=> R0 = 0 to claim, 1 to release
R1 = OS SWI number to claim
R2 = pointer to handler routine
R3 = value to pass in R12 to handler routine
Register or deregister a routine to be called instead of the default handler when an OS SWI is issued.
On 26 bit systems the routine should return with MOVS PC,R14, so the flags should be in R14 and R15. On a 32 bit system the flags in the PSR on return are expected to be valid.
RISC OS 3.50+ include SWIs OS_ClaimSWI and OS_ReleaseSWI, which return 'SWI not known' until Select, when:
SYS "OS_ClaimSWI",x,y,z calls SYS "OS_ClaimOSSWI",0,x,y,z
SYS "OS_ReleaseSWI",x,y,z calls SYS "OS_ClaimOSSWI",1,x,y,z
This describes behaviour in OSes current at Nov 2005. It is likely to change in future OS versions (especially flag usage).
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