source: rtems/c/src/lib/libbsp/i386/pc386/start/start16.s @ ea57368

/*-------------------------------------------------------------------------+
| start16.s v1.0 - PC386 BSP - 1998/04/13
+--------------------------------------------------------------------------+
| This file contains the entry point for the application.
| The name of this entry point is compiler dependent.
| It jumps to the BSP which is responsible for performing all initialization.
+--------------------------------------------------------------------------+
| (C) Copyright 1997 -
| - NavIST Group - Real-Time Distributed Systems and Industrial Automation
|
| http://pandora.ist.utl.pt
|
| Instituto Superior Tecnico * Lisboa * PORTUGAL
+--------------------------------------------------------------------------+
| Disclaimer:
|
| This file is provided "AS IS" without warranty of any kind, either
| expressed or implied.
+--------------------------------------------------------------------------+
| This code is partly based on (from the Linux source tree):
|   video.S - Copyright (C) 1995, 1996 Martin Mares <mj@k332.feld.cvut.cz>
+--------------------------------------------------------------------------*/


/*----------------------------------------------------------------------------+
| Constants
+----------------------------------------------------------------------------*/

.set PROT_CODE_SEG, 0x08        # offset of code segment descriptor into GDT
.set PROT_DATA_SEG, 0x10        # offset of code segment descriptor into GDT
.set CR0_PE,        1           # protected mode flag on CR0 register
.set HDRSTART,      HEADERADDR  # address of start of bin2boot header
.set HDROFF,        0x24        # offset into bin2boot header of start32 addr
.set STACKOFF,      0x200-0x10  # offset to load into %esp, from start of image

/*----------------------------------------------------------------------------+
| A Descriptor table register has the following format: 
+----------------------------------------------------------------------------*/

.set DTR_LIMIT, 0               # offset of two byte limit
.set DTR_BASE,  2               # offset of four byte base address
.set DTR_SIZE,  6               # size of DTR register

/*----------------------------------------------------------------------------+
| CODE section
+----------------------------------------------------------------------------*/

.text

        .globl _start16         # entry point
        .globl start16
start16:
_start16:

.code16

        cli                     # DISABLE INTERRUPTS!!!

        movw    %cs, %ax        #
        movw    %ax, %ds        # set the rest of real mode registers
        movw    %ax, %es        #
        movw    %ax, %ss        #

.code32

        movl    $STACKOFF, %esp # set stack pointer
        movl    $STACKOFF, %ebp #

.code16

#if defined(RTEMS_VIDEO_80x50)
        
        /*---------------------------------------------------------------------+
        | Switch VGA video to 80 lines x 50 columns mode. Has to be done before
        | turning protected mode on since it uses BIOS int 10h (video) services.
        +---------------------------------------------------------------------*/

        movw    $0x0003, %ax    # forced set
        int     $0x10
        movw    $0x1112, %ax    # use 8x8 font
        xorb    %bl, %bl
        int     $0x10
        movw    $0x1201, %ax    # turn off cursor emulation
        movb    $0x34, %bl
        int     $0x10
        movb    $0x01, %ah      # define cursor (scan lines 0 to 7)
        movw    $0x0007, %cx
        int     $0x10

#endif /* RTEMS_VIDEO_80x50 */

        /*---------------------------------------------------------------------+
        | Bare PC machines boot in real mode! We have to turn protected mode on.
        +---------------------------------------------------------------------*/

        lgdt    gdtptr - start16        # load Global Descriptor Table
        lidt    idtptr - start16        # load Interrupt Descriptor Table
        
        movl    %cr0, %eax
        orl     $CR0_PE, %eax
        movl    %eax, %cr0              # turn on protected mode
        
        ljmp    $PROT_CODE_SEG, $1f     # flush prefetch queue, and reload %cs
1:

.code32

        /*---------------------------------------------------------------------+
        | load the other segment registers
        +---------------------------------------------------------------------*/
        movl    $PROT_DATA_SEG, %eax
        movl    %ax, %ds
        movl    %ax, %es
        movl    %ax, %ss
        addl    $start16, %esp          # fix up stack pointer
        addl    $start16, %ebp          # fix up stack pointer

        /*---------------------------------------------------------------------+
        | we have to enable A20 in order to access memory above 1MByte
        +---------------------------------------------------------------------*/
        call    empty_8042
        movb    $0xD1, %al              # command write
        outb    %al, $0x64
        call    empty_8042
        movb    $0xDF, %al              # A20 on
        outb    %al, $0x60
        call    empty_8042

        movl    %cs:HDRSTART + HDROFF, %eax     #
        pushl   %eax                            # jump to start of 32 bit code
        ret                                     #

/*----------------------------------------------------------------------------+
| delay
+------------------------------------------------------------------------------
| Delay is needed after doing I/O. We do it by writing to a non-existent port.
+----------------------------------------------------------------------------*/
        .globl _delay
        .globl delay
delay:
_delay:
        outb    %al, $0xED      # about 1uS delay
        ret

/*----------------------------------------------------------------------------+
| empty_8042
+------------------------------------------------------------------------------
| This routine checks that the keyboard command queue is empty (after emptying
| the output buffers).
| No timeout is used - if this hangs there is something wrong with the machine,
| and we probably couldn't proceed anyway.
+----------------------------------------------------------------------------*/
        .globl _empty_8042
        .globl empty_8042
empty_8042:
_empty_8042:
        call    delay
        inb     $0x64, %al      # 8042 status port
        testb   $0x01, %al      # output buffer?
        jz      no_output
        call    delay
        in      $0x60, %al      # read it
        jmp     empty_8042
no_output:
        test    $0x02, %al      # is input buffer full?
        jnz     empty_8042      # yes - loop
        ret
        
/*----------------------------------------------------------------------------+
| DATA section
+----------------------------------------------------------------------------*/

/**************************
* GLOBAL DESCRIPTOR TABLE *
**************************/

        .align  4
gdtptr:
        /* we use the NULL descriptor to store the GDT pointer - a trick quite
           nifty due to: Robert Collins (rcollins@x86.org) */
        .word   gdtlen - 1
        .long   gdtptr
        .word   0x0000

        /* code segment */
        .word   0xffff, 0
        .byte   0, 0x9f, 0xcf, 0

        /* data segment */
        .word   0xffff, 0
        .byte   0, 0x93, 0xcf, 0

        .set    gdtlen, . - gdtptr      # length of GDT
        
/*************************************
* INTERRUPT DESCRIPTOR TABLE POINTER *
*************************************/

        .align  4
idtptr:
        .word   0x07ff  # limit at maximum (allows all 256 interrupts)
        .word   0, 0    # base at 0