@c
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@c
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@c COPYRIGHT (c) 1988-2002.
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@c COPYRIGHT (c) 1988-2002.
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@c On-Line Applications Research Corporation (OAR).
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@c On-Line Applications Research Corporation (OAR).
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@c All rights reserved.
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@c All rights reserved.
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@c
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@c
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@c memmodel.t,v 1.6 2002/01/17 21:47:46 joel Exp
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@c memmodel.t,v 1.6 2002/01/17 21:47:46 joel Exp
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@c
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@c
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@chapter Memory Model
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@chapter Memory Model
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@section Introduction
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@section Introduction
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A processor may support any combination of memory
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A processor may support any combination of memory
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models ranging from pure physical addressing to complex demand
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models ranging from pure physical addressing to complex demand
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paged virtual memory systems. RTEMS supports a flat memory
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paged virtual memory systems. RTEMS supports a flat memory
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model which ranges contiguously over the processor's allowable
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model which ranges contiguously over the processor's allowable
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address space. RTEMS does not support segmentation or virtual
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address space. RTEMS does not support segmentation or virtual
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memory of any kind. The appropriate memory model for RTEMS
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memory of any kind. The appropriate memory model for RTEMS
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provided by the targeted processor and related characteristics
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provided by the targeted processor and related characteristics
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of that model are described in this chapter.
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of that model are described in this chapter.
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@section Flat Memory Model
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@section Flat Memory Model
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RTEMS supports applications in which the application
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RTEMS supports applications in which the application
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and the executive execute within a single thirty-two bit address
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and the executive execute within a single thirty-two bit address
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space. Thus RTEMS and the application share a common four
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space. Thus RTEMS and the application share a common four
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gigabyte address space within a single space. The PA-RISC
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gigabyte address space within a single space. The PA-RISC
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automatically converts every address from a logical to a
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automatically converts every address from a logical to a
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physical address each time it is used. The PA-RISC uses
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physical address each time it is used. The PA-RISC uses
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information provided in the page table to perform this
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information provided in the page table to perform this
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translation. The following protection levels are assumed:
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translation. The following protection levels are assumed:
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@itemize @bullet
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@itemize @bullet
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@item a single code segment at protection level (0) which
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@item a single code segment at protection level (0) which
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contains all application and executive code.
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contains all application and executive code.
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@item a single data segment at protection level zero (0) which
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@item a single data segment at protection level zero (0) which
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contains all application and executive data.
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contains all application and executive data.
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@end itemize
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@end itemize
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The PA-RISC space registers and associated stack --
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The PA-RISC space registers and associated stack --
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including the stack pointer r27 -- must be initialized when the
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including the stack pointer r27 -- must be initialized when the
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initialize_executive directive is invoked. RTEMS treats the
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initialize_executive directive is invoked. RTEMS treats the
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space registers as system resources shared by all tasks and does
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space registers as system resources shared by all tasks and does
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not modify or context switch them.
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not modify or context switch them.
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This memory model supports a flat 32-bit address
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This memory model supports a flat 32-bit address
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space with addresses ranging from 0x00000000 to 0xFFFFFFFF (4
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space with addresses ranging from 0x00000000 to 0xFFFFFFFF (4
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gigabytes). Each address is represented by a 32-bit value and
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gigabytes). Each address is represented by a 32-bit value and
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memory is addressable. The address may be used to reference a
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memory is addressable. The address may be used to reference a
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single byte, half-word (2-bytes), or word (4 bytes).
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single byte, half-word (2-bytes), or word (4 bytes).
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RTEMS does not require that logical addresses map
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RTEMS does not require that logical addresses map
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directly to physical addresses, although it is desirable in many
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directly to physical addresses, although it is desirable in many
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applications to do so. RTEMS does not need any additional
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applications to do so. RTEMS does not need any additional
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information when physical addresses do not map directly to
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information when physical addresses do not map directly to
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physical addresses. By not requiring that logical addresses map
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physical addresses. By not requiring that logical addresses map
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directly to physical addresses, the memory space of an RTEMS
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directly to physical addresses, the memory space of an RTEMS
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space can be separated from that of a ROM monitor. For example,
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space can be separated from that of a ROM monitor. For example,
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a ROM monitor may load application programs into a separate
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a ROM monitor may load application programs into a separate
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logical address space from itself.
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logical address space from itself.
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RTEMS assumes that the space registers contain the
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RTEMS assumes that the space registers contain the
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selector for the single data segment when a directive is
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selector for the single data segment when a directive is
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invoked. This assumption is especially important when
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invoked. This assumption is especially important when
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developing interrupt service routines.
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developing interrupt service routines.
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