source: xtideuniversalbios/trunk/XTIDE_Universal_BIOS/Src/Handlers/Int19h/BootSector.asm

; 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, 2011-2024 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_AndBoot
;   Parameters:
;       DL:     Drive to boot from (translated, 00h or 80h)
;       DS:     RAMVARS segment
;   Returns:
;       ES:BX:  Ptr to boot sector (if successful)
;       CF:     Cleared if boot sector loaded successfully
;               (only matters when jumping to
;               Int19h_JumpToBootSectorInESBXOrRomBoot)
;   Corrupts registers:
;       AX, CX, DH, SI, DI
;--------------------------------------------------------------------
BootSector_TryToLoadFromDriveDL_AndBoot:
    call    DetectPrint_TryToBootFromDL
    call    BootSector_LoadFirstSectorFromDriveDL
    inc     dx                                              ; Determine if hard drive or floppy drive without changing the CF
    dec     dl
    jnc     SHORT .FirstSectorLoadedToESBX

    ; Do not display timeout error (80h) for floppy drives since
    ; it most likely mean no diskette in drive. This way we do not
    ; display error code every time user intends to boot from hard disk
    ; when A then C boot order is used.
    js      SHORT .PrintFailedToLoadErrorCode               ; Hard Drive
    cmp     ah, RET_HD_TIMEOUT
    je      SHORT BootSector_LoadFirstSectorFromDriveDL.Return
    cmp     ah, RET_HD_NOMEDIA
    je      SHORT BootSector_LoadFirstSectorFromDriveDL.Return
.PrintFailedToLoadErrorCode:
    jmp     DetectPrint_FailedToLoadFirstSector

.FirstSectorLoadedToESBX:
    jns     SHORT Int19h_JumpToBootSectorInESBXOrRomBoot    ; Don't check for boot sector signature for floppy booter games
    cmp     WORD [es:bx+510], 0AA55h                        ; Valid boot sector?
    je      SHORT Int19h_JumpToBootSectorInESBXOrRomBoot    ; With CF cleared
    mov     si, g_szBootSectorNotFound
    jmp     DetectPrint_NullTerminatedStringFromCSSI


;--------------------------------------------------------------------
; BootSector_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
;--------------------------------------------------------------------
BootSector_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:
    mov     ax, 0201h                       ; Read 1 sector
    mov     cx, 1                           ; Cylinder 0, Sector 1
    xor     dh, dh                          ; Head 0
    int     BIOS_DISK_INTERRUPT_13h
    jc      SHORT .FailedToLoadFirstSector
.Return:
    ret

.FailedToLoadFirstSector:
    dec     di                              ; Decrement retry counter (preserve CF)
    jz      SHORT .Return                   ; Loop while retries left

    ; Reset drive and retry
    xor     ax, ax                          ; AH=00h, Disk Controller Reset
    test    dl, dl                          ; Floppy drive?
    eCMOVS  ah, RESET_HARD_DISK             ; AH=0Dh, Reset Hard Disk (Alternate reset)
    int     BIOS_DISK_INTERRUPT_13h
    jmp     SHORT .ReadRetryLoop