source: rtems/doc/porting/cpumodels.t @ 14780ecf

4.104.114.84.95
Last change on this file since 14780ecf was 14780ecf, checked in by Joel Sherrill <joel.sherrill@…>, on Aug 22, 2003 at 1:15:51 PM

2003-08-22 Joel Sherrill <joel@…>

  • cpumodels.t, sourcecode.t: Reconcile differences between CVS trunk and 4.6 branch.
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1@c
2@c  COPYRIGHT (c) 1988-2002.
3@c  On-Line Applications Research Corporation (OAR).
4@c  All rights reserved.
5@c
6@c  $Id$
7@c
8
9@chapter CPU Model Variations
10
11XXX enhance using portability presentation from CS595 class.  See
12general/portability.ppt.
13
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19was written and certainly is out of date now. :)
20
21
22@section Overview of RTEMS Portability
23
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
28project.
29
30@subsection Processor Families
31
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:
37
38@itemize @bullet
39
40@item Motorola ColdFire
41@item Motorola MC68xxx
42@item Motorola MC683xx
43@item Intel ix86 (i386, i486, Pentium 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 Hitachi H8/300
51@item Hitachi SH
52@item OpenCores OR32
53@item Texas Instruments C3x/C4x
54
55@end itemize
56
57
58In addition, there is a port of RTEMS to UNIX that uses standard UNIX
59services to simulate the embedded environment.
60
61Each RTEMS port supplies a well-defined set of services that are the
62foundation for the highly portable RTEMS and POSIX API implementations. 
63When porting to a new processor family, one must provide the processor
64dependent implementation of these services.  This set of processor
65dependent core services includes software to perform interrupt
66dispatching, context switches, and manipulate task register sets.
67
68The RTEMS approach to handling varying processor models reflects the
69approach taken by the designers of the processors themselves.  In each
70processor family, there is a core architecture that must be implemented on
71all processor models within the family to provide any level of
72compatibility.  Many of the modern RISC architectures refer to this as the
73Architectural Definition.  The Architectural Definition is intended to be
74independent of any particular implementation. Additionally, there is a
75feature set which is allowed to vary in a defined way amongst the
76processor models.  These feature sets may be defined as Optional in the
77Architectural Definition, be left as implementation defined
78characteristics, or be processor model specific extensions.  Support for
79floating point, virtual memory, and low power mode are common Optional
80features included in an Architectural Definition.
81
82The processor family dependent software in RTEMS includes a definition of
83which features are present in each supported processor model.  This often
84makes adding support for a new processor model within a supported family
85as simple as determining which features are present in the new processor
86implementation.  If the new processor model varies in a way previously
87unaccounted for, then this must be addressed.  This could be the result of
88a new Optional feature set being added to the Architectural Definition. 
89Alternatively, this particular processor model could have a new and
90different implementation of a feature left as undefined in the
91Architectural Definition.  This would require software to be written to
92utilize that feature.
93
94There is a relatively small set of features that may vary in a processor
95family.  As the number of processor models in the family grow, the
96addition of each new model only requires adding an entry for the new model
97to the single feature table.  It does not require searching for every
98conditional based on processor model and adding the new model in the
99appropriate place.  This significantly eases the burden of adding a new
100processor model as it centralizes and logically simplifies the process.
101
102@subsection Boards
103
104Being portable both between models within a processor family and across
105processor families is not enough to address the needs of embedded systems
106developers.  Custom board development is the norm for embedded systems. 
107Each of these boards is optimized for a particular project.  The processor
108and peripheral set have been chosen to meet a particular set of system
109requirements.  The tools in the embedded systems developers’ toolbox must
110support their project’s unique board.  RTEMS addresses this issue via the
111Board Support Package.
112
113RTEMS segregates board specific code to make it possible for the embedded
114systems developer to easily replace and customize this software.  A
115minimal Board Support Package includes device drivers for a clock tick,
116console I/O, and a benchmark timer (optional) as well as startup and
117miscellaneous support code.  The Board Support Package for a project may
118be extended to include the device drivers for any peripherals on the
119custom board.
120
121@subsection Applications
122
123One important design goal of RTEMS was to provide a bridge between the
124application software and the target hardware.  Most hardware dependencies
125for real-time applications can be localized to the low level device
126drivers which provide an abstracted view of the hardware.  The RTEMS I/O
127interface manager provides an efficient tool for incorporating these
128hardware dependencies into the system while simultaneously providing a
129general mechanism to the application code that accesses them.  A well
130designed real-time system can benefit from this architecture by building a
131rich library of standard application components which can be used
132repeatedly in other real-time projects. The following figure illustrates
133how RTEMS serves as a buffer between the project dependent application
134code and the target hardware.
135
136@section Coding Issues
137
138XXX deal with this as it applies to score/cpu.  Section name may
139be bad.
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