How do I get the program counter of another process (PowerPC)?
Ian Goldby
that wants to read the program counter of other processes in the
system at regular intervals - a kind of specialised profiler. I want
to know how I can retrieve the current program counter of an arbitrary
process in the process table. This is on a PowerPC platform.
So far I have managed this for user-state processes by retrieving a
pointer to the process descriptor of the process in question and then
treating the ps_usp member as copy of the current stack pointer. From
there, according to the documented stack frame format for the PowerPC
processor, the current program counter is *((u_int *)ps_usp + 1).
That seems to work for user-state processes, but not for system-state
processes. In particular, it fails badly for spf_rx.
I have also found that if a process examines its own process
descriptor then ps_usp is never the same as r1, the actual stack
pointer, but this might be because r1 is only saved to ps_usp when the
process is switched out of the CPU (I don't know).
My questions are these:
1a. Is prdesc->ps_usp really the saved value of the stack pointer of
all user-state processes that are currently switched out of the CPU?
If not, what is?
1b. If I am examining the process descriptor from a system alarm
routine, is the process effectively switched out of the CPU - i.e.
assuming the answer to 1a is 'yes', is it still 'yes' in this case?
1c. Will the program counter obtained in the above way be correct for
a user-state process that is currently in sleeping in a device driver,
i.e. in system-state?
2. Is there an equivalent way to get the stack pointer of a system-
state process?
3. How can I distinguish between a user-state process and a system-
state process? There seems to be nothing in the process descriptor to
tell them apart.
4. Is there an easier way of getting the saved program counter than
the above?
N Morrison
> I have a system state process (actually a system state alarm routine)
> that wants to read the program counter of other processes in the
> system at regular intervals - a kind of specialised profiler. I want
> to know how I can retrieve the current program counter of an arbitrary
> process in the process table. This is on a PowerPC platform.
This is with a Microware compiler?
Ian Goldby
> This is with a Microware compiler?
Yes, I'm using Microware OS-9 for PowerPC version 4.2.
Ian Goldby
useful to someone else.
I solved the problem by using an interrupt service routine instead of
a system state alarm. I attach my ISR to the clock ticker IRQ vector
at a higher priority than the clock ticker, and then return from it
EOS_NOTME so that the clock ticker (tkdec) is called immediately after
my ISR.
#define VECTOR T_DEC
#define PRIORITY 8
err = _os_irq(VECTOR, PRIORITY, myIsr, data);
Thus myIsr() is called on every clock tick. On entry to myIsr(), the
special register SRR0 contains the program counter to which the ISR
will return - i.e. the program counter of the next instruction at the
point of the interruption, and SRR1 contains the machine status
register (MSR). Bit MSR_PR is clear if the processor is in system
state and set if in user state. (Yes, it really is that way round.)
The functions _get_srr0() and _get_srr1() are useful here.
The essential header files to look at are regs.h, regsppc.h, and
getset.h.
That's basically it.