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@c  COPYRIGHT (c) 1988-2002.

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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

RTEMS supports the i386 protected mode, flat memory

model with paging disabled.  In this mode, the i386

automatically converts every address from a logical to a

physical address each time it is used.  The i386 uses

information provided in the segment registers and the Global

Descriptor Table to convert these addresses.  RTEMS assumes the

existence of the following segments:

@itemize @bullet

@item a single code segment at protection level (0) which

contains all application and executive code.

@item a single data segment at protection level zero (0) which

contains all application and executive data.

@end itemize

The i386 segment registers and associated selectors

must be initialized when the initialize_executive directive is

invoked.  RTEMS treats the segment registers as system registers

and does not modify or context switch them.

This i386 memory model 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, half-word (2-bytes), or word (4 bytes).

RTEMS does not require that logical addresses map

directly to physical addresses, although it is desirable in many

applications to do so.  If logical and physical addresses are

not the same, then an additional selector will be required so

RTEMS can access the Interrupt Descriptor Table to install

interrupt service routines.  The selector number of this segment

is provided to RTEMS in the CPU Dependent Information Table.

By not requiring that logical addresses map directly

to physical addresses, the memory space of an RTEMS application

can be separated from that of a ROM monitor.  For example, on

the Force Computers CPU386, the ROM monitor loads application

programs into a logical address space where logical address

0x00000000 corresponds to physical address 0x0002000.  On this

board, RTEMS and the application use virtual addresses which do

not map to physical addresses.

RTEMS assumes that the DS and ES registers contain

the selector for the single data segment when a directive is

invoked.   This assumption is especially important when

developing interrupt service routines.