CallBack Handler (changes)

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OS SWI Calls

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» CallBack Handler

CallBack Handler

Use

The CallBack handler is used for handling non-transient CallBacks. After calling OS_SetCallBack, the handler will be called the next time that RISC OS is threaded out (i.e. execution is returning to a user mode task with the supervisor stack empty).

The handler will be called in supervisor mode, or with R12 pointing to the handler workspace. The handler buffer (as specified toIRQ mode, with R12 pointing to the handler workspace. The handler buffer (as specified to OS_ChangeEnvironment / OS_CallBack) will have been filled with the ARM register dump. The buffer has the same size and format as the Exception Registers Block . The register dump reflects the context that would have been restored had the CallBack not been triggered. The typical method used to return from a CallBack handler is to restore the registers from that block, performing an exception “exception return return” into user mode. mode However using a the popular stored use PC/ of CallBack handlers is to implement threading schemes, where you may want to restore a register dump that corresponds to a different thread.PSR. However a popular use of CallBack handlers is to implement threading schemes, where you may want to restore a register dump that corresponds to a different thread.

On ARMv6 and above, non-transient CallBack handlers will typically need to reset the local exclusive monitor to the open state, as described in the ARMv7 compatibility primer. Failure to do this may result in foreground tasks which use the load/store exclusive instructions to malfunction.

Notes

There are two types of CallBack, Transient and Non-transient:

• Transient CallBacks are set up using OS_AddCallBack . Multiple CallBack routines can register via this interface, and they will all be called in turn the next timeRISC OS is threaded out (OS_SetCallBack is not required). Additionally, transient callbacks can occur when the supervisor stack is non-empty.

• Non-transient CallBacks are set up using OS_SetCallBack . Only one routine can be called in this manner (the CallBack environment handler), but this system has the advantage that the callback handler is able to directly control what happens next within the system (e.g. by switching foreground threads).

If both transient and non-transient callbacks are scheduled, the transient callbacks will be called first.

See also

• OS_AddCallBack
• OS_SetCallBack
• OS_CallBack