			FORMAT.DOC
			 11/04/82

The purpose of this file is to document the structure of the
format byte used by Lifeboat, North Star and perhaps others.
The CP/M bios implemented by Lifeboat and North Star contains
several Disk Parameter Blocks that describe the format of the
disk per the CP/M Alteration Guide.  The bios contains a logical
decoder that determines which of the several supported formats
a particular diskette is written in.  It determines this by 
reading the 5CH byte on track 0, sector 0 which is the storage
location for the format byte.  After a determination of which
Disk Parameter Block is applicable to the diskette, the address
of the Block is written into the Disk Parameter Header ( also
described in the CP/M Alteration Guide) for use by the BDOS in
reading and writing of disks.

Here is the structure of the format byte as in these implementations:

	bit	80H	;0 = single density, 1 = double density
	bit	40H	;0 = single sided,   1 = double sided
	bit	20H	;0 = format 1,	     1 = format 2
	bit	10H	;always set to 1
	bit	8H	;not assigned
	bit	4H	;not assigned
	bit	2H	;0 = 48 track/in.,   1 = 96 track/in.
	bit	1H	;always set to 0 (see below)

In the North Star implementation of the bios the 1H bit is used in
a special way as follows:
	If a 48 track/in diskette is mounted in a 96 track/in drive,
the copy of the format byte that is stored at the end of the bios
to indicate the format of the diskette mounted in a particular drive
has its 1H bit set to 1. This allows the bios to use special code
to be able to read 48 tpi data on a 96 tpi drive. Head is double
stepped for each track.  Also, side flipping is done at the right
track based on this info.  To accomplish this, the bios uses an
NDRV byte to indicate which drives are 96 tpi in addition to the
CONFG byte which is structured the same as in the North Star dos.

The 20H bit provides for two alternate formats for each combination
of the other bits.

Thus, the format bytes used in either or both the Lifeboat and North
Star Bios's are as follows:

	10H	single density, single sided, 48 tpi

	90H	double density,	single sided, 48 tpi (format 1)
	B0H	double density,	single sided, 48 tpi (format 2)

	F0H	double density, double sided, 48 tpi (format 2) QUAD
	F2H	double density, double sided, 96 tpi (format 2) OCTAL

In addition to these, there are two others possible:

	D0H	double density, double sided, 48 tpi (format 1) QUAD
	D2H	double density, double sided, 96 tpi (format 1) OCTAL

The actual format for format 1 and format 2 will depend on the par-
ameters contained in the Disk Parameter Block for each format byte
and the use of format 1 and format 2 here is not meant to imply that
they have the same meaning when applied to the different actual formats.

See the file NEWFORM (NEWFRM). ASM for a use of the last two format
bytes.
					RLP   