pSOSystem System Calls - Read

pSOS+ System Calls i_return
i_return Provides an exit from an interrupt service routine.
This call accepts no parameters and cannot be called from a high-level language.
Description
Target
This pSOS+ service entry provides and establishes a standard exit convention for all
Interrupt Service Routines (ISRs). It is available for all processors supported by
pSOSystem. For efficiency reasons, i_return() is different from other system calls
in that it uses a separate, private entry into the pSOS+ kernel. The method of
executing an i_return() depends on the processor and is explained in “Method of
Executing an i_return()” under “Target.”
The i_return() call is used to integrate ISR level processing within pSOS+. The
i_return() call detects when all nested ISRs have exited and control is about to be
passed back to task level execution. At this transition point, it assesses any task
execution state changes that may have taken place during ISR processing, and then
passes control to the appropriate task.
Any ISR that makes system calls that may affect the ready state of a task, must
conclude with an i_return() system call. For those processors which supply an
i_enter() call, i_return() should precede the pSOS+ service calls. Any ISR
which contains an i_enter() must conclude with an i_return(). i_return()
does not accept any input parameters, and it never returns to the caller.
Before it exits, an ISR must restore CPU registers to their state prior to the
interrupt. Processor-specific code examples of ISRs are provided in “Restoring CPU
Registers Prior to Exiting the ISR” under “Target.”
Method of Executing an i_return()
The method of executing an i_return() is processor-specific:
PPC
960
On PowerPC processors, i_return()’s entry point is located at offset
0x58 from the beginning of the pSOS+ kernel code. It is executed by
either a branch (b) or call (bl) instruction.
On 960 processors, i_return() is executed by a calls 13 instruction.
pSOSystem System Calls 1-35
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