src/newlib/libgloss/or1k/README

This document describes the internals of the port for OpenRISC
1000. The API is documented in or1k-support.h as Doxygen comments.

# Data Structures

+----------------+ 0x0
|    vectors     |
+----------------+
|  text,data,..  |
+----------------+
|      bss       |
+----------------+
|      heap      |
|       vv       |
|                |
|       ^^       |
|    stack(s)    |
+----------------+ _or1k_board_mem_base +
		   _or1k_board_mem_size

## Stack and Heap

The stack is allocated at the end of available physical memory which
is defined by each board as _or1k_board_mem_base and
_or1k_board_mem_size. The _or1k_stack_top and _or1k_stack_bottom are
determined by those variables and _or1k_stack_size (which may be
overwritten in _or1k_board_init_early).

A second stack for exceptions is allocated as we allow exceptions to
be arbitrary complex and call C functions etc. It is not an option to
re-use the current software stack as we want to be so generic, that
this can also be a virtual memory stack at moment of exception. The
exception starts below the normal software stack and is
_or1k_exception_stack_size large.

Multicore: For each core a stack and exception stack is allocated and
the stack pointer set at boot. That is: sp(core0) = _or1k_stack_top,
sp(core1) = _or1k_stack_top - _or1k_stack_size, etc.

## _or1k_stack_core (multicore only)

An array of pointers to the software stacks (size:
4*or1k_numcores()). It is dynamically allocated from heap in or1k_init
by calling sbrk(). The pointers contain the values for stack top
pointers as described above. This variable is essentially used on boot
of the slave cores to configure the stack register.

## _or1k_exception_stack_core (multicore only)

An array of pointers to the exception stacks (size:
4*or1k_numcores()). It is allocated identical as the stack_core
array. It is loaded whenever an exception occurs to start with a clean
stack in the exception.

## _or1k_exception_handler_table

A table of function pointers to the handlers of the exceptions. The
generic exception handler checks if an exception handler is registered
and calls it. There are 30 exceptions defined (0x0 is not an exception
vector and 0x100 is reset which is static). This array resides in BSS
and is therefore initialized as 0 (no handler registered) after start.

Multicore: As the number of course is not known at compile time, the
variable is a pointer to and array of arrays (cores x 30) which is
allocated in or1k_init() on heap (using sbrk).

## _or1k_interrupt_handler_table and _or1k_interrupt_handler_table_data_ptr 

The interrupt handlers are stored identical to to the exception handler table.

## _or1k_reent

The struct _or1k_reent contains formerly global data and allows for
reentrancy. In the single core case, this is an allocated object,
while it is a pointer to an array of structs in the multicore library.
It is allocated in _or1k_reent_init() on the heap.