ST20-C1 datasheet(77/205 Pages) STMICROELECTRONICS

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6 Exceptions

The code of the exception handler may access or modify the state of the interrupted or

trapped process. The state of the interrupted or trapped process is stored in the

exception control block, which is located at the initial Wptr of the exception handler.

An exception handler must use

eret to return to the interrupted or trapped process.

6.8.1

Enabling and disabling exceptions

When the exception handler has been created and initialized, the exception can be

enabled. The address of the exception handler control block, ORed with 1 to set the

exception type bit, must be written in the exception vector table at the level for the

exception. To write an entry

control_block with type ExceptionProcessType in the

exception vector table, the following code may be used:

ld control_block; ldc ExceptionProcessType; or;

ld ExceptionBase;

stnl exception_level;

For interrupts, both the

global_interrupt_enable and local_interrupt_enable bits in the

status register must be set. In addition the interrupt controller may need to be initial-

ized, including any interrupt enable bits and masks.

A trap can be disabled by writing

NotProcess into the exception vector table.

Interrupts can be disabled in four ways:

Clearing the status register bit global_interrupt_enable disables all interrupts

until the bit is set by an explicit write to the status register.

Clearing the status register bit local_interrupt_enable disables all interrupts

until the current process is descheduled.

A single exception level can be disabled by writing

NotProcess into the excep-

tion vector table.

The interrupt controller will generally have a means of disabling interrupts indi-

vidually or globally by writing to interrupt controller registers.

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