source: xtideuniversalbios/trunk/XTIDE_Universal_BIOS/Src/VariablesAndDPTs/LbaAssist.asm@ 420

; Project name  :   XTIDE Universal BIOS
; Description   :   Functions for generating L-CHS parameters for
;                   LBA drives.
;
;                   This file is shared with BIOS Drive Information Tool.

;
; 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

;--------------------------------------------------------------------
; LBA assist calculation:
; this is how to fit a big drive into INT13's skimpy size requirements,
; with a maximum of 8.4G available.
;
; If cylinders > 8192
;  Variable CH = Total Sectors / 63
;  Divide (CH – 1) by 1024 (as an assembler bitwise right shift) and add 1
;  Round the result up to the nearest of 16, 32, 64, 128 and 255. This is the value to be used for the number of heads.
;  Divide CH by the number of heads. This is the value to be used for the number of cylinders.
;
; There's a wealth of information at: http://www.mossywell.com/boot-sequence
; they show a slightly different approach to LBA assist calulations, but
; the method here provides compatibility with phoenix BIOS.
;
; LbaAssist_ConvertSectorCountFromBXDXAXtoLbaAssistedCHSinDXAXBLBH:
;   Parameters:
;       BX:DX:AX:   Total number of sectors
;   Returns:
;       DX:AX:  Number of cylinders
;       BH:     Number of sectors per track (always 63)
;       BL:     Number of heads (16, 32, 64, 128 or 255)
;   Corrupts registers:
;       CX
;--------------------------------------------------------------------
LbaAssist_ConvertSectorCountFromBXDXAXtoLbaAssistedCHSinDXAXBLBH:
    push    bp
    push    si

    ; Value CH = Total sector count / 63
    xor     cx, cx
    push    cx                          ; Push zero for bits 48...63
    push    bx
    push    dx
    push    ax                          ; 64-bit sector count now in stack
    mov     cl, LBA_ASSIST_SPT
    mov     bp, sp                      ; SS:BP now points sector count
    call    Math_DivQWatSSBPbyCX        ; Temporary value A now in stack

    ; BX:DX:AX = Value CH - 1
    mov     ax, [bp]
    mov     dx, [bp+2]
    mov     bx, [bp+4]
    sub     ax, BYTE 1                  ; Subtract 1
    sbb     dx, BYTE 0
    sbb     bx, BYTE 0

    ; DX:AX = Number of heads = ((Value CH - 1) / 1024) + 1
    call    Size_DivideSizeInBXDXAXby1024andIncrementMagnitudeInCX
    add     ax, BYTE 1                  ; Add 1
    adc     dx, bx                      ; BX = 0

    ; Heads must be 16, 32, 64, 128 or 255 (round up to the nearest)
    test    dx, dx                      ; 65536 or more heads?
    jnz     SHORT .LimitHeadsTo255
    mov     cx, 16                      ; Min number of heads
.CompareNextValidNumberOfHeads:
    cmp     ax, cx
    jbe     SHORT .NumberOfHeadsNowInCX
    shl     cx, 1                       ; Double number of heads
    test    ch, ch                      ; Reached 256 heads?
    jnz     SHORT .CompareNextValidNumberOfHeads
.LimitHeadsTo255:
    mov     cx, 255
.NumberOfHeadsNowInCX:
    mov     bx, cx                      ; Number of heads are returned in BL
    mov     bh, LBA_ASSIST_SPT          ; Sectors per Track

    ; DX:AX = Number of cylinders = Value CH (without - 1) / number of heads
    call    Math_DivQWatSSBPbyCX
    mov     ax, [bp]
    mov     dx, [bp+2]                  ; Cylinders now in DX:AX

    ; Return LBA assisted CHS
    add     sp, BYTE 8                  ; Clean stack
    pop     si
    pop     bp
    ret