Date   : Thu, 10 May 1984 13:16:01-PDT (Thu)
From   : hplabs!sdcrdcf!sdcsvax!sdccsu3!loral!simard@Ucb-Vax.ARPA
Subject: Re: BIOS Techniques, a question.

	The disk parameter block (DPB) whose address is returned ih
	the HL pair when SELDSK is called contains a pointer to the
	disk parameter header which defines the characteristics
	of the current drive which  are to apply to all further calls to
	SECTRAN, SETSEC (indirectly), READ and WRITE,
	until SELDSK is called again.  As long as the
	DPH/DPB data agree with the disk currently accessed, CP/M will
	be happy.  The only error indications that the BIOS returns
	to the BDOS (main body of CPM) is success (A=0) or failure
	(A<>0) from READ or WRITE, or that SELDSK was asked to select
	an invalid logical drive (HL returned = 0).  If you can determine
	what kind of disk is installed in a physical drive dynamically,
	the check should be placed in SELDSK.  The pointer in the
	DPB to the appropriate DPH would then be set by SELDSK
	to reflect the outcome of the
	check as well as set up the driver for the disk controller
	as appropriate.  If that is not possible,
	then you might want to implement multiple
	logical drives for each physical drive (for example, if your system
	has two physical drives, A: and B: could be SS/SD, C: and D: could
	be the same drives, but SS/DD, etc.).  The operator would select
	the drive according to the type of disk installed. 

	Note: I am referring to DPB's and DPH's from memory.  I
	may have them reversed.  What I refer to as DPB is the
	16-byte block unique to each logical drive, containing
	pointers to the applicable DPH, sector translation table,
	allocation and check vectors, and whose address is returned by
	SELDSK.  The DPH in the above is the
	data set defining the size, reserved tracks, block size,
	etc., and which may be shared by numerous logical drives.
	
	Ray Simard
	Loral Instrumentation, San Diego, CA.