215 lines
14 KiB
HTML
215 lines
14 KiB
HTML
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<TITLE>Int 13/AH=02h
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<META NAME="Description" CONTENT="
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Int 13/AH=02h - DISK - READ SECTOR(S) INTO MEMORY -
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AH = 02h
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AL = number of sectors to read (must be nonzero)
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CH = low eight bits of cylinder number
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CL = sector number 1-63 (bits 0-5)
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high two bits of cylinder (bits 6-7, hard disk only)
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DH = head number
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DL = drive number (bit 7 set for hard disk)
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ES:BX - data buffer
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Return: CF set on error
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if AH = 11h (corrected ECC error), AL = burst length
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<h2>Int 13/AH=02h
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<h3><font color=#C00040>DISK - READ SECTOR(S) INTO MEMORY</font></h3>
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<PRE>
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AH = 02h
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AL = number of sectors to read (must be nonzero)
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CH = low eight bits of cylinder number
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CL = sector number 1-63 (bits 0-5)
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high two bits of cylinder (bits 6-7, hard disk only)
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DH = head number
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DL = drive number (bit 7 set for hard disk)
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ES:BX -> data buffer<br>
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<p><b>Return:</b><br>CF set on error
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if AH = 11h (corrected ECC error), AL = burst length
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CF clear if successful
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AH = status <a href="rb-0606.htm#Table234" tppabs="http://www.ctyme.com/intr/rb-0606.htm#Table234">(see #00234)</a>
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AL = number of sectors transferred (only valid if CF set for some
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BIOSes)
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</PRE>
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<p><b>Notes:</b>
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Errors on a floppy may be due to the motor failing to spin up quickly
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enough; the read should be retried at least three times, resetting
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the disk with AH=00h between attempts.
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Most BIOSes support "multitrack" reads, where the value in AL
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exceeds the number of sectors remaining on the track, in which
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case any additional sectors are read beginning at sector 1 on
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the following head in the same cylinder; the MSDOS CONFIG.SYS command
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MULTITRACK (or the Novell DOS DEBLOCK=) can be used to force DOS to
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split disk accesses which would wrap across a track boundary into two
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separate calls.
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The IBM AT BIOS and many other BIOSes use only the low four bits of
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DH (head number) since the WD-1003 controller which is the standard
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AT controller (and the controller that IDE emulates) only supports
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16 heads.
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AWARD AT BIOS and AMI 386sx BIOS have been extended to handle more
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than 1024 cylinders by placing bits 10 and 11 of the cylinder number
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into bits 6 and 7 of DH.
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Under Windows95, a volume must be locked (see INT 21/AX=440Dh/CX=084Bh)
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in order to perform direct accesses such as INT 13h reads and writes.
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All versions of MS-DOS (including MS-DOS 7 [Windows 95]) have a bug
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which prevents booting on hard disks with 256 heads (FFh), so many
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modern BIOSes provide mappings with at most 255 (FEh) heads.
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Some cache drivers flush their buffers when detecting that DOS is
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bypassed by directly issuing INT 13h from applications. A dummy
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read can be used as one of several methods to force cache
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flushing for unknown caches (e.g. before rebooting).
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<p><b>BUGS:</b>
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When reading from floppies, some AMI BIOSes (around 1990-1991) trash
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the byte following the data buffer, if it is not arranged to an even
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memory boundary. A workaround is to either make the buffer word
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aligned (which may also help to speed up things), or to add a dummy
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byte after the buffer..
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MS-DOS may leave interrupts disabled on return from this function..
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Apparently some BIOSes or intercepting resident software have bugs
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that may destroy DX on return or not properly set the Carry flag.
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At least some Microsoft software frames calls to this function with
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PUSH DX, STC, INT 13h, STI, POP DX..
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On the original IBM AT BIOS (1984/01/10) this function does not disable
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interrupts for harddisks (DL >= 80h). On these machines the MS-DOS/
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PC DOS IO.SYS/IBMBIO.COM installs a special filter to bypass the
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buggy code in the ROM (see CALL F000h:211Eh)
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<p><b>See Also:</b>
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<a href="rb-0608.htm" tppabs="http://www.ctyme.com/intr/rb-0608.htm">AH=03h</a> - <a href="rb-0625.htm" tppabs="http://www.ctyme.com/intr/rb-0625.htm">AH=0Ah</a> - <a href="rb-0617.htm" tppabs="http://www.ctyme.com/intr/rb-0617.htm">AH=06h"V10DISK.SYS"</a> - <a href="rb-0669.htm" tppabs="http://www.ctyme.com/intr/rb-0669.htm">AH=21h"PS/1"</a> - <a href="javascript:if(confirm('http://www.ctyme.com/ \n\nThis file was not retrieved by Teleport Pro, because it is addressed on a domain or path outside the boundaries set for its Starting Address. \n\nDo you want to open it from the server?'))window.location='http://www.ctyme.com/'" tppabs="http://www.ctyme.com/"
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<p><b>See Also:</b>
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<a href="rb-2898.htm" tppabs="http://www.ctyme.com/intr/rb-2898.htm">INT 21/AX=440Dh/CX=084Bh</a> - <a href="rb-6282.htm" tppabs="http://www.ctyme.com/intr/rb-6282.htm">INT 4D/AH=02h</a>
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<p>
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<b>Category: <a href="cat-003.htm" tppabs="http://www.ctyme.com/intr/cat-003.htm">
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Bios</a>
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- <a href="int-13.htm" tppabs="http://www.ctyme.com/intr/int-13.htm">
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Int 13h</a>
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D</a>
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