Documentation: RTSupport Module API Specification
RTSUPPORT MODULE API SPECIFICATION
==================================
The RTSupport module provides facilities supporting real-time code, at a
priority just below that of the interrupt dispatch system, and above that
of any SWIs being handled by the foreground application. Multiple priority
levels are supported, so routines with tighter timing constraints can
pre-empt other real-time routines. Code can be designed either using a
callback model (where the system makes a function call to your entry point)
or using a thread model (where the rest of the system gets CPU time while
your thread sleeps).
Code typically executes in system (SYS) mode and has its own SYS and SVC
stacks, and it may call SWIs (but only re-entrant SWIs) without preventing
higher-priority routines from executing. There is no automatic time-slicing
of routines; it is assumed that the application takes responsibility for
ensuring that the system is not overloaded (although there is a relief valve
to cope with overloaded systems, which ensures that the foreground process
gets a little CPU time at least a few times per second).
Possible future enhancements:
* Per-routine option of maintaining a floating point context for the routine
(traps would need to be disabled while real-time routines execute)
* Per-routine data storage (for example to support C++ exception handling)
* Reusing spare stack frames in the IRQ stack when nearing overflow (useful
for timeslicing schemes)
* The facility to recache the priority lookup file after module
initialisation
RT_Register
(SWI &575C0)
Register a routine with the RTSupport module
On entry
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
On exit
R0 = RTSupport module's handle for this routine
All other registers are preserved
Interrupts
Interrupts are disabled
Fast interrupts are enabled
Processor Mode
Processor is in SVC mode
Re-entrancy
SWI is re-entrant
Use
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.
The routines are called as follows:
On entry:
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
R14_sys = return address (only needed if using the callback model)
SYS mode
First time, IRQs and FIQs enabled;
subsequently IRQ disable state is preserved
Other registers undefined
Static relocation offsets at base of SVC stack are undefined
On exit:
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)
R10 = value to use at next entry
R13_sys = value to use at next entry
R13_svc = value to use at next entry
R14_sys = next entry point (if R0 bit 3 is set)
SYS mode, IRQs enabled or disabled, FIQs enabled
SVC stack may be non-empty
Processor flags, R1-R9 (except where used to return parameters), R11,
R12 and R14_svc may be corrupted
So for example the routine may be implemented as
* Assembler, with the static data referenced using R12 as is conventional
in RISC OS.
* C code interfaced directly, as long as R10 and R13_sys are set up to
describe a standard stack chunk with the reserved words at its base set
up to enable static data relocation, for example by copying them from
the base of the SVC stack prior to the RT_Register call:
_kernel_stack_chunk *chunk = calloc(1, CHUNK_SIZE);
chunk->sc_mark = 0xF60690FF;
chunk->sc_size = CHUNK_SIZE;
memcpy(chunk + 1, _kernel_current_stack_chunk() + 1, 28);
_kernel_swi_regs r = { /* ... */ .r[5] = ((int) chunk) + 560,
.r[6] = ((int) chunk) + CHUNK_SIZE, /* ... */ };
_kernel_swi(RT_Register, &r, &r);
Here is an example callback-model routine written in C that repeatedly
sleeps for a second at a time (assuming the default pollword is never
set):
typedef struct {
unsigned int flags;
unsigned int *pollword;
unsigned int timeout;
} routine_result_t;
__value_in_regs routine_result_t MyRoutine(void *r0) {
unsigned int time;
_swix(OS_ReadMonotonicTime, _OUT(0), &time);
return (routine_result_t) { 1<<2, NULL, time + 100 };
}
* C code interfaced via a veneer: if you want to be able to call
_kernel_raise_error() and functions which rely upon it like exit(),
abort() and assert(), which assume the outermost stack frame was called
as a function returning a _kernel_oserror *, or for use as a threading
library where actions need to be taken when the thread exits (eg
unblocking joining threads) you will need an assembler veneer to the C
code, to patch the r14_sys the C code sees on entry with a pointer to a
suitable cleanup function.
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. However, it is preferred that a string pointer
be used instead: 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
Related SWIs
RT_Deregister
RT_Deregister
(SWI &575C1)
Remove a routine previously registered with the RTSupport module
On entry
R0 = flags (reserved, should be zero)
R1 = routine handle returned from RT_Register
On exit
All registers are preserved
Interrupts
Interrupts are disabled
Fast interrupts are enabled
Processor Mode
Processor is in SVC mode
Re-entrancy
SWI is re-entrant
Use
This SWI can be used to remove a routine from the list held by the
RTSupport module. If the routine in question is currently executing then
this call does not return.
Related SWIs
RT_Register
RT_Yield
(SWI &575C2)
Sleep the current thread and force a test of all pollwords
On entry
R1 = pointer to pollword
On exit
All registers are preserved
Interrupts
Interrupts are disabled
Fast interrupts are enabled
Processor Mode
Processor is in SVC mode
Re-entrancy
SWI is re-entrant
Use
This SWI has two uses: firstly it enables code in real-time context to
sleep, and secondly it enables the foreground process to force a test
of all pollwords (for example if the foreground sets the pollword of a
thread, this may be used to wake the thread rather than having to wait
for the next interrupt). The pollword is usually insignificant for the
foreground process, because by definition control returns to it when no
pollwords are set; however, it may be useful if the foreground is
currently set to a priority above that of a real-time routine.
It is an error to make this call from interrupt context.
While in real-time context, OS_UpCall 6 is interpreted as a call to this
SWI. (UpCall 7 is also intercepted if an attempt is made to remove a
pollword being used by the real-time system.)
Related SWIs
RT_TimedYield
RT_TimedYield
(SWI &575C3)
Perform a time-limited sleep of the current thread and force a test of
all pollwords
On entry
R1 = pointer to pollword
R2 = monotonic time at which to return control even if pollword is unset
On exit
All registers are preserved
Interrupts
Interrupts are disabled
Fast interrupts are enabled
Processor Mode
Processor is in SVC mode
Re-entrancy
SWI is re-entrant
Use
This SWI is similar to RT_Yield except that the sleep does not extend
indefinitely.
Related SWIs
RT_Yield
RT_ChangePriority
(SWI &575C4)
Change the priority of an existing real-time routine
On entry
R0 = flags (reserved, should be zero)
R1 = routine handle returned from RT_Register, or 0 to change the priority
of the foreground process
R2 = new priority (as for RT_Register)
On exit
R0 = previous priority level (as a number from 0-255)
All other registers are preserved
Interrupts
Interrupts are disabled
Fast interrupts are enabled
Processor Mode
Processor is in SVC mode
Re-entrancy
SWI is re-entrant
Use
This SWI allows the priority setting of a routine to be changed on the
fly, for example if process has become more time-critical, or if it is
blocking a routine of a higher priority. It may cause a thread switch
if a different thread is promoted above the priority of the current
thread, or if the current thread is demoted below other unblocked threads.
The priority level of the foreground process may be changed, but this is
only intended for short-term use and not when the foreground is in USR
mode (otherwise system callbacks will also be promoted above some
real-time threads, which is undesirable behaviour).
Related SWIs
RT_Register
RT_ReadInfo
(SWI &575C5)
Read status information
On entry
R0 = reason code
On exit
R0 = value
All other registers are preserved
V flag is set if the reason code was not recognised
Interrupts
Interrupt state is unchanged
Fast interrupts are enabled
Processor Mode
Processor is in SVC mode
Re-entrancy
SWI is re-entrant
Use
This SWI enables you to read current state variables. Reason codes
currently defined are:
reason meaning
0 RTSupport handle for current routine, or
0 if in foreground, or
-1 if in interrupt context
1 Priority setting (integer value) of current routine, or
-1 if in interrupt context
2 Base address of SVC stack for real-time routines
Related SWIs
RT_Register
BJGA, 2004-11-01
