.. SPDX-License-Identifier: CC-BY-SA-4.0


M68xxx and Coldfire Specific Information
****************************************

This chapter discusses the Freescale (formerly Motorola) MC68xxx and Coldfire
architectural dependencies.  The MC68xxx family has a wide variety of CPU
models within it based upon different CPU core implementations.  Ignoring the
Coldfire parts, the part numbers for these models are generally divided into
MC680xx and MC683xx.  The MC680xx models are more general purpose processors
with no integrated peripherals.  The MC683xx models, on the other hand, are
more specialized and have a variety of peripherals on chip including
sophisticated timers and serial communications controllers.

**Architecture Documents**

For information on the MC68xxx and Coldfire architecture, refer to the
following documents available from Freescale website
(http//www.freescale.com/):

- *M68000 Family Reference, Motorola, FR68K/D*.

- *MC68020 User's Manual, Motorola, MC68020UM/AD*.

- *MC68881/MC68882 Floating-Point Coprocessor User's Manual,
  Motorola, MC68881UM/AD*.

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

This section presents the set of features which vary across m68k/Coldfire
implementations that are of importance to RTEMS.  The set of CPU model feature
macros are defined in the file :file:`cpukit/score/cpu/m68k/m68k.h` based upon
the particular CPU model selected on the compilation command line.

BFFFO Instruction
-----------------

The macro ``M68K_HAS_BFFFO`` is set to 1 to indicate that this CPU model has
the bfffo instruction.

Vector Base Register
--------------------

The macro ``M68K_HAS_VBR`` is set to 1 to indicate that this CPU model has a
vector base register (vbr).

Separate Stacks
---------------

The macro ``M68K_HAS_SEPARATE_STACKS`` is set to 1 to indicate that this CPU
model has separate interrupt, user, and supervisor mode stacks.

Pre-Indexing Address Mode
-------------------------

The macro ``M68K_HAS_PREINDEXING`` is set to 1 to indicate that this CPU model
has the pre-indexing address mode.

Extend Byte to Long Instruction
-------------------------------

The macro ``M68K_HAS_EXTB_L`` is set to 1 to indicate that this CPU model has
the extb.l instruction.  This instruction is supposed to be available in all
models based on the cpu32 core as well as mc68020 and up models.

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

The MC68xxx architecture supports a simple yet effective call and return
mechanism.  A subroutine is invoked via the branch to subroutine (``bsr``) or
the jump to subroutine (``jsr``) instructions.  These instructions push the
return address on the current stack.  The return from subroutine (``rts``)
instruction pops the return address off the current stack and transfers control
to that instruction.  It is is important to note that the MC68xxx call and
return mechanism does not automatically save or restore any registers.  It is
the responsibility of the high-level language compiler to define the register
preservation and usage convention.

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

All RTEMS directives are invoked using either a ``bsr`` or ``jsr`` instruction
and return to the user application via the rts instruction.

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

As discussed above, the ``bsr`` and ``jsr`` instructions do not automatically
save any registers.  RTEMS uses the registers D0, D1, A0, and A1 as scratch
registers.  These registers are not preserved by RTEMS directives therefore,
the contents of these registers should not be assumed upon return from any
RTEMS directive.

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

RTEMS assumes that arguments are placed on the current stack before the
directive is invoked via the bsr or jsr instruction.  The first argument is
assumed to be closest to the return address on the stack.  This means that the
first argument of the C calling sequence is pushed last.  The following
pseudo-code illustrates the typical sequence used to call a RTEMS directive
with three (3) arguments:

.. code-block:: c

    push third argument
    push second argument
    push first argument
    invoke directive
    remove arguments from the stack

The arguments to RTEMS are typically pushed onto the stack using a move
instruction with a pre-decremented stack pointer as the destination.  These
arguments must be removed from the stack after control is returned to the
caller.  This removal is typically accomplished by adding the size of the
argument list in bytes to the current stack pointer.

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

The MC68xxx family supports a flat 32-bit address space with addresses ranging
from 0x00000000 to 0xFFFFFFFF (4 gigabytes).  Each address is represented by a
32-bit value and is byte addressable.  The address may be used to reference a
single byte, word (2-bytes), or long word (4 bytes).  Memory accesses within
this address space are performed in big endian fashion by the processors in
this family.

Some of the MC68xxx family members such as the MC68020, MC68030, and MC68040
support virtual memory and segmentation.  The MC68020 requires external
hardware support such as the MC68851 Paged Memory Management Unit coprocessor
which is typically used to perform address translations for these systems.
RTEMS does not support virtual memory or segmentation on any of the MC68xxx
family members.

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

Discussed in this section are the MC68xxx's interrupt response and control
mechanisms as they pertain to RTEMS.

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

Depending on whether or not the particular CPU supports a separate interrupt
stack, the MC68xxx family has two different interrupt handling models.

Models Without Separate Interrupt Stacks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Upon receipt of an interrupt the MC68xxx family members without separate
interrupt stacks automatically use software to switch stacks.

Models With Separate Interrupt Stacks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Upon receipt of an interrupt the MC68xxx family members with separate interrupt
stacks automatically perform the following actions:

- saves the current status register (SR),

- clears the master/interrupt (M) bit of the SR to indicate the switch from
  master state to interrupt state,

- sets the privilege mode to supervisor,

- suppresses tracing,

- sets the interrupt mask level equal to the level of the interrupt being
  serviced,

- pushes an interrupt stack frame (ISF), which includes the program counter
  (PC), the status register (SR), and the format/exception vector offset (FVO)
  word, onto the supervisor and interrupt stacks,

- switches the current stack to the interrupt stack and vectors to an interrupt
  service routine (ISR).  If the ISR was installed with the interrupt_catch
  directive, then the RTEMS interrupt handler will begin execution.  The RTEMS
  interrupt handler saves all registers which are not preserved according to
  the calling conventions and invokes the application's ISR.

A nested interrupt is processed similarly by these CPU models with the
exception that only a single ISF is placed on the interrupt stack and the
current stack need not be switched.

The FVO word in the Interrupt Stack Frame is examined by RTEMS to determine
when an outer most interrupt is being exited. Since the FVO is used by RTEMS
for this purpose, the user application code MUST NOT modify this field.

The following shows the Interrupt Stack Frame for MC68xxx CPU models with
separate interrupt stacks:

+----------------------+-----+
|    Status Register   | 0x0 |
+----------------------+-----+
| Program Counter High | 0x2 |
+----------------------+-----+
| Program Counter Low  | 0x4 |
+----------------------+-----+
| Format/Vector Offset | 0x6 |
+----------------------+-----+


CPU Models Without VBR and RAM at 0
-----------------------------------

This is from a post by Zoltan Kocsi <zoltan@bendor.com.au> and is a nice trick
in certain situations.  In his words:

I think somebody on this list asked about the interupt vector handling w/o VBR
and RAM at 0.  The usual trick is to initialise the vector table (except the
first 2 two entries, of course) to point to the same location BUT you also add
the vector number times 0x1000000 to them. That is, bits 31-24 contain the
vector number and 23-0 the address of the common handler.  Since the PC is 32
bit wide but the actual address bus is only 24, the top byte will be in the PC
but will be ignored when jumping onto your routine.

Then your common interrupt routine gets this info by loading the PC into some
register and based on that info, you can jump to a vector in a vector table
pointed by a virtual VBR:

.. code-block:: c

    //
    //  Real vector table at 0
    //
    .long   initial_sp
    .long   initial_pc
    .long   myhandler+0x02000000
    .long   myhandler+0x03000000
    .long   myhandler+0x04000000
    ...
    .long   myhandler+0xff000000
    //
    // This handler will jump to the interrupt routine   of which
    // the address is stored at VBR[ vector_no ]
    // The registers and stackframe will be intact, the interrupt
    // routine will see exactly what it would see if it was called
    // directly from the HW vector table at 0.
    //
        .comm    VBR,4,2        // This defines the 'virtual' VBR
    // From C: extern void *VBR;
    myhandler:                  // At entry, PC contains the full vector
        move.l  %d0,-(%sp)      // Save d0
        move.l  %a0,-(%sp)      // Save a0
        lea     0(%pc),%a0      // Get the value of the PC
        move.l  %a0,%d0         // Copy it to a data reg, d0 is VV??????
        swap    %d0             // Now d0 is ????VV??
        and.w   #0xff00,%d0     // Now d0 is ????VV00 (1)
        lsr.w   #6,%d0          // Now d0.w contains the VBR table offset
        move.l  VBR,%a0         // Get the address from VBR to a0
        move.l  (%a0,%d0.w),%a0 // Fetch the vector
        move.l  4(%sp),%d0      // Restore d0
        move.l  %a0,4(%sp)      // Place target address to the stack
        move.l  (%sp)+,%a0      // Restore a0, target address is on TOS
        ret                     // This will jump to the handler and
    // restore the stack

(1) If 'myhandler' is guaranteed to be in the first 64K, e.g. just
    after the vector table then that insn is not needed.

There are probably shorter ways to do this, but it I believe is enough to
illustrate the trick. Optimisation is left as an exercise to the reader :-)

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

Eight levels (0-7) of interrupt priorities are supported by MC68xxx family
members with level seven (7) being the highest priority.  Level zero (0)
indicates that interrupts are fully enabled.  Interrupt requests for interrupts
with priorities less than or equal to the current interrupt mask level are
ignored.

Although RTEMS supports 256 interrupt levels, the MC68xxx family only supports
eight.  RTEMS interrupt levels 0 through 7 directly correspond to MC68xxx
interrupt levels.  All other RTEMS interrupt levels are undefined and their
behavior is unpredictable.

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

The default fatal error handler for this architecture disables processor
interrupts to level 7, places the error code in D0, and executes a ``stop``
instruction to simulate a halt processor instruction.

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

SMP is not supported.

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

Thread-local storage is supported.

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

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

An RTEMS based application is initiated or re-initiated when the MC68020
processor is reset.  When the MC68020 is reset, the processor performs the
following actions:

- The tracing bits of the status register are cleared to disable tracing.

- The supervisor interrupt state is entered by setting the supervisor (S) bit
  and clearing the master/interrupt (M) bit of the status register.

- The interrupt mask of the status register is set to level 7 to effectively
  disable all maskable interrupts.

- The vector base register (VBR) is set to zero.

- The cache control register (CACR) is set to zero to disable and freeze the
  processor cache.

- The interrupt stack pointer (ISP) is set to the value stored at vector 0
  (bytes 0-3) of the exception vector table (EVT).

- The program counter (PC) is set to the value stored at vector 1 (bytes 4-7)
  of the EVT.

- The processor begins execution at the address stored in the PC.

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

The address of the application's initialization code should be stored in the
first vector of the EVT which will allow the immediate vectoring to the
application code.  If the application requires that the VBR be some value
besides zero, then it should be set to the required value at this point.  All
tasks share the same MC68020's VBR value.  Because interrupts are enabled
automatically by RTEMS as part of the context switch to the first task, the VBR
MUST be set by either RTEMS of the BSP before this occurs ensure correct
interrupt vectoring.  If processor caching is to be utilized, then it should be
enabled during the reset application initialization code.

In addition to the requirements described in the Board Support Packages chapter
of the Applications User's Manual for the reset code which is executed before
the call to initialize executive, the MC68020 version has the following
specific requirements:

- Must leave the S bit of the status register set so that the MC68020 remains
  in the supervisor state.

- Must set the M bit of the status register to remove the MC68020 from the
  interrupt state.

- Must set the master stack pointer (MSP) such that a minimum stack size of
  MINIMUM_STACK_SIZE bytes is provided for the initialize executive directive.

- Must initialize the MC68020's vector table.
