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@c  COPYRIGHT (c) 1988-2002.

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@c  bsp.t,v 1.9 2002/01/17 21:47:46 joel Exp

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@chapter Board Support Packages

@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.

@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.

@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.

Since the processor is in real mode upon reset, the processor must be

switched to protected mode before RTEMS can execute.  Before switching to

protected mode, at least one descriptor table and two descriptors must be

created.  Descriptors are needed for a code segment and a data segment. (

This will give you the flat memory model.)  The stack can be placed in a

normal read/write data segment, so no descriptor for the stack is needed.

Before the GDT can be used, the base address and limit must be loaded into

the GDTR register using an LGDT instruction.

If the hardware allows an NMI to be generated, you need to create the IDT

and a gate for the NMI interrupt handler.  Before the IDT can be used, the

base address and limit for the idt must be loaded into the IDTR register

using an LIDT instruction.

Protected mode is entered by setting thye PE bit in the CR0 register.

Either a LMSW or MOV CR0 instruction may be used to set this bit. Because

the processor overlaps the interpretation of several instructions, it is

necessary to discard the instructions from the read-ahead cache. A JMP

instruction immediately after the LMSW changes the flow and empties the

processor if intructions which have been pre-fetched and/or decoded.  At

this point, the processor is in protected mode and begins to perform

protected mode application initialization.

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.