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authorChris Johns <chrisj@rtems.org>2016-11-03 16:58:08 +1100
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+.. comment SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. COMMENT: COPYRIGHT (c) 1988-2002.
+.. COMMENT: On-Line Applications Research Corporation (OAR).
+.. COMMENT: All rights reserved.
+.. COMMENT: Jukka Pietarinen <jukka.pietarinen@mrf.fi>, 2008,
+.. COMMENT: Micro-Research Finland Oy
+
+Intel/AMD x86 Specific Information
+##################################
+
+This chapter discusses the Intel x86 architecture dependencies in this port of
+RTEMS. This family has multiple implementations from multiple vendors and
+suffers more from having evolved rather than being designed for growth.
+
+For information on the i386 processor, refer to the following documents:
+
+- *386 Programmer's Reference Manual, Intel, Order No. 230985-002*.
+
+- *386 Microprocessor Hardware Reference Manual, Intel,
+ Order No. 231732-003*.
+
+- *80386 System Software Writer's Guide, Intel, Order No. 231499-001*.
+
+- *80387 Programmer's Reference Manual, Intel, Order No. 231917-001*.
+
+CPU Model Dependent Features
+============================
+
+This section presents the set of features which vary across i386
+implementations and are of importance to RTEMS. The set of CPU model feature
+macros are defined in the :file:`cpukit/score/cpu/i386/i386.h` based upon the
+particular CPU model specified on the compilation command line.
+
+bswap Instruction
+-----------------
+
+The macro ``I386_HAS_BSWAP`` is set to 1 to indicate that this CPU model has
+the ``bswap`` instruction which endian swaps a thirty-two bit quantity. This
+instruction appears to be present in all CPU models i486's and above.
+
+Calling Conventions
+===================
+
+Processor Background
+--------------------
+
+The i386 architecture supports a simple yet effective call and return
+mechanism. A subroutine is invoked via the call (``call``) instruction. This
+instruction pushes the return address on the stack. The return from subroutine
+(``ret``) instruction pops the return address off the current stack and
+transfers control to that instruction. It is is important to note that the
+i386 call and return mechanism does not automatically save or restore any
+registers. It is the responsibility of the high-level language compiler to
+define the register preservation and usage convention.
+
+Calling Mechanism
+-----------------
+
+All RTEMS directives are invoked using a call instruction and return to the
+user application via the ret instruction.
+
+Register Usage
+--------------
+
+As discussed above, the call instruction does not automatically save any
+registers. RTEMS uses the registers EAX, ECX, and EDX as scratch registers.
+These registers are not preserved by RTEMS directives therefore, the contents
+of these registers should not be assumed upon return from any RTEMS directive.
+
+Parameter Passing
+-----------------
+
+RTEMS assumes that arguments are placed on the current stack before the
+directive is invoked via the call instruction. The first argument is assumed
+to be closest to the return address on the stack. This means that the first
+argument of the C calling sequence is pushed last. The following pseudo-code
+illustrates the typical sequence used to call a RTEMS directive with three (3)
+arguments:
+
+.. code-block:: c
+
+ push third argument
+ push second argument
+ push first argument
+ invoke directive
+ remove arguments from the stack
+
+The arguments to RTEMS are typically pushed onto the stack using a push
+instruction. These arguments must be removed from the stack after control is
+returned to the caller. This removal is typically accomplished by adding the
+size of the argument list in bytes to the stack pointer.
+
+Memory Model
+============
+
+Flat Memory Model
+-----------------
+
+RTEMS supports the i386 protected mode, 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. The i386 uses information provided in
+the segment registers and the Global Descriptor Table to convert these
+addresses. RTEMS assumes the existence of the following segments:
+
+- a single code segment at protection level (0) which contains all application
+ and executive code.
+
+- a single data segment at protection level zero (0) which contains all
+ application and executive data.
+
+The i386 segment registers and associated selectors must be initialized when
+the initialize_executive directive is invoked. RTEMS treats the segment
+registers as system registers and does not modify or context switch them.
+
+This i386 memory model supports a flat 32-bit address space with addresses
+ranging from 0x00000000 to 0xFFFFFFFF (4 gigabytes). Each address is
+represented by a 32-bit value and is byte addressable. The address may be used
+to reference a single byte, half-word (2-bytes), or word (4 bytes).
+
+Interrupt Processing
+====================
+
+Although RTEMS hides many of the processor dependent details of interrupt
+processing, it is important to understand how the RTEMS interrupt manager is
+mapped onto the processor's unique architecture. Discussed in this chapter are
+the the processor's response and control mechanisms as they pertain to RTEMS.
+
+Vectoring of Interrupt Handler
+------------------------------
+
+Although the i386 supports multiple privilege levels, RTEMS and all user
+software executes at privilege level 0. This decision was made by the RTEMS
+designers to enhance compatibility with processors which do not provide
+sophisticated protection facilities like those of the i386. This decision
+greatly simplifies the discussion of i386 processing, as one need only consider
+interrupts without privilege transitions.
+
+Upon receipt of an interrupt the i386 automatically performs the following
+actions:
+
+- pushes the EFLAGS register
+
+- pushes the far address of the interrupted instruction
+
+- vectors to the interrupt service routine (ISR).
+
+A nested interrupt is processed similarly by the i386.
+
+Interrupt Stack Frame
+---------------------
+
+The structure of the Interrupt Stack Frame for the i386 which is placed on the
+interrupt stack by the processor in response to an interrupt is as follows:
+
++----------------------+-------+
+| Old EFLAGS Register | ESP+8 |
++----------+-----------+-------+
+| UNUSED | Old CS | ESP+4 |
++----------+-----------+-------+
+| Old EIP | ESP |
++----------------------+-------+
+
+
+Interrupt Levels
+----------------
+
+Although RTEMS supports 256 interrupt levels, the i386 only supports two -
+enabled and disabled. Interrupts are enabled when the interrupt-enable flag
+(IF) in the extended flags (EFLAGS) is set. Conversely, interrupt processing
+is inhibited when the IF is cleared. During a non-maskable interrupt, all
+other interrupts, including other non-maskable ones, are inhibited.
+
+RTEMS interrupt levels 0 and 1 such that level zero
+(0) indicates that interrupts are fully enabled and level one that interrupts
+are disabled. All other RTEMS interrupt levels are undefined and their
+behavior is unpredictable.
+
+Interrupt Stack
+---------------
+
+The i386 family does not support a dedicated hardware interrupt stack. On this
+processor, RTEMS allocates and manages a dedicated interrupt stack. As part of
+vectoring a non-nested interrupt service routine, RTEMS switches from the stack
+of the interrupted task to a dedicated interrupt stack. When a non-nested
+interrupt returns, RTEMS switches back to the stack of the interrupted stack.
+The current stack pointer is not altered by RTEMS on nested interrupt.
+
+Default Fatal Error Processing
+==============================
+
+The default fatal error handler for this architecture disables processor
+interrupts, places the error code in EAX, and executes a HLT instruction to
+halt the processor.
+
+Symmetric Multiprocessing
+=========================
+
+SMP is not supported.
+
+Thread-Local Storage
+====================
+
+Thread-local storage is not implemented.
+
+Board Support Packages
+======================
+
+System Reset
+------------
+
+An RTEMS based application is initiated when the i386 processor is reset. When
+the i386 is reset,
+
+- 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.
+
+- 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.
+
+- 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.
+
+- All bits in the extended flags register (EFLAG) which are not permanently set
+ are cleared. This inhibits all maskable interrupts.
+
+- The Interrupt Descriptor Register (IDTR) is set to point at address zero.
+
+- All segment registers are set to zero.
+
+- 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.
+
+Typically, an intersegment JMP to the application's initialization code is
+placed at address 0xFFFFFFF0.
+
+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 i386 data structures and their contents,
+refer to Intel's 386 Programmer's Reference Manual.