source: xtideuniversalbios/trunk/XTIDE_Universal_BIOS/Src/Handlers/Int19h.asm @ 282

; Project name  :   XTIDE Universal BIOS
; Description   :   Int 19h Handler (Boot Loader).

; Section containing code
SECTION .text

;--------------------------------------------------------------------
; INT 19h handler that properly reboots the computer when
; INT 19h is called.
;
; Int19h_ResetHandler
;   Parameters:
;       Nothing
;   Returns:
;       Never returns (reboots computer)
;--------------------------------------------------------------------
Int19h_ResetHandler:
    mov     ax, BOOT_FLAG_WARM              ; Skip memory tests
    jmp     Reboot_ComputerWithBootFlagInAX


;--------------------------------------------------------------------
; Int19h_BootLoaderHandler
;   Parameters:
;       Nothing
;   Returns:
;       Never returns (loads operating system)
;--------------------------------------------------------------------
ALIGN JUMP_ALIGN
Int19h_BootLoaderHandler:
    sti
    ; Install INT 19h handler for proper reboot
    LOAD_BDA_SEGMENT_TO es, ax
    mov     al, BIOS_BOOT_LOADER_INTERRUPT_19h  ; INT 19h interrupt vector offset
    mov     si, Int19h_ResetHandler             ; INT 19h handler to reboot the system
    call    Interrupts_InstallHandlerToVectorInALFromCSSI
    call    Initialize_AndDetectDrives          ; Installs new boot menu loader
    ; Fall to .PrepareStackAndSelectDriveFromBootMenu

;--------------------------------------------------------------------
; .PrepareStackAndSelectDriveFromBootMenu
;   Parameters:
;       ES:     BDA and interrupt vector segment (zero)
;   Returns:
;       Never returns (loads operating system)
;--------------------------------------------------------------------
.PrepareStackAndSelectDriveFromBootMenu:
    STORE_POST_STACK_POINTER
    SWITCH_TO_BOOT_MENU_STACK
    ; Fall to .InitializeDisplay

;--------------------------------------------------------------------
; .InitializeDisplay
;   Parameters:
;       Nothing
;   Returns:
;       Never returns (loads operating system)
;--------------------------------------------------------------------
.InitializeDisplay:
    ; Change display mode if necessary
    mov     ax, [cs:ROMVARS.wDisplayMode]   ; AH 00h = Set Video Mode
    cmp     al, DEFAULT_TEXT_MODE
    je      SHORT .InitializeDisplayLibrary
    int     BIOS_VIDEO_INTERRUPT_10h
.InitializeDisplayLibrary:
    call    BootMenuPrint_InitializeDisplayContext
    ; Fall to .SelectDriveToBootFrom

;--------------------------------------------------------------------
; .SelectDriveToBootFrom
;   Parameters:
;       Nothing
;   Returns:
;       Never returns (loads operating system)
;--------------------------------------------------------------------
.SelectDriveToBootFrom:
    call    RamVars_GetSegmentToDS
    cmp     WORD [cs:ROMVARS.wfDisplayBootMenu], BYTE 0
    jne     SHORT ProcessBootMenuSelectionsUntilBootableDriveSelected   ; Display boot menu
    ; Fall to BootFromDriveAthenTryDriveC

;--------------------------------------------------------------------
; BootFromDriveAthenTryDriveC
;   Parameters:
;       DS:     RAMVARS segment
;   Returns:
;       Never returns (loads operating system)
;--------------------------------------------------------------------
BootFromDriveAthenTryDriveC:
    xor     dx, dx              ; DL = 00h = Floppy Drive A
    call    BootSector_TryToLoadFromDriveDL
    jc      SHORT Int19hMenu_JumpToBootSector_or_RomBoot
    mov     dl, 80h             ; DL = 80h = First Hard Drive (usually C)
    call    BootSector_TryToLoadFromDriveDL
    jmp     SHORT Int19hMenu_JumpToBootSector_or_RomBoot    ; ROM Boot if error


;--------------------------------------------------------------------
; ProcessBootMenuSelectionsUntilBootableDriveSelected
;   Parameters:
;       DS:     RAMVARS segment
;   Returns:
;       Never returns
;--------------------------------------------------------------------
ProcessBootMenuSelectionsUntilBootableDriveSelected:
    call    BootMenu_DisplayAndReturnSelectionInDX
    call    DriveXlate_ToOrBack                                         ; Translate drive number
    call    BootSector_TryToLoadFromDriveDL
    jnc     SHORT ProcessBootMenuSelectionsUntilBootableDriveSelected   ; Boot failure, show menu again
    ; Fall to Int19hMenu_JumpToBootSector_or_RomBoot
    ; (CF is set or we wouldn't be here, see "jnc" immediately above)

;--------------------------------------------------------------------
; Int19hMenu_JumpToBootSector_or_RomBoot
;
; Switches back to the POST stack, clears the DS and ES registers,
; and either jumps to the MBR (Master Boot Record) that was just read,
; or calls the ROM's boot routine on interrupt 18.
;
;   Parameters:
;       DL:     Drive to boot from (translated, 00h or 80h)
;       CF:     Set for Boot Sector Boot 
;               Clear for Rom Boot
;       ES:BX:  (if CF set) Ptr to boot sector
;
;   Returns:
;       Never returns
;--------------------------------------------------------------------
ALIGN JUMP_ALIGN
Int19hMenu_JumpToBootSector_or_RomBoot:
    mov     cx, es      ; Preserve MBR segment (can't push because of stack change)
    mov     ax, 0       ; NOTE: can't use XOR (LOAD_BDA_SEGMENT_TO) as it impacts CF
    SWITCH_BACK_TO_POST_STACK

; clear segment registers before boot sector or rom call
    mov     ds, ax      
    mov     es, ax
%ifdef USE_386
    mov     fs, ax
    mov     gs, ax
%endif
    jnc     SHORT .romboot

; jump to boot sector
    push    cx          ; sgment address for MBR
    push    bx          ; offset address for MBR
    retf                ; NOTE: DL is set to the drive number

; Boot by calling INT 18h (ROM Basic of ROM DOS)
.romboot:
    int     BIOS_BOOT_FAILURE_INTERRUPT_18h ; Never returns