source: rtems/doc/supplements/i386/bsp.t @ 139b2e4a

@c
@c  COPYRIGHT (c) 1988-1997.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
@c
@c  $Id$
@c

@ifinfo
@node Board Support Packages, Board Support Packages Introduction, Default Fatal Error Processing Default Fatal Error Handler Operations, Top
@end ifinfo
@chapter Board Support Packages
@ifinfo
@menu
* Board Support Packages Introduction::
* Board Support Packages System Reset::
* Board Support Packages Processor Initialization::
@end menu
@end ifinfo

@ifinfo
@node Board Support Packages Introduction, Board Support Packages System Reset, Board Support Packages, Board Support Packages
@end ifinfo
@section Introduction

An RTEMS Board Support Package (BSP) must be designed
to support a particular processor and target board combination.
This chapter presents a discussion of i386 specific BSP issues.
For more information on developing a BSP, refer to the chapter
titled Board Support Packages in the RTEMS
Applications User's Guide.

@ifinfo
@node Board Support Packages System Reset, Board Support Packages Processor Initialization, Board Support Packages Introduction, Board Support Packages
@end ifinfo
@section System Reset

An RTEMS based application is initiated when the i386
processor is reset.  When the i386 is reset,

@itemize @bullet
@item The EAX register is set to indicate the results of the
processor's power-up self test.   If the self-test was not
executed, the contents of this register are undefined.
Otherwise, a non-zero value indicates the processor is faulty
and a zero value indicates a successful self-test.

@item The DX register holds a component identifier and
revision level.  DH contains 3 to indicate an i386 component and
DL contains a unique revision level indicator.

@item Control register zero (CR0) is set such that the
processor is in real mode with paging disabled.   Other portions
of CR0 are used to indicate the presence of a numeric
coprocessor.

@item All bits in the extended flags register (EFLAG) which
are not permanently set are cleared.  This inhibits all maskable
interrupts.

@item The Interrupt Descriptor Register (IDTR) is set to point
at address zero.

@item All segment registers are set to zero.

@item The instruction pointer is set to 0x0000FFF0.   The
first instruction executed after a reset is actually at
0xFFFFFFF0 because the i386 asserts the upper twelve address
until the first intersegment (FAR) JMP or CALL instruction.
When a JMP or CALL is executed, the upper twelve address lines
are lowered and the processor begins executing in the first
megabyte of memory.
@end itemize

Typically, an intersegment JMP to the application's
initialization code is placed at address 0xFFFFFFF0.

@ifinfo
@node Board Support Packages Processor Initialization, Processor Dependent Information Table, Board Support Packages System Reset, Board Support Packages
@end ifinfo
@section Processor Initialization

This initialization code is responsible for
initializing all data structures required by the i386 in
protected mode and for actually entering protected mode.  The
i386 must be placed in protected mode and the segment registers
and associated selectors must be initialized before the
initialize_executive directive is invoked.

The initialization code is responsible for
initializing the Global Descriptor Table such that the i386 is
in the thirty-two bit flat memory model with paging disabled.
In this mode, the i386 automatically converts every address from
a logical to a physical address each time it is used.  For more
information on the memory model used by RTEMS, please refer to
the Memory Model chapter in this document.

If the application requires that the IDTR be some
value besides zero, then it should set it to the required value
at this point.  All tasks share the same i386 IDTR value.
Because interrupts are enabled automatically by RTEMS as part of
the initialize_executive directive, the IDTR MUST be set
properly before this directive is invoked to insure correct
interrupt vectoring.  If processor caching is to be utilized,
then it should be enabled during the reset application
initialization code.  The reset code which is executed before
the call to initialize_executive has the following requirements:

For more information regarding the i386s data
structures and their contents, refer to Intel's 386
Programmer's Reference Manual.