=> R1 = service number
R12 = pointer to private word for module instantiation
R13 = pointer to stack
<= R1 = 0 if service is being claimed, preserved otherwise
R0,R2-R8 altered depending on service call if claimed
R12 may be corrupted
Other registers should be preserved if they are not being used to return values
This entry is called when a service call is issued. Service call code must be reentrant.
Since service calls are made very often they can cause significant slow-downs if not handled in a speed-conscious manner. It is recommended that on reception of service calls that are used by your module your code returns as soon as possible.
From RISC OS 4, the kernel will look for a table of service numbers that the module wants to listen to, which avoids it sending unused ones to the module. It is strongly recommended that this is provided; future versions of RISC OS may even refuse to load modules without it.
This example code shows the recommended way to handle services.
Service entry
EQUD ServiceTable ; offset of the service table
.ServiceCode
MOV r0,r0 ; a magic word
TEQ r1,#Service_One
TEQNE r1,#Service_Two
TEQNE r1,#Service_Three
MOVNE pc,r14
.ServiceDispatch
STMFD r13!,{r0-r3,r14}
TEQ r1,#Service_Two
BEQ Service_Two_Handler
TEQ r1,#Service_Three
BEQ Service_Three_Handler
.Service_One_Handler
...
LDMFD r13!,{r0-r3,pc}
.ServiceTable
EQUD 0 ; flag word, currently zero
EQUD ServiceDispatch ; offset of service dispatch code
EQUD Service_One
EQUD Service_Two
EQUD Service_Three
EQUD 0 ; terminator
The service call numbers must be in ascending order.
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