source: rtems/doc/supplements/c4x/cpumodel.t @ 45d13d56

4.104.114.84.95
Last change on this file since 45d13d56 was 45d13d56, checked in by Joel Sherrill <joel.sherrill@…>, on 09/26/03 at 22:01:50

2003-09-26 Joel Sherrill <joel@…>

  • cpumodel.t: Obsoleting HP PA-RISC port and removing all references.
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1@c
2@c  COPYRIGHT (c) 1988-1999.
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 Dependent Features
10
11@section Introduction
12
13Microprocessors are generally classified into
14families with a variety of CPU models or implementations within
15that family.  Within a processor family, there is a high level
16of binary compatibility.  This family may be based on either an
17architectural specification or on maintaining compatibility with
18a popular processor.  Recent microprocessor families such as the
19SPARC or PowerPC are based on an architectural specification
20which is independent or any particular CPU model or
21implementation.  Older families such as the M68xxx and the iX86
22evolved as the manufacturer strived to produce higher
23performance processor models which maintained binary
24compatibility with older models.
25
26RTEMS takes advantage of the similarity of the
27various models within a CPU family.  Although the models do vary
28in significant ways, the high level of compatibility makes it
29possible to share the bulk of the CPU dependent executive code
30across the entire family.  Each processor family supported by
31RTEMS has a list of features which vary between CPU models
32within a family.  For example, the most common model dependent
33feature regardless of CPU family is the presence or absence of a
34floating point unit or coprocessor.  When defining the list of
35features present on a particular CPU model, one simply notes
36that floating point hardware is or is not present and defines a
37single constant appropriately.  Conditional compilation is
38utilized to include the appropriate source code for this CPU
39model's feature set.  It is important to note that this means
40that RTEMS is thus compiled using the appropriate feature set
41and compilation flags optimal for this CPU model used.  The
42alternative would be to generate a binary which would execute on
43all family members using only the features which were always
44present.
45
46This chapter presents the set of features which vary
47across the various implementations of the C3x/C4x architecture
48that are of importance to rtems.
49the set of cpu model feature macros are defined in the file
50cpukit/score/cpu/c4x/rtems/score/c4x.h and are based upon
51the particular cpu model defined in the bsp's custom configuration
52file as well as the compilation command line.
53
54@section CPU Model Name
55
56The macro @code{CPU_MODEL_NAME} is a string which designates
57the name of this cpu model.  for example, for the c32
58processor, this macro is set to the string "c32".
59
60@section Floating Point Unit
61
62The Texas Instruments C3x/C4x family makes little distinction
63between the various cpu registers.  Although floating point
64operations may only be performed on a subset of the cpu registers,
65these same registers may be used for normal integer operations.
66as a result of this, this port of rtems makes no distinction
67between integer and floating point contexts.  The routine
68@code{_CPU_Context_switch} saves all of the registers that
69comprise a task's context.  the routines that initialize,
70save, and restore floating point contexts are not present
71in this port.
72
73Moreover, there is no floating point context pointer and
74the code in @code{_Thread_Dispatch} that manages the
75floating point context switching process is disabled
76on this port.
77
78This not only simplifies the port, it also speeds up context
79switches by reducing the code involved and reduces the code
80space footprint of the executive on the Texas Instruments
81C3x/C4x.
82
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