ST20-C1 datasheet(74/205 Pages) STMICROELECTRONICS

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6.4 Initial exception handler state

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The control block is at the initial Wptr for the exception handler, so the locations are

word offsets from the initial Wptr of the exception handler. The initial Wptr is the

address of the exception control block which is the address part of the entry in the

exception vector table.

6.4

Initial exception handler state

When the exception handler starts, Wptr is set to the address of the exception control

block. The work space of the exception handler is normally below the control block, so

like a function or procedure call, one of the first actions of the exception handler code

is to adjust the Wptr downwards to create space for local variables using

ajw. The

Wptr must be adjusted back up again before the handler returns.

The initial Iptr of the exception handler is the value loaded from ex.HandlerIptr in the

exception control block. The state of the interrupted or trapped process is saved in the

exception control block. If the exception is an idle trap then the saved state is the state

of the last descheduled process. Initially the status register is set to the values shown

in Table 6.4.

If the exception handler is interrupting a user process, then the address of the

exception control block (i.e. the user process state) is left in Tdesc. If necessary, the

exception handler can save this address. If a sequence of nested interrupts has

Field or bit

Value

mac_count

As in the interrupted or trapped process.

mac_buffer

As in the interrupted or trapped process.

mac_scale

As in the interrupted or trapped process.

mac_mode

As in the interrupted or trapped process.

global_interrupt_enable

False if exception is a trap, otherwise preserved.

local_interrupt_enable

As in the interrupted or trapped process.

overflo w

False.

underflo w

False.

carry

False.

user_mode

False.

interrupt_mode

True if any interrupts are running, false otherwise.

trap_mode

True if exception is a trap, false otherwise.

sleep

False.

reserved

Undefined.

start_next_task

False.

timeslice_enable

False.

timeslice_count

As in the interrupted or trapped process.

Table 6.4 Exception handler initial status register

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