source: xtideuniversalbios/trunk/XTIDE_Universal_BIOS/Src/VariablesAndDPTs/LbaAssist.asm@ 370

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

; 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.
;
; total LBAs (as obtained by words 60+61)
; divided by 63 (sectors per track) (save as value A)
; Sub 1 from A
; divide A by 1024 + truncate.
; == total number of heads to use.
; add 1
; this value must be either 16, 32, 64, 128, or 256 (round up)
; then take the value A above and divide by # of heads
; to get the # of cylinders to use.
;
;
; so a LBA28 drive will have 268,435,456 as maximum LBAs
;
; 10000000h / 63   = 410410h (total cylinders or tracks)
;   410410h / 1024 = 1041h, which is way more than 256 heads, but 256 is max.
;   410410h / 256  = 4104h 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
;
; we're using the values from 60+61 here because we're topping out at 8.4G
; anyway, so there's no need to use the 48bit LBA values.
;
; 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 A = 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 = Number of heads =  A / 1024
    mov     ax, [bp]
    mov     dx, [bp+2]
    mov     bx, [bp+4]
    call    Size_DivideSizeInBXDXAXby1024andIncrementMagnitudeInCX

    ; Heads must be 16, 32, 64, 128 or 256 (round up)
    mov     bx, 256                     ; Max number of heads
    test    dx, dx                      ; 65536 or more heads?
    jnz     SHORT .GetNumberOfCylinders
    mov     cx, 128                     ; Half BX for rounding up
.FindMostSignificantBitForHeadSize:
    cmp     ax, cx
    jae     SHORT .GetNumberOfCylinders
    shr     cx, 1
    shr     bx, 1                       ; Halve number of heads
    jmp     SHORT .FindMostSignificantBitForHeadSize

    ; DX:AX = Number of cylinders = A / number of heads
.GetNumberOfCylinders:
    mov     cx, bx
    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
    sub     bl, bh                      ; Limit heads to 255
    mov     bh, LBA_ASSIST_SPT
    pop     si
    pop     bp
    ret