source: rtems/doc/porting/cpumodels.t @ 37b30dc
Last change on this file since 37b30dc was 37b30dc, checked in by Joel Sherrill <joel.sherrill@…>, on Feb 14, 2003 at 7:50:11 PM

2003-02-14 Joel Sherrill <joel@…>

  • cpumodels.t: Removed a29k reference and updated.
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2@c  COPYRIGHT (c) 1988-2002.
3@c  On-Line Applications Research Corporation (OAR).
4@c  All rights reserved.
6@c  $Id$
9@chapter CPU Model Variations
11XXX enhance using portability presentation from CS595 class.  See
14Since the text in the next section was written, RTEMS view of
15portability has grown to distinguish totally portable, CPU
16family dependent, CPU model dependent, peripheral chip dependent
17and board dependent.  This text was part of a larger paper that
18did not even cover portability completely as it existed when this
19was written and certainly is out of date now. :)
22@section Overview of RTEMS Portability
24RTEMS was designed to be a highly portable, reusable software component. 
25This reflects the fundamental nature of embedded systems in which hardware
26components, ranging from boards to peripherals to even the processor
27itself, are selected specifically to meet the requirements of a particular
30@subsection Processor Families
32Since there are a wide variety of embedded systems, there are a wide
33variety of processors targeting embedded systems. RTEMS alleviates some of
34the burden on the embedded systems programmer by providing a consistent,
35high-performance environment regardless of the target processor.  RTEMS
36has been ported to a variety of microprocessor families including:
38@itemize @bullet
40@item Motorola ColdFire
41@item Motorola MC68xxx
42@item Motorola MC683xx
43@item Intel ix86 (i386 and above)
44@item Intel i960
45@item ARM
46@item MIPS
47@item PowerPC 4xx, 5xx, 6xx, 7xx, 8xx, and 84xx
48@item SPARC
49@item Hewlett-Packard PA-RISC
50@item OpenCores OR32
51@item Texas Instruments C3x/C4x
53@end itemize
56In addition, there is a port of RTEMS to UNIX that uses standard UNIX
57services to simulate the embedded environment.
59Each RTEMS port supplies a well-defined set of services that are the
60foundation for the highly portable RTEMS and POSIX API implementations. 
61When porting to a new processor family, one must provide the processor
62dependent implementation of these services.  This set of processor
63dependent core services includes software to perform interrupt
64dispatching, context switches, and manipulate task register sets.
66The RTEMS approach to handling varying processor models reflects the
67approach taken by the designers of the processors themselves.  In each
68processor family, there is a core architecture that must be implemented on
69all processor models within the family to provide any level of
70compatibility.  Many of the modern RISC architectures refer to this as the
71Architectural Definition.  The Architectural Definition is intended to be
72independent of any particular implementation. Additionally, there is a
73feature set which is allowed to vary in a defined way amongst the
74processor models.  These feature sets may be defined as Optional in the
75Architectural Definition, be left as implementation defined
76characteristics, or be processor model specific extensions.  Support for
77floating point, virtual memory, and low power mode are common Optional
78features included in an Architectural Definition.
80The processor family dependent software in RTEMS includes a definition of
81which features are present in each supported processor model.  This often
82makes adding support for a new processor model within a supported family
83as simple as determining which features are present in the new processor
84implementation.  If the new processor model varies in a way previously
85unaccounted for, then this must be addressed.  This could be the result of
86a new Optional feature set being added to the Architectural Definition. 
87Alternatively, this particular processor model could have a new and
88different implementation of a feature left as undefined in the
89Architectural Definition.  This would require software to be written to
90utilize that feature.
92There is a relatively small set of features that may vary in a processor
93family.  As the number of processor models in the family grow, the
94addition of each new model only requires adding an entry for the new model
95to the single feature table.  It does not require searching for every
96conditional based on processor model and adding the new model in the
97appropriate place.  This significantly eases the burden of adding a new
98processor model as it centralizes and logically simplifies the process.
100@subsection Boards
102Being portable both between models within a processor family and across
103processor families is not enough to address the needs of embedded systems
104developers.  Custom board development is the norm for embedded systems. 
105Each of these boards is optimized for a particular project.  The processor
106and peripheral set have been chosen to meet a particular set of system
107requirements.  The tools in the embedded systems developers’ toolbox must
108support their project’s unique board.  RTEMS addresses this issue via the
109Board Support Package.
111RTEMS segregates board specific code to make it possible for the embedded
112systems developer to easily replace and customize this software.  A
113minimal Board Support Package includes device drivers for a clock tick,
114console I/O, and a benchmark timer (optional) as well as startup and
115miscellaneous support code.  The Board Support Package for a project may
116be extended to include the device drivers for any peripherals on the
117custom board.
119@subsection Applications
121One important design goal of RTEMS was to provide a bridge between the
122application software and the target hardware.  Most hardware dependencies
123for real-time applications can be localized to the low level device
124drivers which provide an abstracted view of the hardware.  The RTEMS I/O
125interface manager provides an efficient tool for incorporating these
126hardware dependencies into the system while simultaneously providing a
127general mechanism to the application code that accesses them.  A well
128designed real-time system can benefit from this architecture by building a
129rich library of standard application components which can be used
130repeatedly in other real-time projects. The following figure illustrates
131how RTEMS serves as a buffer between the project dependent application
132code and the target hardware.
134@section Coding Issues
136XXX deal with this as it applies to score/cpu.  Section name may
137be bad.
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