similar to the REENTER command on the RT11 operating system on Digital computers.

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