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Exceptions

This chapter describes exceptions and how to use them. The architecture of the

ST20-C1, including the registers and memory arrangement, are described in Chapter

3. A full list of constants and data structures is given in Appendix A.

exception is an exceptional event detected by the ST20-C1 core, which causes a

context switch from the normal flow of an executing program. The event triggering the

exception may be generated by software inside the core, in which case it is called a

trap. Otherwise, the event may be a hardware signal from outside the core, in which

case it is called an interrupt, except that the interrupt may attempt to perform a sched-

uling action, which may cause a trap.

When an exception occurs the CPU changes context to an

exception handler, which is

a section of code only executed when an exception occurs. The process state

registers (Areg, Breg, Creg, Iptr, Wptr, Status and Tdesc) are saved while the

exception handler is running, and restored when it returns. Exception handlers for

traps are called

trap handlers and exception handlers for interrupts are called interrupt

handlers. Normal processes which are not exception handlers are known as user

processes.

The exception handler is a transient process. Each exception handler starts execution

with a standard initial state, runs to completion and terminates with an empty work-

space. When the triggering event occurs again, the handler is restarted from its

standard initial state and again runs to completion and terminates.

Exception handlers may be nested to arbitrary depth, but they are not re-entrant, so

care should be taken to ensure that the exception which caused the handler to run

cannot occur while the handler is running, trapped or interrupted. The nesting of

exceptions is illustrated in Figure 6.1.

Figure 6.1 Nested exceptions

User

process

User

process

Exception 1 taken - state of user process saved

Exception 2 taken - state of exception 1 saved

Exception 2 returns - state of exception 1 restored

Exception 1 returns - state of user process restored

Exception 1 executing

Exception 2 executing

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