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@c  memmodel.t,v 1.6 2002/01/17 21:47:46 joel Exp

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@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 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.