ST20-GP1 datasheet(25/116 Pages) STMICROELECTRONICS

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ST20-GP1

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Trap groups encoding is shown in Table 4.4 below. These codes are used to identify trap groups to

various instructions.

In addition to the trap groups mentioned above, the CauseError ﬂag in the Status register is used

to signal when a trap condition has been activated by the

causeerror instruction. It can be used to

indicate when trap conditions have occurred due to the user setting them, rather than by the

system.

4.6.2

Events that can cause traps

Table 4.4 summarizes the events that can cause traps and gives the encoding of bits in the trap

Status and Enable words.

Trap group

Code

Breakpoint

CPU Errors

System operations

Scheduler

Table 4.3 Trap group codes

Trap cause

Status/Enable

codes

Trap

group

Comments

Breakpoint

When a process executes the breakpoint instruction (

j0) then it traps

to its trap handler.

IntegerError

Integer error other than integer overﬂow – e.g. explicitly checked or

explicitly set error.

Overflow

Integer overﬂow or integer division by zero.

IllegalOpcode

Attempt to execute an illegal instruction. This is signalled when

opr

(operate) is executed with an invalid operand.

LoadTrap

When the trap descriptor is read with the

ldtraph (load trap handler)

instruction or when the trapped process status is read with the

ldtrapped (load trapped) instruction.

StoreTrap

When the trap descriptor is written with the

sttraph (store trap handler)

instruction or when the trapped process status is written with the

sttrapped (store trapped) instruction.

InternalChannel

Scheduler trap from internal channel.

ExternalChannel

Scheduler trap from external channel.

Timer

Scheduler trap from timer alarm.

Timeslice

Scheduler trap from timeslice.

Run

Scheduler trap from

runp (run process) or startp (start process).

Signal

Scheduler trap from

signal.

ProcessInterrupt

Start executing a process at a new priority level.

QueueEmpty

Caused by no process active at a priority level.

CauseError

15 (Status only)

Any,

encoded

0-3

Signals that the

causeerror instruction set the trap ﬂag.

Table 4.4 Trap causes and Status/Enable codes

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