st20-gp1 STMicroelectronics, st20-gp1 Datasheet - Page 26

• Manufacturer Part Number: st20-gp1
• Description: Gps Processor
• Manufacturer: STMicroelectronics

ST20-GP1
4.6.3
For each trap handler there is a trap handler structure and a trapped process structure. Both the
trap handler structure and the trapped process structure are in memory and can be accessed via
instructions, see Section 4.6.4.
The trap handler structure specifies what should happen when a trap condition is present, see
Table 4.6.
The trapped process structure saves some of the state of the process that was running when the
trap was taken, see Table 4.7.
In addition, for each priority, there is an Enables register and a Status register. The Enables
register contains flags to enable each cause of trap. The Status register contains flags to indicate
which trap conditions have been detected. The Enables and Status register bit encodings are
given in Table 4.4.
A trap will be taken at an interruptible point if a trap is set and the corresponding trap enable bit is
set in the Enables register. If the trap is not enabled then nothing is done with the trap condition. If
the trap is enabled then the corresponding bit is set in the Status register to indicate the trap
condition has occurred.
When a process takes a trap the processor saves the existing Iptr, Wptr, Status and Enables in
the trapped process structure. It then loads Iptr, Wptr and Status from the equivalent trap handler
structure and ANDs the value in Enables with the value in the structure. This allows the user to
disable various events while in the handler, in particular a trap handler must disable all the traps of
its trap group to avoid the possibility of a handler trapping to itself.
The trap handler then executes. The values in the trapped process structure can be examined
using the ldtrapped instruction (see Section 4.6.4). When the trap handler has completed its
operation 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).
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Iptr
Wptr
Status
Enables
Iptr
Wptr
Status
Enables
Trap handlers
Comments
Iptr of trap handler process.
Wptr of trap handler process.
Contains the Status register that the trap handler starts with.
Contains a word which encodes the trap enable and global interrupt masks which will be
ANDed with the existing masks to allow the trap handler to disable various events while it
runs.
Comments
Points to the instruction after the one that caused the trap condition.
Wptr of the process that was running when the trap was taken.
The relevant trap bit is set, see Table 4.5 for trap codes.
Interrupt enables.
Table 4.6 Trapped process structure
Table 4.5 Trap handler structure
Base + 3
Base + 2
Base + 1
Base + 0
Base + 3
Base + 2
Base + 1
Base + 0