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Basic I/O System (BIOS) Functions
5-12
SELECT DISK DRIVE
ENTRY
CL: Disk Drive
OOH-OFFH
FUNCTION 9
SELDSK
RETURN
BX:DPH
Address
This function selects the disk drive given by register CL for further operations.
Register CL can contain a value ranging from 0 for Drive A to 15 for Drive P.
Recall, however, that the APC is configured to support diskette drives A-D (OH-
3H) and hard disk drives E-H (4H-7H) only. SELDSK returns the base address of
the selected drive's Disk Parameter Header (DPH) in BX.
On entry to SELDSK it is possible to determine whether it is the first time the
specified disk has been selected. Register DL, bit 0 (least significant bit) is zero if the
drive has not been previously selected. This information can be used to set up a
dynamic disk definition table.
This routine does not change the contents of the header and associated tables. For
hard disk the routine checks SYSUNITID for the hard disk unit number. For both
diskette and hard disk, if the call attempts to select a nonexistent drive, SELDSK
returns OOH in BX as an error indicator.
Although this operation must return the header address on each call, it is advisable
to postpone the actual physical disk select operation until an I/O function (seek,
read, or write) is performed. This is due to the fact that disk select operations may
take place without a subsequent disk operation and thus disk access may be
substantially slower.
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