When OS_Claim adds a routine to a vector, it automatically removes any earlier instances of the same routine from the chain. If you don't want this to happen, use OS_AddToVector instead. The routines are called in reverse order to the order in which they called OS_Claim.
Let's give a small example for you to imagine how this works roughly (don't count on correct presentation, my (Epic's) own knowledge of the OS is not 100%, yet... ;-)
A program does an SWI "XYZ" (which has been claimed)
RISC OS has a list for this vector, which could be explained like this:
SWI "XYZ" goes here: STMFD R13!,{R14}
BL Last_Client
BL ...
BL Client_#1
BL ROM-Routine
LDMFD R13!,{PC}
Your own routine can do one of the following:
· Replace the original routine, passing control directly back to the caller: By intercepting.
· Do some processing before calling the standard routine, which then passes control back to the caller: By passing on the call.
· Call the standard routine, process some of the results it returns, and then pass control back to the caller; by a bit more sophisticated stack pushing. For example, push the address of the part of your routine which will do the post-processing on the stack, then pass on the call. When the next (or the RISC OS) routine on the chain has finished, this post-routine will be called.
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