ST20-GP1 datasheet(27/116 Pages) STMICROELECTRONICS

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ST20-GP1

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4.6.4

Trap instructions

Trap handlers and trapped processes can be set up and examined via the

ldtraph, sttraph,

ldtrapped and sttrapped instructions. Table 4.8 describes the instructions that may be used when

dealing with traps.

The ﬁrst four instructions transfer data to/from the trap handler structures or trapped process

structures from/to an area in memory. In these instructions Areg contains the trap group code (see

Table 4.4) and Breg points to the 4 word area of memory used as the source or destination of the

transfer. In addition Creg contains the priority of the handler to be installed/examined in the case of

ldtraph or sttraph. ldtrapped and sttrapped apply only to the current priority.

If the

LoadTrap trap is enabled then ldtraph and ldtrapped do not perform the transfer but set the

LoadTrap trap ﬂag. If the

StoreTrap trap is enabled then sttraph and sttrapped do not perform the

transfer but set the StoreTrap trap ﬂag.

The trap enable masks are encoded by an array of bits (see Table 4.5) which are set to indicate

which traps are enabled. This array of bits is stored in the lower half-word of the Enables register.

There is an Enables register for each priority. Traps are enabled or disabled by loading a mask into

Areg with bits set to indicate which traps are to be affected and the priority to affect in Breg.

Executing

trapenb ORs the mask supplied in Areg with the trap enables mask in the Enables

trapdis negates the mask supplied in Areg and ANDs it

with the trap enables mask in the Enables register for the priority in Breg. Both instructions return

the previous value of the trap enables mask in Areg.

4.6.5

Restrictions on trap handlers

There are various restrictions that must be placed on trap handlers to ensure that they work

correctly.

Trap handlers must not deschedule or timeslice. Trap handlers alter the Enables masks,

therefore they must not allow other processes to execute until they have completed.

Trap handlers must have their Enable masks set to mask all traps in their trap group to

avoid the possibility of a trap handler trapping to itself.

Trap handlers must terminate via the

tret (trap return) instruction. The only exception to this

is that a scheduler kernel may use

restart to return to a previously shadowed process.

Instruction

Meaning

Use

ldtraph

load trap handler

load the trap handler from memory to the trap handler descriptor

sttraph

store trap handler

store an existing trap handler descriptor to memory

ldtrapped

load trapped

load replacement trapped process status from memory

sttrapped

store trapped

store trapped process status to memory

trapenb

trap enable

enable traps

trapdis

trap disable

disable traps

tret

trap return

used to return from a trap handler

causeerror

cause error

program can simulate the occurrence of an error

Table 4.7 Instructions which may be used when dealing with traps

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