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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 applications in which the application

and the executive execute within a single thirty-two bit address

space.  Thus RTEMS and the application share a common four

gigabyte address space within a single space.  The PA-RISC

automatically converts every address from a logical to a

physical address each time it is used.  The PA-RISC uses

information provided in the page table to perform this

translation.  The following protection levels are assumed:

@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 PA-RISC space registers and associated stack --

including the stack pointer r27 -- must be initialized when the

initialize_executive directive is invoked.  RTEMS treats the

space registers as system resources shared by all tasks and does

not modify or context switch them.

This 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

memory is 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.  RTEMS does not need any additional

information when physical addresses do not map directly to

physical addresses.  By not requiring that logical addresses map

directly to physical addresses, the memory space of an RTEMS

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

a ROM monitor may load application programs into a separate

logical address space from itself.

RTEMS assumes that the space registers contain the

selector for the single data segment when a directive is

invoked.   This assumption is especially important when

developing interrupt service routines.