Changeset 419 in xtideuniversalbios for trunk/XTIDE_Universal_BIOS/Src/VariablesAndDPTs/LbaAssist.asm

Timestamp:

May 6, 2012, 2:34:35 PM (13 years ago)

Author:

aitotat@…

google:author:

aitotat@gmail.com

Message:

Changes to XTIDE Universal BIOS:

• IDE transfers now properly return number of sectors transferred when no errors.
• Tweaked L-CHS parameters for LBA drives.

File:

• trunk/XTIDE_Universal_BIOS/Src/VariablesAndDPTs/LbaAssist.asm (modified) (3 diffs)

trunk/XTIDE_Universal_BIOS/Src/VariablesAndDPTs/LbaAssist.asm

@@ -29 +29 @@
 ; 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.
+; 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.
 ;
-;
-; 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
+; 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.
+; the method here provides compatibility with phoenix BIOS.
 ;
 ; LbaAssist_ConvertSectorCountFromBXDXAXtoLbaAssistedCHSinDXAXBLBH:
@@ -67 +53 @@
     push    si
 
-    ; Value A = Total sector count / 63
+    ; Value CH = Total sector count / 63
     xor     cx, cx
-    push    cx      ; Push zero for bits 48...63
+    push    cx                          ; Push zero for bits 48...63
     push    bx
     push    dx
-    push    ax                      ; 64-bit sector count now in stack
+    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
+    mov     bp, sp                      ; SS:BP now points sector count
+    call    Math_DivQWatSSBPbyCX        ; Temporary value A now in stack
 
-    ; BX = Number of heads =  A / 1024
+    ; 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 256 (round up)
-    mov     bx, 256                     ; Max number of heads
+    ; Heads must be 16, 32, 64, 128 or 255 (round up to the nearest)
     test    dx, dx                      ; 65536 or more heads?
-    jnz     SHORT .GetNumberOfCylinders
-    mov     cx, 128                     ; Half BX for rounding up
-.FindMostSignificantBitForHeadSize:
+    jnz     SHORT .LimitHeadsTo255
+    mov     cx, 16                      ; Min number of heads
+.CompareNextValidNumberOfHeads:
     cmp     ax, cx
-    jae     SHORT .GetNumberOfCylinders
-    shr     cx, 1
-    shr     bx, 1                       ; Halve number of heads
-    jmp     SHORT .FindMostSignificantBitForHeadSize
+    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 = A / number of heads
-.GetNumberOfCylinders:
-    mov     cx, bx
+    ; DX:AX = Number of cylinders = Value CH (without - 1) / number of heads
     call    Math_DivQWatSSBPbyCX
     mov     ax, [bp]
@@ -104 +99 @@
     ; 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