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CMOS Memory Map v1.25 June, 1995
[Last Change: 6/04/95]

Compiled from multiple sources by Padgett Peterson
Corrections/additions/comments to: [email protected]

No guarantees of any kind.

Copyrights/Trademarks belong to whoever they may belong to.

Found: Algorithm used by IBM in calculating CRC checksums for PS/2
(see bytes 32h-33h). Complex (recursive part is 12 lines of
assembly) and not yet validated for every model.


Background

The CMOS (complementary metal oxide semiconductor) memory is actually
a 64 or 128 byte battery-backed RAM memory module that is a part of the
system clock chip. Some IBM PS/2 models have the capability for a
2k (2048 byte) CMOS ROM Extension.

First used with clock-calender cards for the IBM PC-XT, when the PC/AT
(Advanced Technology) was introduced in 1985, the Motorola MC146818
became a part of the motherboard. Since the clock only uses fourteen of
the RAM bytes, the rest are available for storing system configuration data.

Interestingly, the original IBM-PC/AT (Advanced Technology) standard for
the region 10h-3Fh is nearly universal with one notable exception: The
IBM PS/2 systems deviate considerably (Note: AMSTRAD 8086 machines were
among the first to actively use the CMOS memory available and since they
*predate* the AT, do not follow the AT standard).

This is just another example of how IBM created a standard, lost control
of it, tried to replace it, failed and lost market share in the process.

Originally, the IBM PC/AT only made use of a small portion of CMOS memory
and was defined in the IBM PC/AT Technical Reference Manual, specifically
bytes 10h, 12h, 14h-18h, 2Eh-33h. The balance was left undefined but was
quickly appropriated by various BIOS manufacturers for such user-selectable
options such as wait states, clock speeds, initial boot drive selection, and
password storage.

Later, as CMOS memory requirements grew, newer clock chips with 128
bytes of RAM came into use. However the fact remains that once the AT
standard was established, only IBM has tried to change the definitions
of that first description.

Accessing the CMOS

The CMOS memory exists outside of the normal address space and cannot
contain directly executable code. It is reachable through IN and OUT
commands at port number 70h (112d) and 71h (113d). To read a CMOS byte,
an OUT to port 70h is executed with the address of the byte to be read and
an IN from port 71h will then retrieve the requested information. The
following BASIC fragment will read 128 CMOS bytes and print them to the
screen in 8 rows of 16 values.

CMOS RAM space has an upper limit of 128 bytes because of the structure
of port 70: only bits 0-6 are used for addressing, bit 7 is used to
enable (0) or disable (1) Non-Maskable Interrupts (NMI) and explains why
IBM uses 80h OR

to read/write data & follows with a "throw-away"
call.

Note that if the CMOS only has 64 bytes available, addressing will
generally wrap and addresses from 40h-7Fh will mirror 00h-3Fh. Output will
be hexadecimal.

10 CLS
20 FOR i = 0 TO &H7F
30 OUT &H70, i
40 PRINT USING "\ \"; HEX$(INP(&H71));
50 NEXT i
60 PRINT " "

Note: where not otherwise noted, all data points are expressed as BYTES
these are eight bit values and are read from MSB to LSB e.g.
0000 0000 0101 1010 binary would be written as 5Ah
7654 3210 where only some bits are used this is represented with
Xs e.g bits 5-3 would be shown as 00xx x000

Note: the entries for AMI WinBIOS also apply to AMIBIOS with core dates of
12/15/95 or later



Organization of CMOS Memory - Clock

00h-0Eh is defined by the clock hardware and all must follow it. Other
manufacturers generally follow the same format as specified for the
region 10h - 2Fh. Some also follow the IBM format for 30h-33h but not all
(Zenith in particular is different).

The first fourteen bytes are dedicated to the MC146818 chip clock functions
and consist of ten read/write data registers and four status registers, two
of which are read/write and two of which are read only.

The format of the ten clock data registers (bytes 00h-09h) is:

00h Seconds (BCD 00-59, Hex 00-3B) Note: Bit 7 is read only
01h Second Alarm (BCD 00-59, Hex 00-3B; "don't care" if C0-FF)
02h Minutes (BCD 00-59, Hex 00-3B)
03h Minute Alarm (BCD 00-59, Hex 00-3B; "don't care" if C0-FF))
04h Hours (BCD 00-23, Hex 00-17 if 24 hr mode)
(BCD 01-12, Hex 01-0C if 12 hr am)
(BCD 81-92. Hex 81-8C if 12 hr pm)
05h Hour Alarm (same as hours; "don't care" if C0-FF))
06h Day of Week (01-07 Sunday=1)
07h Date of Month (BCD 01-31, Hex 01-1F)
08h Month (BCD 01-12, Hex 01-0C)
09h Year (BCD 00-99, Hex 00-63)

BCD/Hex selection depends on Bit 2 of register B (0Bh)
12/24 Hr selection depends on Bit 1 of register B (0Bh)
Alarm will trigger when contents of all three Alarm byte registers
match their companions.

The following is the on-chip status register information.

0Ah Status Register A (read/write) (usu 26h)
Bit 7 - (1) time update cycle in progress, data ouputs undefined
(bit 7 is read only)
Bit 6,5,4 - 22 stage divider. 010b - 32.768 Khz time base (default)
Bit 3-0 - Rate selection bits for interrupt.
0000b - none
0011b - 122 microseconds (minimum)
1111b - 500 milliseconds
0110b - 976.562 microseconds (default)

0Bh Status Register B (read/write)
Bit 7 - 1 enables cycle update, 0 disables
Bit 6 - 1 enables periodic interrupt
Bit 5 - 1 enables alarm interrupt
Bit 4 - 1 enables update-ended interrupt
Bit 3 - 1 enables square wave output
Bit 2 - Data Mode - 0: BCD, 1: Binary
Bit 1 - 24/12 hour selection - 1 enables 24 hour mode
Bit 0 - Daylight Savings Enable - 1 enables

0Ch Status Register C (Read only)
Bit 7 - Interrupt request flag - 1 when any or all of bits 6-4 are
1 and appropriate enables (Register B) are set to 1. Generates
IRQ 8 when triggered.
Bit 6 - Periodic Interrupt flag
Bit 5 - Alarm Interrupt flag
Bit 4 - Update-Ended Interrupt Flag
Bit 3-0 ???

0Dh Status Register D (read only)
Bit 7 - Valid RAM - 1 indicates batery power good, 0 if dead or
disconnected.
Bit 6-0 ???

---------------------------------------------

Organization of CMOS Memory - non-Clock

The last two bytes in the first hexadecimal decade (hexade ?) were not
specified in the PC/AT but may have the following use on some systems:

0Eh (PS/2) Diagnostic Status Byte
Bit 7 - When set (1) indicates clock has lost power
Bit 6 - (1) indicates incorrect checksum
Bit 5 - (1) indicates that equipment configuration is incorrect
power-on check requires that atleast one floppy be installed
Bit 4 - (1) indicates error in memory size
Bit 3 - (1) indicates that controller or disk drive failed initialization
Bit 2 - (1) indicates that time is invalid
Bit 1 - (1) indicates installed adaptors do not match configuration
Bit 0 - (1) indicates a time-out while reading adaptor ID

0Eh (AMSTRAD) 6 BYTEs time and date machine last used

---------------------------------------------
0Fh Reset Code (IBM PS/2 "Shutdown Status Byte")
00h-03h perform power-on reset
00h software reset or unexpected reset
01h reset after memory size check in real/virtual mode
02h reset after successful memory test in real/virtual mode
03h reset after failed memory test in real/virtual mode
04h INT 19h reboot
05h flush keyboard and jump via 40h:67h
06h reset (after successful test in virtual mode)
07h reset (after failed test in virtual mode)
08h used by POST during protected-mode RAM test
09h used for INT 15/87h (block move) support
0Ah jump via 40h:67h
0Bh-FFh perform power-on reset

---------------------------------------------

The second group of values extends from address 10h to 2Dh. The word at

2Eh-2Fh is a byte-wise summation of the values in these bytes. Most BIOSes
will generate a CMOS Checksum error if this value is invalid however many
programs ignore the checksum and report the apparent value. The current
version of MSD reports my XT as having 20+ MB of extended memory.

Where a definiton appears universal, no identification is made. Where
the definition is thought to be specific to a manufacturer/model (AMI,
AMSTRAD, IBM AT, IBM PS/2) the identification is enclosed in parens. The
AMSTAD definitions appear to relate to 8088/8086 (PC and PC/XT class)
mchines only. AT class machines appear to adhere to IBM PC/AT fornat.

---------------------------------------------
10h - Floppy Drive Type

Bits 7-4 - First Floppy Disk Drive Type
0h No Drive
1h 360 KB 5.25 Drive
2h 1.2 MB 5.25 Drive - note: not listed in PS/2 technical manual
3h 720 KB 3.5 Drive
4h 1.44 MB 3.5 Drive
5h 2.88 MB 3.5 drive
6h-Fh unused

Bits 3-0 Second Floppy Disk Drive Type (bit settings same as A)

Hence a PC having a 5 1/4 1.2 Mb A: drive and a 1.44 Mb B: drive will
have a value of 24h in byte 10h. With a single 1.44 drive: 40h.

---------------------------------------------
11h - (IBM-PS/2) First Fixed Disk Drive Type Byte (00-FFh) Note: if IBM
ESDI or SCSI drive controller is used, CMOS drive type will be zero (00 -
no drive) and INT 13h will be directed to controller ROM.

11h - (older AMI) Keyboard Typematic Data
Bit 7 Enable Typematic (1 = On)
Bits 6-5 Typematic Delay (wait before begin repeating)
00b 250 ms
01b 500 ms
10b 750 ms
11b 100 ms
Bits 4-0 Typematic Rate char/sec e.g. 01010b = 12.0 cps
00000b - 300 01000b - 159 10000b - 75 11000b - 37
00001b - 267 01001b - 133 10001b - 67 11001b - 33
00010b - 240 01010b - 120 10010b - 60 11010b - 30
00011b - 218 01011b - 109 10011b - 55 11011b - 27
00100b - 200 01100b - 100 10100b - 50 11100b - 25
00101b - 185 01101b - 92 10101b - 46 11101b - 23
00110b - 171 01110b - 86 10110b - 43 11110b - 21
00111b - 160 01111b - 80 10111b - 40 11111b - 20

11h - (AMI) Advanced Setup Options
bit 7 mouse enabled
bit 6 test memory above 1 megabyte
bit 5 generate clicks during memory test
bit 4 enable memory parity check
bit 3 display key for Setup while booting
bit 2 store user-defined disk data at top of memory instead of 0030h:0000h
bit 1 request F1 keypress on boot error

11h - (AMI WinBIOS) Boot Options
bit 7 systems boots with high CPU speed
bit 6 memory test above 1MB enabled
bit 5 memory test tick sound enabled
bit 4 floppy drive seek at boot enabled
bit 3 "Hit " message enabled
bit 2 BIOS extended RAM area takes 1K at top of memory instead of 30h:0000h
bit 1 wait for F1 key on error
bit 0 NumLock enabled at boot
see also 13h"AMI"

11h - (AWARD) Configuration Bits
Bit 7 NumLock ON at reboot.
Bit 6 IDE Block Mode enabled.
Bit 5 ???
Bit 4 Shadow ROM BIOS at CC00-CFFF
Bit 3 Shadow ROM BIOS at C800-CBFF
Bit 2 ???
Bit 1 BIOS Password Enabled
Bit 0 0 = Password controls BIOS Setup Only
1 = Password required to enter System

---------------------------------------------
12h - (IBM PS/2) Second Fixed Disk Drive Type (00-FFh) - see 11h
12h - Hard Disk Data
Bits 7-4 First Hard Disk Drive
00 No drive
01-0Eh Hard drive Type 1-14
0Fh Hard Disk Type 16-255 (actual Hard Drive Type is in CMOS RAM 1Ah)
Bits 3-0 Second Hard Disk Drive Type (same as above except extrnded type
will be found in 1Bh).
A PC with a single type 2 (20 Mb ST-225) hard disk will have 20h in byte 12h
Note: some PCs utilizing external disk controller ROMs will use type 0 to
disable ROM BIOS (e.g. Zenith 248 with Plus HardCard).

---------------------------------------------
13h (AMI) Advanced Setup Options
bit 7 Mouse Enabled (1 = On)
bit 6 Test Memory above 1 MB (1 = On)
bit 5 Memory Test Tick Sound (1 = On)
bit 4 Memory Parity Error Check (1 = On)
bit 3 Press to Disable Memory Test (1 = On)
bit 2 User-Defined Hard Disk (1 = Type 47 data area at address 0:300h)
bit 1 Wait for Message if Error (1 = On)
bit 0 Turn Num Lock On at boot (1 = On)
see also 11h"WinBIOS"

13h (AMI WinBIOS) Peripheral Options
bits 7-5 typematic rate
000-111 = 6,8,10,12,15,20,24,30 cps
bit 4: numeric processor test enabled

13h (PS/2 MCA) internal POST operations
bit 7 POST sets VGA pel information
bit 6 RTC battery OK
bit 5 invoke ROM BASIC from POST
bit 4 POST sets typematic to 30cps/250ms delay instead of 10.9cps/500ms
bit 3-2 unused or unknown
bit 1 network password installed
bit 0 power-on password installed

13h - (AWARD) Configuration Bits
bit 7 Set Keyboard Typematic Rate
bit 4-6 Keyboard Repeat Rate
000 = 6 cps
001 = 8 cps
010 = 10 cps
011 = 12 cps
100 = 15 cps
101 = 20 cps
110 = 24 cps
111 = 30 cps
bit 2-3 Keyboard Typematic Delay
00 = 250 Msec
01 = 500 Msec
10 = 750 Msec
11 = 1000 Msec
bit 1 ???
bit 0 Boot Up Floppy Seek

---------------------------------------------
14h - Equipment Byte
Bits 7-6 Number of Floppy Drives (system must have at least one)
00b 1 Drive
01b 2 Drives
10b ??? 3 Drives
11b ??? 4 Drives
Bits 5-4 Monitor Type
00b Not CGA or MDA (observed for EGA & VGA)
01b 40x25 CGA
10b 80x25 CGA
11b MDA (Monochrome)
Bit 3 Display Enabled (1 = On) (turned off to enable boot of rackmount)
Bit 2 Keyboard Enabled (1 = On) (turned off to enable boot of rackmount)
Bit 1 Math coprocessor Installed (1 = On)
Bit 0 Floppy Drive Installed (1 = On) (turned off for rackmount boot)

14h - (AMSTRAD) BYTE user RAM checksum
LSB of sum of all user bytes should be AAh

---------------------------------------------
15h - Base Memory in K, Low Byte
15h - (AMSTRAD) WORD Enter key scancode/ASCII code
default: 1C0Dh - emulates Return key

---------------------------------------------
16h Base Memory in K, High Byte
The value in 15h-16h should be the same as in 0:413h and that returned by
Int 12h. A PC having 640k (280h) of conventional memory will return 80h in
byte 15h and 02h in byte 16h.

---------------------------------------------
17h - Extended Memory in K, Low Byte
17h - (AMSTRAD) WORD Forward delete key scancode/ASCII code
default: 2207h - emulates ^G (bell/beep)

---------------------------------------------
18h - Extended Memory in K, High Byte (some systems will only accommodate
15 Mb extended or 16 Mb total) Format is the same as in 15h-16h

---------------------------------------------
19h - First Extended Hard Disk Drive Type (not in original AT
specification but now nearly universally used except for PS/2).
0-Fh unused (would not require extension. Note: this has the effect of
making type 0Fh (15d) unavailable.
10h-FFh First Extended Hard Drive Type 16d-255d
19h - (MCA) Slot 0 adapter card ID
19h - (AMI)
bits 3-0: ???
bits 7-4: ???

For most manufacturers the last drive type (typically either 47d or 49d)
is "user defined" and parameters are stored elsewhere in the CMOS.

19h - (AMSTRAD) WORD Joystick fire button 1 scancode/ASCII code
default: FFFFh - (no translation)

---------------------------------------------
1Ah - Second Extended Hard Disk Drive Type (see 19h above)
1Ah - (MCA) Slot 0 adapter card ID

---------------------------------------------
1Bh - (MCA) Slot 1 adapter card ID
1Bh - (AMI) First Hard Disk (type 47) user defined: # of Cylinders, LSB
1Bh - (AMSTRAD) WORD Joystick fire button 2 scancode/ASCII code
default: FFFFh - (no translation)
1Bh - (PHOENIX) LSB of Word to 82335 RC1 roll compare register

1Bh - (AWARD) Configuration Bits
Bit 7-4 ???
Bit 3 Shadow ROM BIOS at DC00-DFFF
Bit 2 " " " " D800-DBFF
Bit 1 " " " " D400-D7FF
Bit 0 " " " " D000-D3FF

---------------------------------------------
1Ch - (MCA) Slot 1 adapter card ID
1Ch - (AMI) First Hard Disk user defined: # of Cylinders, High Byte
1Ch - (PHOENIX) MSB of Word to 82335 RC1 roll compare register

1Ch,1Dh - (AWARD) Password
Stored as a checksum or CRC using unknown algorithm. (See byte 11h
to enable)

---------------------------------------------
1Dh - (MCA) Slot 2 adapter card ID
1Dh - (AMI) First Hard Disk user defined: Number of Heads
1Dh - (AMSTRAD) WORD mouse button 1 scancode/ASCII code
default: FFFFh - (no translation)
1Dh - (Zenith Z-200 monitor) Boot Drive Selection
Bits 6-5 (0xx0 0000)
00 - MFM Monitor
01 - First floppy drive (A:)
10 - First fixed disk (C:)
11 - First floppy drive (A:). If not there then First fixed disk (C:)
(this is the default).
1Dh - (PHOENIX) LSB of Word to 82335 RC2 roll compare register
1Dh - (AWARD) MSB of password checksum (see byte 1Ch)

---------------------------------------------
1Eh - (MCA) Slot 2 adapter card ID
1Eh - (AMI) First Hard Disk user defined: Write Precompensation Cylinder,
Low Byte
1Eh - (PHOENIX) MSB of Word to 82335 RC2 roll compare register
1Eh - (AWARD) 2nd Hard Disk user defined (type 48): # of Cylinders Low Byte

---------------------------------------------
1Fh - (MCA) Slot 3 adapter card ID
1Fh - (AMI) First Hard Disk user defined: Write Precompensation Cylinder,
High Byte
1Fh - (AMSTRAD) WORD mouse button 2 scancode/ASCII code
default: FFFFh - (no translation)
1Fh - (AWARD) 2nd Hard Disk user defined (type 48): # of Cylinders High Byte

---------------------------------------------
20h - (MCA) Slot 3 adapter card ID
20h - (AMI) First Hard Disk user defined: Control Byte
bits 7-6 no retries (1)
bit 5 bad sector map at last cylinder+1
bit 4 unused (0)
bit 3 more than 8 heads
bits 2-0 unused (0)
20h - (AMI WinBIOS) First Hard Disk user defined: Landing Zone, Low Byte
20h - (PHOENIX) First user defined hard disk (type 48) Cylinders LSB
20h - (AWARD) 2nd Hard Disk user defined (type 48): Number of Heads

---------------------------------------------
21h - (MCA) Programmable Option Select configuration byte 2
21h - (AMI) First Hard Disk user defined: Landing Zone, Low Byte
21h - (AMI WinBIOS) First Hard Disk user defined: Landing Zone, High Byte
21h - (AMSTRAD) BYTE mouse X scaling factor, default: 0Ah
21h - (PHOENIX) First user defined hard disk (type 48) Cylinders MSB
21h - (AWARD) 2nd Hard Disk user defined (type 48): Write Precomp Low Byte

---------------------------------------------
22h - (MCA) Programmable Option Select configuration byte 3
22h - (AMI) First Hard Disk user defined: Landing Zone, High Byte
22h - (AMI WinBIOS) First Hard Disk user defined: # of Sectors per track
22h - (AMSTRAD) BYTE mouse Y scaling factor default: 0Ah
22h - (PHOENIX) First user defined hard disk (type 48) of Heads
22h - (AWARD) 2nd Hard Disk user defined (type 48): Write Precomp High Byte

---------------------------------------------
23h - (MCA) Programmable Option Select configuration byte 4
23h - (AMI) First Hard Disk user defined: # of Sectors per track
23h - (AMI WinBIOS) Second Hard Disk user defined: # of Cylinders, Low Byte
23h - (AMSTRAD) BYTE initial VDU mode and drive count default: 20h
bit 7: enables extended serial flow control
(NB this is buggy)
bit 6: set if two floppy drives installed
bits 5 & 4: (from Amstrad 1640 tech ref)
0 0 Internal video adapter
0 1 CGA card added; 40 x 25 mode
1 0 CGA card added; 80 x 25 mode
1 1 mono card added; 80 x 25 mode
23h - (PHOENIX) First user defined hard disk (type 48) Write Precomp. LSB
23h - (AWARD) 2nd Hard Disk user defined (type 48): Landing Zone Low Byte

---------------------------------------------
24h - (MCA) Programmable Option Select configuration byte 5
24h - (AMI) Second Hard Disk user defined: # of Cylinders, Low Byte
24h - (AMI WinBIOS) Second Hard Disk user defined: # of Cylinders, High Byte
24h - (AMSTRAD) BYTE initial VDU character attribute, default: 7h
24h - (PHOENIX) First user defined hard disk (type 48) Write Precomp. MSB
24h - (AWARD) 2nd Hard Disk user defined (type 48): Landing Zone High Byte

---------------------------------------------
25h - (AMI) Second Hard Disk user defined: # of Cylinders, High Byte
25h - (AMI WinBIOS) Second Hard Disk user defined: Number of Heads
25h - (AMSTRAD) BYTE size of RAM disk in 2K blocks
default: 0 - only used by the RAMDISK software supplied.
25h - (PHOENIX) First user defined hard disk (type 48) Parking zone LSB
25h - (AWARD) 2nd Hard Disk user defined (type 48): Sectors per Track

---------------------------------------------
26h - (AMI) Second Hard Disk user defined: Number of Heads
26h - (AMI WinBIOS) Second Hard Disk user defined: Write Precompensation
Cylinder, Low Byte
26h - (AMSTRAD) BYTE initial system UART setup byte
default: E3h - format as for Int 14h fn 0
26h - (PHOENIX) First user defined hard disk (type 48) Parking zone MSB
26h - (AWARD) 1st Hard Disk user defined (type 49): # of Cylinders Low Byte

---------------------------------------------
27h - (AMI) Second Hard Disk user defined: Write Precompensation Cylinder,
Low Byte
27h - (AMI WinBIOS) Second Hard Disk user defined: Write Precompensation
Cylinder, High Byte
27h - (AMSTRAD) BYTE initial external UART setup byte
default: E3h - format as for Int 14h fn 0
27h - (PHOENIX) First user defined hard disk (type 48) Sectors per track
27h - (AWARD) 1st Hard Disk user defined (type 49): # of Cylinders High Byte

---------------------------------------------
28h - (AMI) Second Hard Disk user defined: Write Precompensation Cylinder,
High Byte
28h - (AMI WinBIOS) Second Hard Disk user defined: Landing Zone, Low Byte
28h - (HP Vectra) checksum over bytes 29h-2Dh
28h - (AWARD) 1st Hard Disk user defined (type 49): Number of Heads

28h-3Fh (AMSTRAD) 24 BYTEs user applications default: zeroes

---------------------------------------------
29h - (AMI) Second Hard Disk user defined: Control Byte (80h if # of heads
is equal or greater than 8)
29h - (AMI WinBIOS) Second Hard Disk user defined: Landing Zone, High Byte
29h - (PHOENIX) LSB word to Intel 82335 CC0 compare register
29h - (AWARD) 1st Hard Disk user defined (type 49): Write Precomp Low Byte
29h - (HP Vectra) officially reserved "CMOS_HPCONFIG"
bit 7: include byte 2Ch in checksum (default = 0)
bit 6: select second ROM video adapter as primary (default = 0)
bits 5-1: reserved
bit 0: manufacturing test enabled

---------------------------------------------
2Ah - (AMI) Second Hard Disk user defined: Landing Zone, Low Byte
2Ah - (AMI WinBIOS) Second Hard Disk user defined: # of Sectors per track
2Ah - (HP Vectra) officially reserved
2Ah - (PHOENIX) MSB word to Intel 82335 CC0 compare register
2Ah - (AWARD) 1st Hard Disk user defined (type 49): Write Precomp High Byte

---------------------------------------------
2Bh - (AMI) Second Hard Disk user defined: Landing Zone, High Byte
2Bh - (AMI WinBIOS) IDE and shadowing control
bit 7: LBA mode enabled
bit 6: IDE block mode enabled
bit 5: 32-bit transfer enabled
bit 4: unused
bit 3: shadowing of DC00h enabled
bit 2: shadowing of D800h enabled
bit 1: shadowing of D400h enabled
bit 0: shadowing of D000h enabled
2Bh - (HP Vectra) officially reserved
2Bh - (PHOENIX) LSB word to Intel 82335 CC1 compare register
2Bh - (AWARD) 1st Hard Disk user defined (type 49): Landing Zone Low Byte

---------------------------------------------
2Ch - (AMI) Second Hard Disk user defined: # of Sectors per track
2Ch - (AMI WinBIOS) cache control
bit 7: external RAM cache enabled
bit 6: internal RAM cache enabled
bit 5: shadowing of E000h enabled
bit 4: shadowing of CC00h enabled
bit 3: shadowing of C800h enabled
bit 2: shadowing of C400h (video ROM) enabled
bit 1: shadowing of C000h (video ROM) enabled
bit 0: shadowing of system BIOS (F000h, 64K) enabled
2Ch - (HP Vectra) officially reserved
2Ch - (COMPAQ) bit 6: 0 - numlock OFF on boot, 1 - numlock ON at boot
2Ch - (PHOENIX) MSB word to Intel 82335 CC1 compare register
2Ch - (AWARD) 1st Hard Disk user defined (type 49): Landing Zone High Byte

---------------------------------------------
2Dh - (AMI) Configuration Options
bit 7 Weitek Installed
bit 6 Floppy Drive Seek - turn off for fast boot
bit 5 Boot Order 0 - Drive C:, then A:
1 - Drive A:, then C:
bit 4 Boot Speed (0 - Low; 1 - High)
bit 3 External Cache Enable (1 = On)
bit 2 Internal Cache Enable (1 = On)
bit 1 Use Fast Gate A20 after boot (1 = On)
bit 0 Turbo Switch (1 = On)
2Dh - (AMI WinBIOS)
bit 7 Weitek Installed
bit 6 bootsector virus protection enabled
bit 5 mouse enabled
bit 4 password checking (0 setup, 1 always)
bit 3 parity error check enabled
bits 2-1 boot order (00 = C:A:, 01 = A:C:)
bit 0 turbo switch enabled
2Dh - (HP Vectra) officially reserved
2Dh - (PHOENIX) Checks for values AAh or CCh
2Dh - (AWARD) 1st Hard Disk user defined (type 49): Sectors per Track

---------------------------------------------
2Eh - Standard CMOS Checksum, High Byte
2Fh - Standard CMOS Checksum, Low Byte

2Eh and 2Fh are as defined by the original IBM PC/AT specification and
represent a byte-wise additive sum of the values in locations 10h-2Dh only,
00h-0Fh and 30h-33h are not included. This definition is used by most
clone manufacturers including AMI, Compaq, Tandon, NEC, and Zenith. The
IBM PS/2 line does not follow this standard with the range 19h-31h being
undefined. On the original HP Vectra, this checksum only covers locations
10h to 20h, with a separate checksum for bytes 29h-2Ch (see offset 28h).


---------------------------------------------
30h - Extended Memory in K, Low Byte

---------------------------------------------
31h - Extended Memory in K, High Byte
(this appears to mirror the value in bytes 17h-18h.)

---------------------------------------------
32h - Century Byte (BCD value for the century - currently 19h)
32h - (IBM-PS2) Configuration CRC low byte. CRC for range 10h-31h

---------------------------------------------
33h - Information Flag
bit 7 128K (??? believe this indicates the presence of the special 128k
memory expansion board for the AT to boost the "stock" 512k
to 640k - all machines surveyed have this bit set)
bits 6-0 ???
33h - (IBM PS/2) Configuration CRC high byte (see entry for 32h)
33h - (PHOENIX) Bit 4 (000x 0000) bit 4 from Intel CPU register CP0
33h - (AMI WinBIOS)
bit 7: IBM-defined top 128K present
bits 6-4 CPU internal clock frequency
000-011 = 25, 33, 40, 50 MHz
100 = 60/66 MHz
101 = 75 MHz
110 = 80 MHz
111 = 90/100 MHz
bits 2-1 CPU internal clock multiplier
00-11 = 1,2,3,4
bit 0 FlashROM programming enabled (Ctrl-Home pressed at power on)
Note: this location is not included in any CMOS checksum fields

---------------------------------------------
34h - (AMI) Shadowing & Boot Password
Bits 7-6 Password Selection
00b Disable 10b Reserved
01b Set 11b Boot
Bit 5 C8000h Shadow ROM (Bit 1 = On)
Bit 4 CC000h Shadow ROM (Bit 1 = On)
Bit 3 D0000h Shadow ROM (Bit 1 = On)
Bit 2 D4000h Shadow ROM (Bit 1 = On)
Bit 1 D8000h Shadow ROM (Bit 1 = On)
Bit 0 DC000h Shadow ROM (Bit 1 = On)
24h - (AMI WinBIOS) system-specific information (bits 3-1)

34h-3Ah - (AWARD) ??? unused ??? Defaults to all FFh's.

---------------------------------------------
35h - (AMI) Shadowing
Bit 7 E0000h Shadow ROM (Bit 1 = On)
Bit 6 E4000h Shadow ROM (Bit 1 = On)
Bit 5 E8000h Shadow ROM (Bit 1 = On)
Bit 4 EC000h Shadow ROM (Bit 1 = On)
Bit 3 F0000h Shadow ROM (Bit 1 = On)
Bit 2 C0000h Shadow ROM (Bit 1 = On)
Bit 1 C4000h Shadow ROM (Bit 1 = On)
Bit 0 Reserved
35h - (AMI WinBIOS) extended memory size in 64K blocks, low byte
35h - (PHOENIX) Second user defined hard disk (type 48) Cylinders LSB
NOTE: used only when PS/2 style password is NOT in effect.

---------------------------------------------
36h - (PHOENIX) Second user defined hard disk (type 48) Cylinders MSB
NOTE: used only when PS/2 style password is NOT in effect.
36h - (AWARD)
bit 6: IDE 32-bit transfer mode
36h - (AMI)
bits 1-0: ???
bits 3-2: ???
36h - (AMI WinBIOS) extended memory size in 64K blocks, high byte

---------------------------------------------
37h - (IBM PS/2) Date Century Byte
37h - (PHOENIX) Second user defined hard disk (type 48) # of heads
NOTE: used only when PS/2 style password is NOT in effect.
37h - (AMI WinBIOS)
bits 7-4 password seed
bits 3-0 WinBIOS/AMIBIOS setup color options

---------------------------------------------
38h-3Dh (AMI) Encrypted Password

38h-3Fh ??? (IBM PS/2) Encrypted Password. Initialized to 00h in all
bytes. Will accept from 1-7 scan codes.

38h - (PHOENIX) Second user defined hard disk (type 48) Write Precomp. LSB
NOTE: used only when PS/2 style password is NOT in effect.

---------------------------------------------
39h - (PHOENIX) Second user defined hard disk (type 48) Write Precomp. MSB
NOTE: used only when PS/2 style password is NOT in effect.

---------------------------------------------
3Ah - (PHOENIX) Second user defined hard disk (type 48) Parking Zone LSB
NOTE: used only when PS/2 style password is NOT in effect.

---------------------------------------------
3Bh - (PHOENIX) Second user defined hard disk (type 48) Parking Zone MSB
NOTE: used only when PS/2 style password is NOT in effect.

3Bh - (AWARD) Configuration Bits
Bit 4-7 Screen Colors Used in Setup
0 Yellow/White on Blue (Default)
1 Magenta/White on Blue
2 Yellow/Black on Green
3 Yellow/Green on Cyan
4 Black/Yellow on Cyan
5 Brown/White on Cyan
6 White/Green on Red
7 White/White on Red
8 Green/White on Magenta
9 Yellow/Red on Magenta
Ah Red/White on Grey
Bh Yellow/White on Grey
Ch Cyan/White on Grey
Dh Cyan/Yellow on Black
Eh White on Black (Monochrome)
Fh Green/Red on Black
Bit 3 ??? Default = 0
Bit 2 ??? Default = 0
Bit 1 ??? Default = 1
Bit 0 Enable External Cache

---------------------------------------------
3Ch - (PHOENIX) Second user defined hard disk (type 48) Sectors per track
NOTE: used only when PS/2 style password is NOT in effect.

3Ch - (AWARD) Boot Configuration Bits
Bit 7 1 = Disable Virus Warning on Boot
Bit 6,5 ???
Bit 4 Quick POST Enabled
Bit 3,2 ???
Bit 1 Enable Turbo Switch Input
Bit 0 0 = Boot from A, then C
1 = Boot from C, then A

---------------------------------------------
3Dh - (AWARD) ???

---------------------------------------------
3Eh - (AMI) Extended CMOS Checksum, High Byte (includes 34h - 3Dh)

3Eh - (AWARD) Boot Configuration Bits
Bit 7 Shadow Video BIOS at C000h
Bit 6,5 ???
Bit 4 Swap Floppy Drive
Bit 3 ???
Bit 2 Don't Halt on Diskette Errors at Boot
Bit 1 Don't Halt on Keyboard Errors at Boot
Bit 0 Never Halt for any error at Boot

---------------------------------------------
3Fh - (AMI) Extended CMOS Checksum, Low Byte (includes 34h - 3Dh)
3Fh - (AWARD) ???

---------------------------------------------

End of original 64 CMOS RAM bytes. Many modern chips now contain 128
bytes and the IBM PS/2 has provision for 2k of "Expansion CMOS".
The AMI HI-FLEX description is below. If the chip does have only
64 bytes, addresses will wrap so that requests for bytes 40h-7Fh will
return the same values as 00h-3Fh.

---------------------------------------------
40h-55h - (AMI WinBIOS) PCI BIOS setup data

---------------------------------------------
40h ???

40h - (AWARD) Motherboard Chipset (SiS 85C501/85C502 shown)
Bit 7-1 ???
Bit 0 Automatic Configuration Enabled (Default: 1=enabled)

---------------------------------------------
41h - (AMI)
Bits 7-6 IOR/IOW Wait states
Bits 5-4 16-bit DMA Wait States
Bits 3-2 8-bit DMA Wait States
Bit 1 EMR bit
Bit 0 DMA Clock Source

---------------------------------------------
42h-43h ???

42h-44h - (AWARD) ??? chipset setup ???

---------------------------------------------
44h - (AMI)
Bit 4 NMI Power Fail Warning
Bit 3 NMI Local Bus Timeout

---------------------------------------------
45h - (AMI)
Bits 7-6 AT Bus 32-Bit Delay
Bits 5-4 AT Bus 16-Bit Delay
Bits 3-2 AT Bus 8-Bit Delay
Bits 1-0 AT Bus I/O Delay

45h - (AWARD) Motherboard Chipset (SiS 85C501/85C502 shown)
Bit 7 System BIOS Cacheable (Default: 1=enabled)
Bit 6 Video BIOS Cacheable (Default: 1=enabled)
Bit 6-0 ???

---------------------------------------------
46h - (AMI)
Bits 7-6 AT Bus 32 Bit Wait States
Bits 5-4 AT Bus 16 Bit Wait States
Bits 3-2 AT Bus 8 Bit Wait States
Bits 1-0 AT Bus Clock Source

46h thru 47h - (AWARD) ??? chipset setup ???

---------------------------------------------
47h-50h ???

48h-4Fh - (AWARD) ??? unused ??? Defaults to all FFh's.

---------------------------------------------
50h - (AWARD) PCI Bus Slot 1 Latency Timer 0-255 (default: 0)

---------------------------------------------
51h - (AMI)
Bit 7 Bank 0/1 RAS Precharge
Bit 6 Bank 0/1 Access Wait States
Bits 3-2 Bank 0/1 Wait States

51h - (AWARD) PCI Bus Setup
Bit 7 PIRQ0# Interrupt Triggering
0 = Edge Sensitive,
1 = Level Sensitive
Bit 6-2 ??? Default: all 1's
Bit 0-1 Slot 1 IRQ Setup
00 = A-PIRQ0 (Default)
01 = B-PIRQ1
10 = C-PIRQ2
11 = D-PIRQ3

---------------------------------------------
52h ???
52h - (AWARD) PCI Bus Slot 2 Latency Timer 0-255 (default: 0)

---------------------------------------------
53h - (AMI)
Bit 7 Bank 2/3 RAS Precharge
Bit 6 Bank 2/3 Access Wait States
Bits 3-2 Bank 2/3 Wait States

53h - (AWARD) PCI Bus Setup
Bit 7 PIRQ1# Interrupt Triggering
0 = Edge Sensitive,
1 = Level Sensitive
Bit 6-2 ??? Default: all 1's
Bit 0-1 Slot 2 IRQ Setup
00 = A-PIRQ1 (Default)
01 = B-PIRQ2
10 = C-PIRQ3
11 = D-PIRQ0

---------------------------------------------
54h-7Fh ???
54h - (AWARD) PCI Bus Slot 3 Latency Timer 0-255 (default: 0)

---------------------------------------------
55h - (AWARD) PCI Bus Setup
Bit 7 PIRQ2# Interrupt Triggering
0 = Edge Sensitive,
1 = Level Sensitive
Bit 6-2 ??? Default: all 1's
Bit 0-1 Slot 3 IRQ Setup
00 = A-PIRQ2 (Default)
01 = B-PIRQ3
10 = C-PIRQ0
11 = D-PIRQ1

---------------------------------------------
56h - (AWARD) ?? reserved for PCI Bus Slot 4 Latency Timer ??

---------------------------------------------
57h - (AWARD) PCI Bus Setup
Bit 7 PIRQ3# Interrupt Triggering
0 = Edge Sensitive,
1 = Level Sensitive
Bit 6-0 ??not used Default: all 1's

---------------------------------------------
58h - (AWARD) ?? reserved for PCI Bus Slot 5 Latency Timer ??
bit 3: onboard CMD IDE Mode 3

---------------------------------------------
59h - (AWARD) ?? reserved for PCI Bus Setup ??

---------------------------------------------
5Ah - (AWARD) PCI Bus IRQ Setup
Bit 4-7 PIRQ1# Interrupt Line (0=none, Bh=IRQ11, etc)
Bit 0-3 PIRQ0# Interrupt Line " " "

---------------------------------------------
5Bh - (AWARD) PCI Bus IRQ Setup
Bit 4-7 PIRQ3# Interrupt Line (0=none, Bh=IRQ11, etc)
Bit 0-3 PIRQ2# Interrupt Line " " "

---------------------------------------------
5Ch-5Fh - (AWARD) ??? unused ??? Defaults to all FFh's.

---------------------------------------------
60h-77h - (AMI WinBIOS) PCI chipset-specific setup information

---------------------------------------------
60h - (AWARD) Power Management
Bit 7 ???
Bit 6 Video Off Method
1 = V/H SYNC + Blank (default)
0 = Blank Screen
Bit 4,5 Video Off Option
00 = Always On (default)
01 = Suspend -> Off
10 = Suspend, Standby -> Off
11 = All Modes -> Off
Bit 3 PM Control by APM (1=Yes)
Bit 2 ???
Bit 0,1 Power Management Setup
00 User Defined
01 Disabled (default)
10 Minimum Power Savings (40 Minutes for all events)
11 Maximum Power Savings (20 Seconds for all events)

---------------------------------------------
61h - (AWARD) Power Management
Bit 7 PM Event on HDD Ports Activity (1=enable)
Bit 6 PM Event on LPT Port Activity (1=enable)
Bit 5 PM Event on COM Port Activity (1=enable)
Bit 4 HDD Power Down on Suspend
Bit 0-3 HDD Power Down Time
0 Disabled
1-15 Time in Minutes

---------------------------------------------
62h - (AWARD) Power Management
Bit 7-4 Standby Mode Setting (for User Defined)
0 Disabled
1 20 Seconds
2 1 Minute
3 5 Minutes
4 10 Minutes
5 15 Minutes
6 20 Minutes
7 30 Minutes
8 40 Minutes
Bit 0-3 Doze Mode Setting (for User Defined)
(See Standby Mode above)

---------------------------------------------
63h - (AWARD) Power Management
Bit 7 Disable PM Event on IRQ3 Activity (COM2) (1=disable)
Bit 6 PM Event on VGA Activity (1=enable)
Bit 5 ??? (Defaults to 1)
Bit 4 PM Event on PCI/ISA Master Activity (1=enable)
Bit 0-3 Suspend Mode Setting (for User Defined)
(See Standby Mode above)

---------------------------------------------
64h - (AWARD) Power Management
Bit 7 Disable PM Event on IRQ11 Activity (1=disable)
Bit 6 Disable PM Event on IRQ10 Activity (1=disable)
Bit 5 Disable PM Event on IRQ9 Activity (IRQ2 Redir) (1=disable)
Bit 4 Disable PM Event on IRQ8 Activity (RTC Alarm) (1=disable)
Bit 3 Disable PM Event on IRQ7 Activity (LPT1) (1=disable)
Bit 2 Disable PM Event on IRQ6 Activity (Floppy) (1=disable)
Bit 1 Disable PM Event on IRQ5 Activity (LPT2) (1=disable)
Bit 0 Disable PM Event on IRQ4 Activity (COM1) (1=disable)

---------------------------------------------
65h - (AWARD) Power Management
Bit 7-4 ??? may be unused. Defaults to all 1's
Bit 3 Disable PM Event on IRQ15 Activity (1=disable)
Bit 2 Disable PM Event on IRQ14 Activity (Hard Disk) (1=disable)
Bit 1 Disable PM Event on IRQ13 Activity (Coprocessor) (1=disable)
Bit 0 Disable PM Event on IRQ12 Activity (PS/2 Mouse) (1=disable)

---------------------------------------------
66h-79h - (AWARD) ??? unused ??? Defaults to all FFh's.

---------------------------------------------
68h-6Fh - (AWARD) IDE hard disk params for first drive on secondary IDE port

---------------------------------------------
70h-77h - (AWARD) IDE hard disk params for second drive on secondary IDE port


---------------------------------------------
78h-7Dh - (AMI WinBIOS) used by BIOS as scratch RAM

---------------------------------------------
7Ah - (AWARD) Extended CMOS Checksum, High Byte

---------------------------------------------
7Bh - (AWARD) Extended CMOS Checksum, Low Byte
Award's extended checksum is the arithmetic sum of all the bytes
from 40h (64 decimal) through 79h (121 decimal).

---------------------------------------------
7Eh-7Fh - (AMI WinBIOS) used as scratch RAM by power management code

---------------------------------------------

Revision History

v1.25 June 1995 Added AMI WinBIOS info from Daniel Miller (Ralf)
v1.24 Jan, 1995 Added Award info from Tim Farley (Ralf)
v1.23 June, 1994 Added some MCA info from _The_Undocumented_PC_
v1.22 Feb, 1994 Added NMI mask note
v1.21 Jan, 1994 Added note for PS/2 checksum found
v1.20 Sept, 1993 PHOENIX data from Wim Osterholt added
additional AMI data from Howie ([email protected])
v1.15 June, 1993 AMSTRAD data updated
v1.1 June, 1993 AMSTRAD & PS/2 data added
v1.0 June, 1993 First release: Motorola MC 146818, PC-AT & AMI
"Hi-Flex" information baselined