rtems-docs/cpu_supplement/powerpc.rst

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.. COMMENT: COPYRIGHT (c) 1988-2002.
.. COMMENT: On-Line Applications Research Corporation (OAR).
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PowerPC Specific Information
############################

This chapter discusses the PowerPC architecture dependencies in this port of
RTEMS.  The PowerPC family has a wide variety of implementations by a range of
vendors.  Consequently, there are many, many CPU models within it.

It is highly recommended that the PowerPC RTEMS application developer obtain
and become familiar with the documentation for the processor being used as well
as the specification for the revision of the PowerPC architecture which
corresponds to that processor.

**PowerPC Architecture Documents**

For information on the PowerPC architecture, refer to the following documents
available from Motorola and IBM:

- *PowerPC Microprocessor Family: The Programming Environment*
  (Motorola Document MPRPPCFPE-01).

- *IBM PPC403GB Embedded Controller User's Manual*.

- *PoweRisControl MPC500 Family RCPU RISC Central Processing
  Unit Reference Manual* (Motorola Document RCPUURM/AD).

- *PowerPC 601 RISC Microprocessor User's Manual*
  (Motorola Document MPR601UM/AD).

- *PowerPC 603 RISC Microprocessor User's Manual*
  (Motorola Document MPR603UM/AD).

- *PowerPC 603e RISC Microprocessor User's Manual*
  (Motorola Document MPR603EUM/AD).

- *PowerPC 604 RISC Microprocessor User's Manual*
  (Motorola Document MPR604UM/AD).

- *PowerPC MPC821 Portable Systems Microprocessor User's Manual*
  (Motorola Document MPC821UM/AD).

- *PowerQUICC MPC860 User's Manual*
  (Motorola Document MPC860UM/AD).

Motorola maintains an on-line electronic library for the PowerPC at the
following URL:

-  http://www.mot.com/powerpc/library/library.html

This site has a a wealth of information and examples.  Many of the manuals are
available from that site in electronic format.

**PowerPC Processor Simulator Information**

PSIM is a program which emulates the Instruction Set Architecture of the
PowerPC microprocessor family.  It is reely available in source code form under
the terms of the GNU General Public License (version 2 or later).  PSIM can be
integrated with the GNU Debugger (gdb) to execute and debug PowerPC executables
on non-PowerPC hosts.  PSIM supports the addition of user provided device
models which can be used to allow one to develop and debug embedded
applications using the simulator.

The latest version of PSIM is included in GDB and enabled on pre-built binaries
provided by the RTEMS Project.

CPU Model Dependent Features
============================

This section presents the set of features which vary across PowerPC
implementations and are of importance to RTEMS.  The set of CPU model feature
macros are defined in the file ``cpukit/score/cpu/powerpc/powerpc.h`` based
upon the particular CPU model specified on the compilation command line.

Alignment
---------

The macro PPC_ALIGNMENT is set to the PowerPC model's worst case alignment
requirement for data types on a byte boundary.  This value is used to derive
the alignment restrictions for memory allocated from regions and partitions.

Cache Alignment
---------------

The macro PPC_CACHE_ALIGNMENT is set to the line size of the cache.  It is used
to align the entry point of critical routines so that as much code as possible
can be retrieved with the initial read into cache.  This is done for the
interrupt handler as well as the context switch routines.

In addition, the "shortcut" data structure used by the PowerPC implementation
to ease access to data elements frequently accessed by RTEMS routines
implemented in assembly language is aligned using this value.

Maximum Interrupts
------------------

The macro PPC_INTERRUPT_MAX is set to the number of exception sources supported
by this PowerPC model.

Has Double Precision Floating Point
-----------------------------------

The macro PPC_HAS_DOUBLE is set to 1 to indicate that the PowerPC model has
support for double precision floating point numbers.  This is important because
the floating point registers need only be four bytes wide (not eight) if double
precision is not supported.

Critical Interrupts
-------------------

The macro PPC_HAS_RFCI is set to 1 to indicate that the PowerPC model has the
Critical Interrupt capability as defined by the IBM 403 models.

Use Multiword Load/Store Instructions
-------------------------------------

The macro PPC_USE_MULTIPLE is set to 1 to indicate that multiword load and
store instructions should be used to perform context switch operations.  The
relative efficiency of multiword load and store instructions versus an
equivalent set of single word load and store instructions varies based upon the
PowerPC model.

Instruction Cache Size
----------------------

The macro PPC_I_CACHE is set to the size in bytes of the instruction cache.

Data Cache Size
---------------

The macro PPC_D_CACHE is set to the size in bytes of the data cache.

Debug Model
-----------

The macro PPC_DEBUG_MODEL is set to indicate the debug support features present
in this CPU model.  The following debug support feature sets are currently
supported:

*``PPC_DEBUG_MODEL_STANDARD``*
    indicates that the single-step trace enable (SE) and branch trace enable
    (BE) bits in the MSR are supported by this CPU model.

*``PPC_DEBUG_MODEL_SINGLE_STEP_ONLY``*
    indicates that only the single-step trace enable (SE) bit in the MSR is
    supported by this CPU model.

*``PPC_DEBUG_MODEL_IBM4xx``*
    indicates that the debug exception enable (DE) bit in the MSR is supported
    by this CPU model.  At this time, this particular debug feature set has
    only been seen in the IBM 4xx series.

Low Power Model
~~~~~~~~~~~~~~~

The macro PPC_LOW_POWER_MODE is set to indicate the low power model supported
by this CPU model.  The following low power modes are currently supported.

*``PPC_LOW_POWER_MODE_NONE``*
    indicates that this CPU model has no low power mode support.

*``PPC_LOW_POWER_MODE_STANDARD``*
    indicates that this CPU model follows the low power model defined for the
    PPC603e.

Multilibs
=========

The following multilibs are available:

# ``.``: 32-bit PowerPC with FPU

# ``nof``: 32-bit PowerPC with software floating point support

# ``m403``: Instruction set for PPC403 with FPU

# ``m505``: Instruction set for MPC505 with FPU

# ``m603e``: Instruction set for MPC603e with FPU

# ``m603e/nof``: Instruction set for MPC603e with software floating
  point support

# ``m604``: Instruction set for MPC604 with FPU

# ``m604/nof``: Instruction set for MPC604 with software floating point
  support

# ``m860``: Instruction set for MPC860 with FPU

# ``m7400``: Instruction set for MPC7500 with FPU

# ``m7400/nof``: Instruction set for MPC7500 with software floating
  point support

# ``m8540``: Instruction set for e200, e500 and e500v2 cores with
  single-precision FPU and SPE

# ``m8540/gprsdouble``: Instruction set for e200, e500 and e500v2 cores
  with double-precision FPU and SPE

# ``m8540/nof/nospe``: Instruction set for e200, e500 and e500v2 cores
  with software floating point support and no SPE

# ``me6500/m32``: 32-bit instruction set for e6500 core with FPU and
  AltiVec

# ``me6500/m32/nof/noaltivec``: 32-bit instruction set for e6500 core
  with software floating point support and no AltiVec

Calling Conventions
===================

RTEMS supports the Embedded Application Binary Interface (EABI) calling
convention.  Documentation for EABI is available by sending a message with a
subject line of "EABI" to eabi@goth.sis.mot.com.

Programming Model
-----------------

This section discusses the programming model for the PowerPC architecture.

Non-Floating Point Registers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The PowerPC architecture defines thirty-two non-floating point registers
directly visible to the programmer.  In thirty-two bit implementations, each
register is thirty-two bits wide.  In sixty-four bit implementations, each
register is sixty-four bits wide.

These registers are referred to as ``gpr0`` to ``gpr31``.

Some of the registers serve defined roles in the EABI programming model.  The
following table describes the role of each of these registers:

+---------------+----------------+------------------------------+
| Register Name | Alternate Name |         Description          |
+---------------+----------------+------------------------------+
|      r1       |      sp        |         stack pointer        |
+---------------+----------------+------------------------------+
|               |                |  global pointer to the Small |
|      r2       |      na        |     Constant Area (SDA2)     |
+---------------+----------------+------------------------------+
|    r3 - r12   |      na        | parameter and result passing |
+---------------+----------------+------------------------------+
|               |                |  global pointer to the Small |
|      r13      |      na        |         Data Area (SDA)      |
+---------------+----------------+------------------------------+

Floating Point Registers
~~~~~~~~~~~~~~~~~~~~~~~~

The PowerPC architecture includes thirty-two, sixty-four bit floating point
registers.  All PowerPC floating point instructions interpret these registers
as 32 double precision floating point registers, regardless of whether the
processor has 64-bit or 32-bit implementation.

The floating point status and control register (fpscr) records exceptions and
the type of result generated by floating-point operations.  Additionally, it
controls the rounding mode of operations and allows the reporting of floating
exceptions to be enabled or disabled.

Special Registers
~~~~~~~~~~~~~~~~~

The PowerPC architecture includes a number of special registers which are
critical to the programming model:

*Machine State Register*
    The MSR contains the processor mode, power management mode, endian mode,
    exception information, privilege level, floating point available and
    floating point excepiton mode, address translation information and the
    exception prefix.

*Link Register*
    The LR contains the return address after a function call.  This register
    must be saved before a subsequent subroutine call can be made.  The use of
    this register is discussed further in the *Call and Return Mechanism*
    section below.

*Count Register*
    The CTR contains the iteration variable for some loops.  It may also be
    used for indirect function calls and jumps.

Call and Return Mechanism
-------------------------

The PowerPC architecture supports a simple yet effective call and return
mechanism.  A subroutine is invoked via the "branch and link" (``bl``) and
"brank and link absolute" (``bla``) instructions.  This instructions place the
return address in the Link Register (LR).  The callee returns to the caller by
executing a "branch unconditional to the link register" (``blr``) instruction.
Thus the callee returns to the caller via a jump to the return address which is
stored in the LR.

The previous contents of the LR are not automatically saved by either the
``bl`` or ``bla``.  It is the responsibility of the callee to save the contents
of the LR before invoking another subroutine.  If the callee invokes another
subroutine, it must restore the LR before executing the ``blr`` instruction to
return to the caller.

It is important to note that the PowerPC subroutine call and return mechanism
does not automatically save and restore any registers.

The LR may be accessed as special purpose register 8 (``SPR8``) using the "move
from special register" (``mfspr``) and "move to special register" (``mtspr``)
instructions.

Calling Mechanism
-----------------

All RTEMS directives are invoked using the regular PowerPC EABI calling
convention via the ``bl`` or``bla`` instructions.

Register Usage
--------------

As discussed above, the call instruction does not automatically save any
registers.  It is the responsibility of the callee to save and restore any
registers which must be preserved across subroutine calls.  The callee is
responsible for saving callee-preserved registers to the program stack and
restoring them before returning to the caller.

Parameter Passing
-----------------

RTEMS assumes that arguments are placed in the general purpose registers with
the first argument in register 3 (``r3``), the second argument in general
purpose register 4 (``r4``), and so forth until the seventh argument is in
general purpose register 10 (``r10``).  If there are more than seven arguments,
then subsequent arguments are placed on the program stack.  The following
pseudo-code illustrates the typical sequence used to call a RTEMS directive
with three (3) arguments:

.. code-block:: c

    load third argument into r5
    load second argument into r4
    load first argument into r3
    invoke directive

Memory Model
============

Flat Memory Model
-----------------

The PowerPC architecture supports a variety of memory models.  RTEMS supports
the PowerPC using a flat memory model with paging disabled.  In this mode, the
PowerPC automatically converts every address from a logical to a physical
address each time it is used.  The PowerPC uses information provided in the
Block Address Translation (BAT) to convert these addresses.

Implementations of the PowerPC architecture may be thirty-two or sixty-four
bit.  The PowerPC architecture supports a flat thirty-two or sixty-four bit
address space with addresses ranging from 0x00000000 to 0xFFFFFFFF (4
gigabytes) in thirty-two bit implementations or to 0xFFFFFFFFFFFFFFFF in
sixty-four bit implementations.  Each address is represented by either a
thirty-two bit or sixty-four bit value and is byte addressable.  The address
may be used to reference a single byte, half-word (2-bytes), word (4 bytes), or
in sixty-four bit implementations a doubleword (8 bytes).  Memory accesses
within the address space are performed in big or little endian fashion by the
PowerPC based upon the current setting of the Little-endian mode enable bit
(LE) in the Machine State Register (MSR).  While the processor is in big endian
mode, memory accesses which are not properly aligned generate an "alignment
exception" (vector offset 0x00600).  In little endian mode, the PowerPC
architecture does not require the processor to generate alignment exceptions.

The following table lists the alignment requirements for a variety of data
accesses:

+--------------+-----------------------+
|   Data Type  | Alignment Requirement |
+--------------+-----------------------+
|     byte     |          1            |
|   half-word  |          2            |
|     word     |          4            |
|  doubleword  |          8            |
+--------------+-----------------------+

Doubleword load and store operations are only available in PowerPC CPU models
which are sixty-four bit implementations.

RTEMS does not directly support any PowerPC Memory Management Units, therefore,
virtual memory or segmentation systems involving the PowerPC are not supported.

Interrupt Processing
====================

Although RTEMS hides many of the processor dependent details of interrupt
processing, it is important to understand how the RTEMS interrupt manager is
mapped onto the processor's unique architecture. Discussed in this chapter are
the PowerPC's interrupt response and control mechanisms as they pertain to
RTEMS.

RTEMS and associated documentation uses the terms interrupt and vector.  In the
PowerPC architecture, these terms correspond to exception and exception
handler, respectively.  The terms will be used interchangeably in this manual.

Synchronous Versus Asynchronous Exceptions
------------------------------------------

In the PowerPC architecture exceptions can be either precise or imprecise and
either synchronous or asynchronous.  Asynchronous exceptions occur when an
external event interrupts the processor.  Synchronous exceptions are caused by
the actions of an instruction. During an exception SRR0 is used to calculate
where instruction processing should resume.  All instructions prior to the
resume instruction will have completed execution.  SRR1 is used to store the
machine status.

There are two asynchronous nonmaskable, highest-priority exceptions system
reset and machine check.  There are two asynchrononous maskable low-priority
exceptions external interrupt and decrementer.  Nonmaskable execptions are
never delayed, therefore if two nonmaskable, asynchronous exceptions occur in
immediate succession, the state information saved by the first exception may be
overwritten when the subsequent exception occurs.

The PowerPC arcitecure defines one imprecise exception, the imprecise floating
point enabled exception.  All other synchronous exceptions are precise.  The
synchronization occuring during asynchronous precise exceptions conforms to the
requirements for context synchronization.

Vectoring of Interrupt Handler
------------------------------

Upon determining that an exception can be taken the PowerPC automatically
performs the following actions:

- an instruction address is loaded into SRR0

- bits 33-36 and 42-47 of SRR1 are loaded with information specific to the
  exception.

- bits 0-32, 37-41, and 48-63 of SRR1 are loaded with corresponding bits from
  the MSR.

- the MSR is set based upon the exception type.

- instruction fetch and execution resumes, using the new MSR value, at a
  location specific to the execption type.

If the interrupt handler was installed as an RTEMS interrupt handler, then upon
receipt of the interrupt, the processor passes control to the RTEMS interrupt
handler which performs the following actions:

- saves the state of the interrupted task on it's stack,

- saves all registers which are not normally preserved by the calling sequence
  so the user's interrupt service routine can be written in a high-level
  language.

- if this is the outermost (i.e. non-nested) interrupt, then the RTEMS
  interrupt handler switches from the current stack to the interrupt stack,

- enables exceptions,

- invokes the vectors to a user interrupt service routine (ISR).

Asynchronous interrupts are ignored while exceptions are disabled.  Synchronous
interrupts which occur while are disabled result in the CPU being forced into
an error mode.

A nested interrupt is processed similarly with the exception that the current
stack need not be switched to the interrupt stack.

Interrupt Levels
----------------

The PowerPC architecture supports only a single external asynchronous interrupt
source.  This interrupt source may be enabled and disabled via the External
Interrupt Enable (EE) bit in the Machine State Register (MSR).  Thus only two
level (enabled and disabled) of external device interrupt priorities are
directly supported by the PowerPC architecture.

Some PowerPC implementations include a Critical Interrupt capability which is
often used to receive interrupts from high priority external devices.

The RTEMS interrupt level mapping scheme for the PowerPC is not a numeric level
as on most RTEMS ports.  It is a bit mapping in which the least three
significiant bits of the interrupt level are mapped directly to the enabling of
specific interrupt sources as follows:

*Critical Interrupt*
    Setting bit 0 (the least significant bit) of the interrupt level enables
    the Critical Interrupt source, if it is available on this CPU model.

*Machine Check*
    Setting bit 1 of the interrupt level enables Machine Check execptions.

*External Interrupt*
    Setting bit 2 of the interrupt level enables External Interrupt execptions.

All other bits in the RTEMS task interrupt level are ignored.

Default Fatal Error Processing
==============================

The default fatal error handler for this architecture performs the following
actions:

- places the error code in r3, and

- executes a trap instruction which results in a Program Exception.

If the Program Exception returns, then the following actions are performed:

- disables all processor exceptions by loading a 0 into the MSR, and

- goes into an infinite loop to simulate a halt processor instruction.

Symmetric Multiprocessing
=========================

SMP is supported.  Available platforms are the Freescale QorIQ P series (e.g.
P1020) and T series (e.g. T2080, T4240).

Thread-Local Storage
====================

Thread-local storage is supported.

Board Support Packages
======================

System Reset
------------

An RTEMS based application is initiated or re-initiated when the PowerPC
processor is reset.  The PowerPC architecture defines a Reset Exception, but
leaves the details of the CPU state as implementation specific.  Please refer
to the User's Manual for the CPU model in question.

In general, at power-up the PowerPC begin execution at address 0xFFF00100 in
supervisor mode with all exceptions disabled.  For soft resets, the CPU will
vector to either 0xFFF00100 or 0x00000100 depending upon the setting of the
Exception Prefix bit in the MSR.  If during a soft reset, a Machine Check
Exception occurs, then the CPU may execute a hard reset.

Processor Initialization
------------------------

If this PowerPC implementation supports on-chip caching and this is to be
utilized, then it should be enabled during the reset application initialization
code.  On-chip caching has been observed to prevent some emulators from working
properly, so it may be necessary to run with caching disabled to use these
emulators.

In addition to the requirements described in the*Board Support Packages*
chapter of the RTEMS C Applications User's Manual for the reset code which is
executed before the call to ``rtems_initialize_executive``, the PowrePC version
has the following specific requirements:

- Must leave the PR bit of the Machine State Register (MSR) set to 0 so the
  PowerPC remains in the supervisor state.

- Must set stack pointer (sp or r1) such that a minimum stack size of
  MINIMUM_STACK_SIZE bytes is provided for the RTEMS initialization sequence.

- Must disable all external interrupts (i.e. clear the EI (EE) bit of the
  machine state register).

- Must enable traps so window overflow and underflow conditions can be properly
  handled.

- Must initialize the PowerPC's initial Exception Table with default handlers.