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6 Exceptions

bit 0 of the descriptor, and so can be either

ExceptionProcessType (which has the

value 1) or

UserProcessType (which has the value 0). The type allows each exception

level to be assigned to one of the following:

• an exception handler. The entry for an exception handler is the address of the

exception control block (as described in section 6.3) bitwise ORed with

ExceptionProcessType to indicate an exception.

• a user process waiting for a peripheral (as described in section 4.11).The entry

for a user process is the task descriptor (i.e. the address of the process control

block) bitwise ORed with

UserProcessType to indicate a user process.

• the null entry NotProcess. The CPU treats null entries as disabled exceptions.

When the exception is triggered, the CPU looks in the table entry for the requested

exception level. If the value in the table is

NotProcess then no exception or trap is

taken and the CPU continues with the default behavior. Otherwise, if bit 0 is

UserProcessType then the address part of the value is assumed to be a valid task

descriptor of a process. If bit 0 is

ExceptionProcessType, then the address part is

assumed to be a pointer to a valid exception control block.

Using user processes as exceptions to handle peripherals is described in section

4.11. The rest of this chapter refers only to exceptions.

Typically a separate descriptor is used for each interrupt level and system call, so that

this scheme provides a system of vectored interrupts and system calls. The exception

handler is then executed, and can itself be interrupted or trapped.

6.3

Exception control block and the saved state

When an exception is taken, the state is saved in the exception control block. The

evaluation stack, status register, Iptr, Wptr and Tdesc are automatically saved on

taking the exception and restored on returning. This is done to enable any exception

handler to have direct access to the state of the underlying process and is required for

some software scheduler implementations.

The constants in Table 6.3 define the locations in the control block.

Word offset

Name

Purpose

ex.HandlerIptr

Exception handler instruction pointer.

ex.InterptdStatus

Interrupted or trapped process status register.

ex.InterptdTdesc

Interrupted or trapped process task descriptor.

ex.InterptdIptr

Interrupted or trapped process instruction pointer.

ex.InterptdWptr

Interrupted or trapped process workspace pointer.

ex.InterptdCreg

Interrupted or trapped process Creg.

ex.InterptdBreg

Interrupted or trapped process Breg.

ex.InterptdAreg

Interrupted or trapped process Areg.

Table 6.3 Exception control block

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