ST20-GP6 datasheet(28/123 Pages) STMICROELECTRONICS

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ST20-GP6

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tion it returns to the trapped process via the

tret (trap return) instruction. This reloads the values

saved in the trapped process structure and clears the trap ﬂag in

Status.

Note that when a trap handler is started,

Areg, Breg and Creg are not saved. The trap handler

must save the

Areg, Breg, Creg registers using stl (store local).

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 4.7 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 struc-

tures from/to an area in memory. In these instructions

Areg contains the trap group code (see

Table 4.3) 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 4.4) 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.Exe-

cuting

trapenb ORs the mask supplied in

Areg with the trap enables mask in the Enables register

for the priority in

Breg. Executing 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 pre-

vious value of the trap enables mask in

Areg.

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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