IBM 770 manual Bits 5, System Board, 26. Hard Disk Drive Type Byte, Equipment Byte

Hard	Disk Drive Type	Byte		(Hex 012): This			byte defines	the	type
of	hard disk	drive		installed. Hex			00 indicates	that	no hard disk drive
is	installed.
	Bit	Drive	Type
	7–4	Hard	disk		drive	0
	3–0	Hard	disk		drive	1

Figure 2-26. Hard Disk Drive Type Byte

Reserved	Bytes		(Hex	013): These bytes are reserved.
Equipment	Byte		(Hex	014):		This	byte defines the basic equipment
in the		system		for the power-on diagnostic tests.
	Bit			Description
	7,	6		Number	of	diskette drives
	5,	4		Display	operating mode
	3,	2		Reserved
	1			Coprocessor presence
	0			Diskette		drive	0 presence
Figure		2-27. Equipment Byte

Bits 7, 6 These bits indicate the number of installed diskette drives.

Bits 7,6		Number	of Diskette Drives
0	0	One	drive
0	1	Reserved
1	0	Reserved
1	1	Reserved

Figure 2-28. Installed Diskette Drive Bits

Bits 5, 4		These bits indicate the operating mode of the display
		attached		to	the video port.
Bits		5,4	Display	Operating Mode
0	0		Reserved
0	1		40-column		mode
1		0	80-column		mode
1		1	Monochrome mode

Figure 2-29. Display Operating Mode Bits

System Board 2-27