=> R0 = flags (reserved, should be zero)
R1 = pointer to default entry point for routine
R2 = handle to pass in R0 to routine
R3 = handle to pass in R12 to routine
R4 = default pointer to pollword for routine
R5 = initial value of R10 for routine
R6 = initial value of R13_sys for routine
R7 = initial priority for routine: integer between 1-255, or a pointer to a string
<= R0 = RTSupport module's handle for this routine
This SWI adds a new routine to the list of routines to potentially be executed each time the interrupt dispatcher is unthreaded (but before transient and non-transient callbacks are executed, if the dispatcher is returning to USR mode). Each routine remains registered until removed by a call to RT_Deregister, but it may sleep for a time by use of a pollword. It is recommended that if it is known that a routine will sleep for an extended time, that a Deregister/Register pair should be used instead, to reduce cache thrashing during interrupt handling (when the RTSupport module has to check all possible pollwords).
Each time the RTSupport module gains control of the CPU, it scans its list of routines in decreasing priority order for routines that are unblocked (ie where the pollword is non-zero). If a real-time routine was interrupted, then routines of a higher priority are executed, followed by the interrupted routine, and then equal and lower priority routines are examined. (However, note that if and only if routines have their priorities changed dynamically, it is possible that while a routine is executing, a second routine at the same priority level may be in a pre-empted state.) If the RTSupport module encounters two unblocked routines at the same priority level, then the calling order is undefined.
Priority values
The priority passed in R7 can be an integer from 1 (lowest priority) to 255 (highest priority). Priority 0 is reserved for the exclusive use of the foreground process.
Priority strings (preferred)
The string is used to look up a priority in the file RTSupport:Priorities. This enables easy tuning of priority levels for a given system, without the need for each real-time component to be individually configurable. For example, the file might be as follows:
Critical:224
AudioFill:192
VideoPaint:160
Normal:128
AudioDecode:96
VideoDecode:64
Coroutine:32
An RT routine
=> R0 = contents of R2 when registered
R10 = first time, this is contents of R5 when registered; subsequently, it is preserved from the previous call
R11 = 0 (to support C code)
R12 = contents of R3 when registered
R13_sys = first time, this is contents of R6 when registered; subsequently, it is preserved from the previous call
R13_svc = first time, this is the top of an 8K stack based at a megabyte boundary (the same logical address is used for all real-time routine SVC stacks, but it differs from the SVC stack address used by the foreground process); subsequently, it is preserved from the previous call. Note that static relocation offsets at base of SVC stack are undefined
R14_sys = return address (only needed if using the callback model)
PSR = SYS mode, IRQs and FIQs enabled; subsequently IRQ disable state is preserved
<= R0 = flags:
bit 0 set => rescan all higher priority routines (for example,
if this routine unblocked a semaphore that a
higher priority routine may be sleeping on)
otherwise rescan all equal priority routines,
including this routine, before descending
bit 1 set => R1 contains new pollword pointer (otherwise the
pollword given at registration is used)
bit 2 set => R2 contains a monotonic time after which control
should be returned even if the pollword is not
set
bit 3 set => R14_sys contains the address to enter next time
(otherwise the address used at registration is
re-entered)
other bits are reserved and will currently be zero
R1 = pollword pointer (if R0 bit 1 is set)
R2 = timeout monotonic time (if R0 bit 2 is set)
R3-R9 = may be corrupted
R10 = value to use at next entry
R11-R12 = may be corrupted
R13_sys = value to use at next entry
R13_svc = value to use at next entry; SVC stack may be non-empty
R14_sys = next entry point (if R0 bit 3 is set)
R14_svc = may be corrupted
PSR = SYS mode, IRQs enabled or disabled, FIQs enabled, flags may be corrupted
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