memmodel.t

RTEMS (Real-Time Executive for Multiprocessor Systems) is a free open source real-time operating sys

@c
@c  COPYRIGHT (c) 1988-2002.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
@c
@c  $Id: memmodel.t,v 1.8 2002/01/17 21:47:46 joel Exp $
@c

@chapter Memory Model

@section Introduction

A processor may support any combination of memory
models ranging from pure physical addressing to complex demand
paged virtual memory systems.  RTEMS supports a flat memory
model which ranges contiguously over the processor's allowable
address space.  RTEMS does not support segmentation or virtual
memory of any kind.  The appropriate memory model for RTEMS
provided by the targeted processor and related characteristics
of that model are described in this chapter.

@section 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:

@ifset use-ascii
@example
@group
          +--------------+-----------------------+
          |   Data Type  | Alignment Requirement |
          +--------------+-----------------------+
          |     byte     |          1            |
          |   half-word  |          2            |
          |     word     |          4            |
          |  doubleword  |          8            |
          +--------------+-----------------------+
@end group
@end example
@end ifset

@ifset use-tex
@sp 1
@tex
\centerline{\vbox{\offinterlineskip\halign{
\vrule\strut#&
\hbox to 1.75in{\enskip\hfil#\hfil}&
\vrule#&
\hbox to 1.75in{\enskip\hfil#\hfil}&
\vrule#\cr
\noalign{\hrule}
&\bf Data Type &&\bf Alignment Requirement&\cr\noalign{\hrule}
&byte&&1&\cr\noalign{\hrule}
&half-word&&2&\cr\noalign{\hrule}
&word&&4&\cr\noalign{\hrule}
&doubleword&&8&\cr\noalign{\hrule}
}}\hfil}
@end tex
@end ifset
 
@ifset use-html
@html
<CENTER>
  <TABLE COLS=2 WIDTH="60%" BORDER=2>
<TR><TD ALIGN=center><STRONG>Data Type</STRONG></TD>
    <TD ALIGN=center><STRONG>Alignment Requirement</STRONG></TD></TR>
<TR><TD ALIGN=center>byte</TD>
    <TD ALIGN=center>1</TD></TR>
<TR><TD ALIGN=center>half-word</TD>
    <TD ALIGN=center>2</TD></TR>
<TR><TD ALIGN=center>word</TD>
    <TD ALIGN=center>4</TD></TR>
<TR><TD ALIGN=center>doubleword</TD>
    <TD ALIGN=center>8</TD></TR>
  </TABLE>
</CENTER>
@end html
@end ifset

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.