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6 Exceptions

Exception handler code is completed by executing the

eret instruction, which restores

the state of the interrupted or trapped process. When an interrupt handler executes

eret, it also signals to the interrupt controller that the interrupt has completed. This

allows the interrupt handler to start a lower priority waiting interrupt if required.

The exception instructions are listed in Table 6.1.

6.1

Exception levels

All exception handlers are identified by an integer called the

exception level. Exception

levels 0 to

HighestException (255) are available for user-defined exceptions, while

system exceptions have negative levels, as defined in Table 6.2.

Any exception may be triggered from software with the

ecall instruction. User-defined

exceptions can be interrupt handlers, user processes waiting for DMA peripherals or

trap handlers used as system calls by executing

ecall.

System exceptions are traps which may be triggered automatically when the CPU is in

certain states. They are intended mainly for operating system kernels to trap sched-

uling events and for debuggers to trap breakpoints. The circumstances in which each

system trap is taken are as follows:

• el_breakpoint_trap

This trap is taken when either a

breakpoint instruction is executed or a diag-

nostic controller (DCU) signals to the CPU requesting a breakpoint. If the trap

is null then the process continues. This trap is used by debuggers.

Mnemonic

Name

ecall

exception call

eret

exception return

breakpoint

Table 6.1 Exception instructions

Exception level

Name

Circumstances when taken if not null

0 - 255

Interrupt, system call, DMA user process.

-1

el_breakpoint_trap

breakpoint instruction executed or DCU break-

point request.

-2

el_illegal_instr_trap

Illegal op-code encountered.

-3

el_idle_trap

CPU becomes idle.

-4

el_schedule_exception_trap

Schedule a user process as an exception.

-5

el_run_trap

Execute a

run instruction.

-6

el_stop_trap

Execute a

stop instruction.

-7

el_timeslice_trap

Take a timeslice.

Table 6.2 Exception levels

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