What are CPU exceptions

CPU Exceptions is a low-level mechanism for invoking some exception handler, as defined by the X86 ISA architecture. The exception handler (i.e. an machine code-level function) can run at a different privilege level, with a different stack, etc. wrt. to the program that was running when the exception occurred.

Exceptions can be triggered either asynchronously by an interrupt (typically initiated by some peripheral hardware device) or synchronously as a side-effect of a CPU instruction (also called "software interrupts"). Synchronous exceptions can further be classified into explicit exceptions that are raised by some variant of the INT instruction, and implicit exceptions that are raised to signal some condition, such as division-by-zero or page-fault.

CPU exceptions vs. Common Lisp conditions

From a birds-eye perspective, it seems clear that certain implicit CPU exceptions must be mapped to corresponding Lisp-level conditions being signaled. For example, the division-by-zero CPU exception must trigger a division-by-zero error condition. (A conceivable but infeasible alternative would be that the compiler protects each division operation by explicitly testing the divisor for zero.) This is however not to say that that all CPU exceptions must be mapped to Lisp conditions. It is entirely reasonable that e.g. hardware interrupts are dealt with by handling the CPU exceptions as such.

The Movitz run-time also uses explicit CPU exceptions as a mechanism for triggering Lisp conditions. This mechanism is used primarily because it is (memory) space efficient: Only a two-byte instruction is required for raising an arbitrary CPU exception. However, the overhead (in terms of CPU cycles, say, in the hundreds) is not negligible, so this mechanism should only be used for situations that are truly exceptional.

The Movitz run-time core is intended to be orthogonal to a series of architectural design choices that interfere with how CPU exception handlers can operate (wrt. privilege levels, stacks, etc.). Therefore, the Movitz run-time specifies the meaning of certain CPU exceptions, and leaves it to particular applications systems to ensure that these CPU exceptions are dealt with appropriately (e.g. preferably in compliance with ANSI Common Lisp).

CPU exception handling in Movitz

The following describes mechanisms that primarily apply to a los0-like run-time, where all code runs at the same privilege level and on the same stack.

(setf (exception-handler <interrupt>) <function>)

<function> should take 2 arguments: (exception-vector exception-frame)