2.7. Bochs's CMOS map

In addition to the default CMOS RAM layout, the Bochs BIOS uses some additional registers for harddisk parameters and the boot sequence. The following table shows all CMOS registers and their meaning.

S	- set by the emulator (Bochs)
Q	- set by the emulator (Qemu)
B	- set by the bios
U	- unused by the bios

0x00	S	rtc seconds
0x01	B	second alarm
0x02	S	rtc minutes
0x03	B	minute alarm
0x04	S	rtc hours
0x05	B	hour alarm

0x06	S,U	day of week
0x07	S,B	date of month
0x08	S,B	month
0x09	S,B	year

0x0a	S,B	status register A
0x0b	S,B	status register B
0x0c	S	status register C
0x0d	S	status register D

0x0f	S	shutdown status
	0x00: normal startup
	0x09: normal
	0x0d+: normal
	0x05: eoi ?
	else: unimpl

0x10	S	fd drive type (2 nibbles: high=fd0, low=fd1)
	1: 360K 5.25"
	2: 1.2MB 5.25"
	3: 720K 3.5"
	4: 1.44MB 3.5"
	5: 2.88MB 3.5"

!0x11	configuration bits!!

0x12	S	how many disks first (hd type)

!0x13	advanced configuration bits!!

0x14	S,U	equipment byte (?)
	bits	where		what
	7-6	floppy.cc
	5-4	vga.cc		0 = vga
	2	keyboard.cc	1 = enabled
	0	floppy.cc

0x15	S,U	base memory - low
0x16	S,U	base memory - high

0x17	S,U	extended memory in k - low
0x18	S,U	extended memory in k - high

0x19	S	hd0: extended type
0x1a	S	hd1: extended type

0x1b	S,U	hd0:cylinders - low
0x1c	S,U	hd0:cylinders - high
0x1d	S,U	hd0:heads
0x1e	S,U	hd0:write pre-comp - low
0x1f	S,U	hd0:write pre-comp - high
0x20	S,U	hd0:retries/bad_map/heads>8
0x21	S,U	hd0:landing zone - low
0x22	S,U	hd0:landing zone - high
0x23	S,U	hd0:sectors per track

0x24	S,U	hd1:cylinders - low
0x25	S,U	hd1:cylinders - high
0x26	S,U	hd1:heads
0x27	S,U	hd1:write pre-comp - low
0x28	S,U	hd1:write pre-comp - high
0x29	S,U	hd1:retries/bad_map/heads>8
0x2a	S,U	hd1:landing zone - low
0x2b	S,U	hd1:landing zone - high
0x2c	S,U	hd1:sectors per track

0x2d	S	boot from (bit5: 0:hd, 1:fd)

0x2e	S,U	standard cmos checksum (0x10->0x2d) - high
0x2f	S,U	standard cmos checksum (0x10->0x2d) - low

0x30	S	extended memory in k - low
0x31	S	extended memory in k - high

0x32	S	rtc century

0x34	S	extended memory in 64k - low
0x35	S	extended memory in 64k - high

0x37	S	ps/2 rtc century (copy of 0x32, needed for winxp)

0x38	S	eltorito boot sequence + boot signature check
	0	floppy boot signature check (1: disabled, 0: enabled)
	7-4	boot drive #3 (0: unused, 1: fd, 2: hd, 3:cd, else: fd)

0x39	S	ata translation policy - ata0 + ata1
	1-0	ata0-master (0: none, 1: LBA, 2: LARGE, 3: R-ECHS)
	3-2	ata0-slave
	5-4	ata1-master
	7-6	ata1-slave

0x3a	S	ata translation policy - ata2 + ata3 (see above)

0x3b	S	ata biosdetect flags - ata0 + ata1 (unimplemented)
	1-0	ata0-master (0: auto, 1: cmos, 2: none)
	3-2	ata0-slave
	5-4	ata1-master
	7-6	ata1-slave

0x3c	S	ata biosdetect flags - ata2 + ata3 (unimplemented)

0x3d	S	eltorito boot sequence (see above)
	3-0	boot drive #1
	7-4	boot drive #2

0x3f	S	BIOS options
	0	fastboot (skip boot menu delay)
	7-1	reserved

0x5b	S 	extra memory above 4GB
0x5c	S 	extra memory above 4GB
0x5d	S 	extra memory above 4GB
0x5f	Q	number of processors