[358] | 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 | ; Section containing code |
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| 8 | SECTION .text |
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| 9 | |
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| 10 | ;-------------------------------------------------------------------- |
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| 11 | ; LBA assist calculation: |
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| 12 | ; this is how to fit a big drive into INT13's skimpy size requirements, |
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| 13 | ; with a maximum of 8.4G available. |
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| 14 | ; |
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| 15 | ; total LBAs (as obtained by words 60+61) |
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| 16 | ; divided by 63 (sectors per track) (save as value A) |
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| 17 | ; Sub 1 from A |
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| 18 | ; divide A by 1024 + truncate. |
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| 19 | ; == total number of heads to use. |
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| 20 | ; add 1 |
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| 21 | ; this value must be either 16, 32, 64, 128, or 256 (round up) |
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| 22 | ; then take the value A above and divide by # of heads |
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| 23 | ; to get the # of cylinders to use. |
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| 24 | ; |
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| 25 | ; |
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| 26 | ; so a LBA28 drive will have 268,435,456 as maximum LBAs |
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| 27 | ; |
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| 28 | ; 10000000h / 63 = 410410h (total cylinders or tracks) |
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| 29 | ; 410410h / 1024 = 1041h, which is way more than 256 heads, but 256 is max. |
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| 30 | ; 410410h / 256 = 4104h cylinders |
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| 31 | ; |
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| 32 | ; there's a wealth of information at: http://www.mossywell.com/boot-sequence |
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| 33 | ; they show a slightly different approach to LBA assist calulations, but |
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| 34 | ; the method here provides compatibility with phoenix BIOS |
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| 35 | ; |
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| 36 | ; we're using the values from 60+61 here because we're topping out at 8.4G |
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| 37 | ; anyway, so there's no need to use the 48bit LBA values. |
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| 38 | ; |
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| 39 | ; LbaAssist_ConvertSectorCountFromBXDXAXtoLbaAssistedCHSinDXAXBLBH: |
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| 40 | ; Parameters: |
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| 41 | ; BX:DX:AX: Total number of sectors |
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| 42 | ; Returns: |
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| 43 | ; DX:AX: Number of cylinders |
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| 44 | ; BH: Number of sectors per track (always 63) |
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| 45 | ; BL: Number of heads (16, 32, 64, 128 or 255) |
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| 46 | ; Corrupts registers: |
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| 47 | ; CX |
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| 48 | ;-------------------------------------------------------------------- |
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| 49 | LbaAssist_ConvertSectorCountFromBXDXAXtoLbaAssistedCHSinDXAXBLBH: |
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| 50 | push bp |
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| 51 | push si |
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| 52 | |
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| 53 | ; Value A = Total sector count / 63 |
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| 54 | xor cx, cx |
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| 55 | push cx ; Push zero for bits 48...63 |
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| 56 | push bx |
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| 57 | push dx |
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| 58 | push ax ; 64-bit sector count now in stack |
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| 59 | mov cl, LBA_ASSIST_SPT |
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| 60 | mov bp, sp ; SS:BP now points sector count |
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| 61 | call Math_DivQWatSSBPbyCX ; Temporary value A now in stack |
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| 62 | |
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| 63 | ; BX = Number of heads = A / 1024 |
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| 64 | mov ax, [bp] |
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| 65 | mov dx, [bp+2] |
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| 66 | mov bx, [bp+4] |
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| 67 | call Size_DivideSizeInBXDXAXby1024andIncrementMagnitudeInCX |
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| 68 | |
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| 69 | ; Heads must be 16, 32, 64, 128 or 256 (round up) |
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| 70 | mov bx, 256 ; Max number of heads |
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| 71 | test dx, dx ; 65536 or more heads? |
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| 72 | jnz SHORT .GetNumberOfCylinders |
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| 73 | mov cx, 128 ; Half BX for rounding up |
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| 74 | .FindMostSignificantBitForHeadSize: |
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| 75 | cmp ax, cx |
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| 76 | jae SHORT .GetNumberOfCylinders |
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| 77 | shr cx, 1 |
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| 78 | shr bx, 1 ; Halve number of heads |
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| 79 | jmp SHORT .FindMostSignificantBitForHeadSize |
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| 80 | |
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| 81 | ; DX:AX = Number of cylinders = A / number of heads |
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| 82 | .GetNumberOfCylinders: |
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| 83 | mov cx, bx |
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| 84 | call Math_DivQWatSSBPbyCX |
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| 85 | mov ax, [bp] |
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| 86 | mov dx, [bp+2] ; Cylinders now in DX:AX |
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| 87 | |
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| 88 | ; Return LBA assisted CHS |
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| 89 | add sp, BYTE 8 ; Clean stack |
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| 90 | sub bl, bh ; Limit heads to 255 |
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| 91 | mov bh, LBA_ASSIST_SPT |
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| 92 | pop si |
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| 93 | pop bp |
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| 94 | ret |
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