source: xtideuniversalbios/trunk/XTIDE_Universal_BIOS/Src/Boot/BootSector.asm@ 390

; Project name  :   XTIDE Universal BIOS
; Description   :   Reading and jumping to boot sector.

;
; XTIDE Universal BIOS and Associated Tools 
; Copyright (C) 2009-2010 by Tomi Tilli, 2011-2012 by XTIDE Universal BIOS Team.
;
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation; either version 2 of the License, or
; (at your option) any later version.
; 
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
; GNU General Public License for more details.
; Visit http://www.gnu.org/licenses/old-licenses/gpl-2.0.html               
;       

; Section containing code
SECTION .text

;--------------------------------------------------------------------
; BootSector_TryToLoadFromDriveDL
;   Parameters:
;       DL:     Drive to boot from (translated, 00h or 80h)
;       DS:     RAMVARS segment
;   Returns:
;       ES:BX:  Ptr to boot sector (if successful)
;       CF:     Set if boot sector loaded successfully
;               Cleared if failed to load boot sector
;   Corrupts registers:
;       AX, CX, DH, SI, DI, (DL if failed to read boot sector)
;--------------------------------------------------------------------
BootSector_TryToLoadFromDriveDL:
    call    DetectPrint_TryToBootFromDL
    call    LoadFirstSectorFromDriveDL
    jc      SHORT .FailedToLoadFirstSector

    test    dl, dl
    jns     SHORT .AlwaysBootFromFloppyDriveForBooterGames
    cmp     WORD [es:bx+510], 0AA55h        ; Valid boot sector?
    jne     SHORT .FirstHardDiskSectorNotBootable
.AlwaysBootFromFloppyDriveForBooterGames:
    stc
    ret
.FailedToLoadFirstSector:
    call    DetectPrint_FailedToLoadFirstSector
    clc
    ret
.FirstHardDiskSectorNotBootable:
    mov     si, g_szBootSectorNotFound
    call    DetectPrint_NullTerminatedStringFromCSSIandSetCF
    clc
    ret

;--------------------------------------------------------------------
; LoadFirstSectorFromDriveDL
;   Parameters:
;       DL:     Drive to boot from (translated, 00h or 80h)
;   Returns:
;       AH:     INT 13h error code
;       ES:BX:  Ptr to boot sector (if successful)
;       CF:     Cleared if read successful
;               Set if any error
;   Corrupts registers:
;       AL, CX, DH, DI
;--------------------------------------------------------------------
LoadFirstSectorFromDriveDL:
    LOAD_BDA_SEGMENT_TO es, bx              ; ES:BX now points to...
    mov     bx, BOOTVARS.rgbBootSect        ; ...boot sector location
    mov     di, BOOT_READ_RETRY_TIMES       ; Initialize retry counter

.ReadRetryLoop:
    call    .ResetBootDriveFromDL
    call    .LoadFirstSectorFromDLtoESBX
    jnc     SHORT .Return
    dec     di                              ; Decrement retry counter
    jnz     SHORT .ReadRetryLoop            ; Loop while retries left
.Return:
    ret

;--------------------------------------------------------------------
; .ResetBootDriveFromDL
;   Parameters:
;       DL:     Drive to boot from (translated, 00h or 80h)
;   Returns:
;       AH:     INT 13h error code
;       CF:     Cleared if read successful
;               Set if any error
;   Corrupts registers:
;       AL
;--------------------------------------------------------------------
.ResetBootDriveFromDL:
    xor     ax, ax                          ; AH=0h, Disk Controller Reset
    test    dl, dl                          ; Floppy drive?
    jns     SHORT .SkipAltReset
    mov     ah, 0Dh                         ; AH=Dh, Reset Hard Disk (Alternate reset)
.SkipAltReset:
    int     BIOS_DISK_INTERRUPT_13h
    ret

;--------------------------------------------------------------------
; .LoadFirstSectorFromDLtoESBX
;   Parameters:
;       DL:     Drive to boot from (translated, 00h or 80h)
;       ES:BX:  Destination buffer for boot sector
;   Returns:
;       AH:     INT 13h error code
;       ES:BX:  Ptr to boot sector
;       CF:     Cleared if read successful
;               Set if any error
;   Corrupts registers:
;       AL, CX, DH
;--------------------------------------------------------------------
.LoadFirstSectorFromDLtoESBX:
    mov     ax, 0201h                       ; Read 1 sector
    mov     cx, 1                           ; Cylinder 0, Sector 1
    xor     dh, dh                          ; Head 0
    int     BIOS_DISK_INTERRUPT_13h
    ret