| 1 | ; Project name : XTIDE Universal BIOS
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| 2 | ; Description : Functions for generating L-CHS parameters for
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| 3 | ; LBA drives.
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| 4 | ;
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| 5 | ; This file is shared with BIOS Drive Information Tool.
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| 6 |
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| 7 | ;
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| 8 | ; XTIDE Universal BIOS and Associated Tools
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| 9 | ; Copyright (C) 2009-2010 by Tomi Tilli, 2011-2012 by XTIDE Universal BIOS Team.
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| 10 | ;
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| 11 | ; This program is free software; you can redistribute it and/or modify
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| 12 | ; it under the terms of the GNU General Public License as published by
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| 13 | ; the Free Software Foundation; either version 2 of the License, or
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| 14 | ; (at your option) any later version.
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| 15 | ;
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| 16 | ; This program is distributed in the hope that it will be useful,
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| 17 | ; but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 18 | ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 19 | ; GNU General Public License for more details.
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| 20 | ; Visit http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
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| 21 | ;
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| 22 |
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| 23 | ; Section containing code
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| 24 | SECTION .text
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| 25 |
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| 26 | ;--------------------------------------------------------------------
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| 27 | ; LBA assist calculation:
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| 28 | ; this is how to fit a big drive into INT13's skimpy size requirements,
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| 29 | ; with a maximum of 8.4G available.
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| 30 | ;
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| 31 | ; total LBAs (as obtained by words 60+61)
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| 32 | ; divided by 63 (sectors per track) (save as value A)
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| 33 | ; Sub 1 from A
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| 34 | ; divide A by 1024 + truncate.
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| 35 | ; == total number of heads to use.
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| 36 | ; add 1
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| 37 | ; this value must be either 16, 32, 64, 128, or 256 (round up)
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| 38 | ; then take the value A above and divide by # of heads
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| 39 | ; to get the # of cylinders to use.
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| 40 | ;
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| 41 | ;
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| 42 | ; so a LBA28 drive will have 268,435,456 as maximum LBAs
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| 43 | ;
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| 44 | ; 10000000h / 63 = 410410h (total cylinders or tracks)
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| 45 | ; 410410h / 1024 = 1041h, which is way more than 256 heads, but 256 is max.
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| 46 | ; 410410h / 256 = 4104h cylinders
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| 47 | ;
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| 48 | ; there's a wealth of information at: http://www.mossywell.com/boot-sequence
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| 49 | ; they show a slightly different approach to LBA assist calulations, but
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| 50 | ; the method here provides compatibility with phoenix BIOS
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| 51 | ;
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| 52 | ; we're using the values from 60+61 here because we're topping out at 8.4G
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| 53 | ; anyway, so there's no need to use the 48bit LBA values.
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| 54 | ;
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| 55 | ; LbaAssist_ConvertSectorCountFromBXDXAXtoLbaAssistedCHSinDXAXBLBH:
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| 56 | ; Parameters:
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| 57 | ; BX:DX:AX: Total number of sectors
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| 58 | ; Returns:
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| 59 | ; DX:AX: Number of cylinders
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| 60 | ; BH: Number of sectors per track (always 63)
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| 61 | ; BL: Number of heads (16, 32, 64, 128 or 255)
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| 62 | ; Corrupts registers:
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| 63 | ; CX
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| 64 | ;--------------------------------------------------------------------
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| 65 | LbaAssist_ConvertSectorCountFromBXDXAXtoLbaAssistedCHSinDXAXBLBH:
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| 66 | push bp
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| 67 | push si
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| 68 |
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| 69 | ; Value A = Total sector count / 63
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| 70 | xor cx, cx
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| 71 | push cx ; Push zero for bits 48...63
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| 72 | push bx
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| 73 | push dx
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| 74 | push ax ; 64-bit sector count now in stack
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| 75 | mov cl, LBA_ASSIST_SPT
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| 76 | mov bp, sp ; SS:BP now points sector count
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| 77 | call Math_DivQWatSSBPbyCX ; Temporary value A now in stack
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| 78 |
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| 79 | ; BX = Number of heads = A / 1024
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| 80 | mov ax, [bp]
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| 81 | mov dx, [bp+2]
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| 82 | mov bx, [bp+4]
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| 83 | call Size_DivideSizeInBXDXAXby1024andIncrementMagnitudeInCX
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| 84 |
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| 85 | ; Heads must be 16, 32, 64, 128 or 256 (round up)
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| 86 | mov bx, 256 ; Max number of heads
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| 87 | test dx, dx ; 65536 or more heads?
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| 88 | jnz SHORT .GetNumberOfCylinders
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| 89 | mov cx, 128 ; Half BX for rounding up
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| 90 | .FindMostSignificantBitForHeadSize:
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| 91 | cmp ax, cx
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| 92 | jae SHORT .GetNumberOfCylinders
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| 93 | shr cx, 1
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| 94 | shr bx, 1 ; Halve number of heads
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| 95 | jmp SHORT .FindMostSignificantBitForHeadSize
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| 96 |
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| 97 | ; DX:AX = Number of cylinders = A / number of heads
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| 98 | .GetNumberOfCylinders:
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| 99 | mov cx, bx
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| 100 | call Math_DivQWatSSBPbyCX
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| 101 | mov ax, [bp]
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| 102 | mov dx, [bp+2] ; Cylinders now in DX:AX
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| 103 |
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| 104 | ; Return LBA assisted CHS
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| 105 | add sp, BYTE 8 ; Clean stack
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| 106 | sub bl, bh ; Limit heads to 255
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| 107 | mov bh, LBA_ASSIST_SPT
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| 108 | pop si
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| 109 | pop bp
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| 110 | ret
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