;; @file
;  This is the code that goes from real-mode to protected mode.
;  It consumes the reset vector, configures the stack.
;
; Copyright (c) 2015, Intel Corporation. All rights reserved.<BR>
; SPDX-License-Identifier: BSD-2-Clause-Patent
;;

;
; Define assembler characteristics
;
.586p
.xmm
.model flat, c

EXTRN   TempRamInitApi:NEAR
EXTRN   FspInitApi:NEAR

;
; Contrary to the name, this file contains 16 bit code as well.
;
_TEXT_REALMODE      SEGMENT PARA PUBLIC USE16 'CODE'
                    ASSUME  CS:_TEXT_REALMODE, DS:_TEXT_REALMODE

;----------------------------------------------------------------------------
;
; Procedure:    _ModuleEntryPoint
;
; Input:        None
;
; Output:       None
;
; Destroys:     Assume all registers
;
; Description:
;
;   Transition to non-paged flat-model protected mode from a
;   hard-coded GDT that provides exactly two descriptors.
;   This is a bare bones transition to protected mode only
;   used for a while in PEI and possibly DXE.
;
;   After enabling protected mode, a far jump is executed to
;   transfer to PEI using the newly loaded GDT.
;
; Return:       None
;
;----------------------------------------------------------------------------
align 16
_ModuleEntryPoint      PROC C PUBLIC
  ;
  ; Load the GDT table in GdtDesc
  ;
  mov     esi, OFFSET GdtDesc
  db      66h
  lgdt    fword ptr cs:[si]

  ;
  ; Transition to 16 bit protected mode
  ;
  mov     eax, cr0                   ; Get control register 0
  or      eax, 00000003h             ; Set PE bit (bit #0) & MP bit (bit #1)
  mov     cr0, eax                   ; Activate protected mode

  ;
  ; Now we're in 16 bit protected mode
  ; Set up the selectors for 32 bit protected mode entry
  ; 
  mov     ax, SYS_DATA_SEL
  mov     ds, ax
  mov     es, ax
  mov     fs, ax
  mov     gs, ax
  mov     ss, ax

  ;
  ; Transition to Flat 32 bit protected mode
  ; The jump to a far pointer causes the transition to 32 bit mode
  ;
  mov esi, offset ProtectedModeEntryLinearAddress
  jmp     fword ptr cs:[si]

_ModuleEntryPoint   ENDP

_TEXT_REALMODE      ENDS

.code 
;
; Protected mode portion initializes stack, configures cache, and calls C entry point
;

;----------------------------------------------------------------------------
;
; Procedure:    ProtectedModeEntryPoint
;
; Input:        Executing in 32 Bit Protected (flat) mode
;               cs: 0-4GB
;               ds: 0-4GB
;               es: 0-4GB
;               fs: 0-4GB
;               gs: 0-4GB
;               ss: 0-4GB
;
; Output:       This function never returns
;
; Destroys:
;               ecx
;               edi
;               esi
;               esp
;
; Description:
;               Perform any essential early platform initilaisation
;               Setup a stack
;
;----------------------------------------------------------------------------

ProtectedModeEntryPoint PROC NEAR C PUBLIC
  ;
  ; Dummy function. Consume 2 API to make sure they can be linked.
  ;
  mov  eax, TempRamInitApi
  mov  eax, FspInitApi

  ; Should never return
  jmp  $

ProtectedModeEntryPoint ENDP

;
; ROM-based Global-Descriptor Table for the PEI Phase
;
align 16
PUBLIC  BootGdtTable

;
; GDT[0]: 0x00: Null entry, never used.
;
NULL_SEL        equ     $ - GDT_BASE        ; Selector [0]
GDT_BASE:
BootGdtTable    DD      0
                DD      0
;
; Linear code segment descriptor
;
LINEAR_CODE_SEL equ     $ - GDT_BASE        ; Selector [0x8]
        DW      0FFFFh                      ; limit 0xFFFF
        DW      0                           ; base 0
        DB      0
        DB      09Bh                        ; present, ring 0, data, expand-up, not-writable
        DB      0CFh                        ; page-granular, 32-bit
        DB      0
;
; System data segment descriptor
;
SYS_DATA_SEL    equ     $ - GDT_BASE        ; Selector [0x10]
        DW      0FFFFh                      ; limit 0xFFFF
        DW      0                           ; base 0
        DB      0
        DB      093h                        ; present, ring 0, data, expand-up, not-writable
        DB      0CFh                        ; page-granular, 32-bit
        DB      0

GDT_SIZE        EQU     $ - BootGDTtable    ; Size, in bytes

;
; GDT Descriptor
;
GdtDesc:                                    ; GDT descriptor
        DW      GDT_SIZE - 1                ; GDT limit
        DD      OFFSET BootGdtTable         ; GDT base address

ProtectedModeEntryLinearAddress   LABEL   FWORD
ProtectedModeEntryLinearOffset    LABEL   DWORD
  DD      OFFSET ProtectedModeEntryPoint  ; Offset of our 32 bit code
  DW      LINEAR_CODE_SEL
  
END
