workspace structure

[Joel Sherrill]

Catalin Morosan wrote:
> Hi.
>
> I've been trying to understand what exactly is the structure of the
> workspace in memory, especially where are the information for the
> threads and where are the stacks kept. I understand that the workspace
> is allocated at the end of the memory and is like a separate heap.
>
> After doing some debugging of an application with 4 tasks I deduced this:
>
> ---beginning of workspace
> ...
> Task 4 stack
> Task 3 stack
> Task 2 stack
> Task 1 stack
> Task idle stack
> ..
> Task 1 control block
> Task 2 control block
> Task 3 control block
> Task 4 control block
> Task idle control block
> ..
> --- end of workspace
>
> Is this accurate?
>   
I don't think it is 100% accurate even ignoring the ...'s

First in older RTEMS versions, the heap allocated memory
from the end of a free block -- now it does it from the beginning
of a free block so there is a chance of realloc() working.

Each class of control block is allocated as a contiguous
array.  So your task 1..4 CB's are contiguous.

Internal tasks such as IDLE are allocated first so (in current
RTEMS), the IDLE TCB address will be lower than the first
Classic API task.  See this I printed by breaking at Init

(gdb) p _Thread_Executing
$1 = (Thread_Control *) 0x7dc2e8
(gdb) p _Thread_Idle
$2 = (Thread_Control *) 0x7dc148

Notice IDLE is lower than Init (Classic API task 1)

The stacks are not allocated until the task is created which
(except for IDLE) is potentially long after RTEMS is initialized.
Again in gdb at the same point. 

(gdb) p _Thread_Executing->Start.Initial_stack.area
$4 = (void *) 0x7df2c0
(gdb) p _Thread_Idle->Start.Initial_stack.area
$5 = (void *) 0x7dcd48

IDLE's stack was allocated before  the Classic API init task.

The layout of Workspace memory is strictly a function of the
order of initialization inside RTEMS and the order in which
your application creates object such as tasks (stacks) and
message queues (buffers).

In summary, the only thing vaguely guaranteed is that the
control blocks for objects within a single class were
allocated as an array.
> Also, I don't understand why it has this structure because
> _Workspace_Allocate() takes only the size of memory to be allocated and
> in turn, it calls _Heap_Allocate() with the size and the information for
> the workspace. Doesn't allocation start from the beginning of the
> workspace and continues to the end? Why is the task idle control block
> after the control blocks of the other tasks although the idle task is
> created first? etc. There is something I am missing but I can't see it
> right now.
>
>   
All control blocks are allocated then the stacks are
allocated as the TCBs are used.

For all non-system tasks, the TCB is allocated during init and
the stack is allocated at create time so they are not even
close to contiguous in memory.
> Thanks for any input.
>
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>   


-- 
Joel Sherrill, Ph.D.             Director of Research & Development
joel.sherrill at OARcorp.com        On-Line Applications Research
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