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1994-01-22
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540 lines
CMOS Memory Map v1.21 January, 1994
Compiled from multiple sources by Padgett Peterson
Corrections/additions/comments to: padgett@tccslr.dnet.mmc.com
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.
Information Wanted: In calculating/writing information to the CMOS
in a PS/2, The IBM BIOS ORs the actual location with 80h (the
floppy disk drive byte at 10h is addressed as 90h). Since the
CMOS addresses wrap, this winds up at the same place (10h) on
all PS/2s checked thusfar. Anyone know why IBM does this ?
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.
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
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)
02h Minutes (BCD 00-59, Hex 00-3B))
03h Minute Alarm (BCD 00-59, Hex 00-3B)
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)
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")
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 1/4 Drive
2h 1.2 MB 5 1/4 Drive - note: not listed in PS/2 technical manual
3h 720 KB 3 1/2 Drive
4h 1.44 MB 3 1/2 Drive
5h-Fh unused (??? 5h: 2.88 Mb 3 1/2 Drive ???)
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 - (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
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 <Esc> 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 <F1> Message if Error (1 = On)
Bit 0 Turn Num Lock On at boot (1 = On)
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
making type 0Fh (15d) unavailable.
10h-FFh First Extended Hard Drive Type 16d-255d
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)
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
1Ch - (AMI) First Hard Disk user defined: # of Cylinders, High Byte
1Ch - (PHOENIX) MSB of Word to 82335 RC1 roll compare register
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
1Eh - (AMI) First Hard Disk user defined: Write Precompensation Cylinder,
Low Byte
1Eh - (PHOENIX) MSB of Word to 82335 RC2 roll compare register
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)
20h - (AMI) First Hard Disk user defined: Control Byte (80h if # of heads
is equal or greater than 8)
20h - (PHOENIX) First user defined hard disk (type 48) Cylinders LSB
21h - (AMI) First Hard Disk user defined: Landing Zone, Low Byte
21h - (AMSTRAD) BYTE mouse X scaling factor, default: 0Ah
21h - (PHOENIX) First user defined hard disk (type 48) Cylinders MSB
22h - (AMI) First Hard Disk user defined: Landing Zone, High Byte
22h - (AMSTRAD) BYTE mouse Y scaling factor default: 0Ah
22h - (PHOENIX) First user defined hard disk (type 48) of Heads
23h - (AMI) First Hard Disk user defined: # of Sectors per track
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
24h - (AMI) Second Hard Disk user defined: # of Cylinders, Low Byte
24h - (AMSTRAD) BYTE initial VDU character attribute, default: 7h
24h - (PHOENIX) First user defined hard disk (type 48) Write Precomp. MSB
25h - (AMI) Second Hard Disk user defined: # of Cylinders, High Byte
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
26h - (AMI) Second Hard Disk user defined: Number of Heads
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
27h - (AMI) Second Hard Disk user defined: Write Precompensation Cylinder,
Low 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
28h - (AMI) Second Hard Disk user defined: Write Precompensation Cylinder,
High Byte
28h - (HP Vectra) checksum over words 29h-2Dh
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 - (PHOENIX) LSB word to Intel 82335 CC0 compare register
2Ah - (AMI) Second Hard Disk user defined: Landing Zone, Low Byte
2Ah - (PHOENIX) MSB word to Intel 82335 CC0 compare register
2Bh - (AMI) Second Hard Disk user defined: Landing Zone, High Byte
2Bh - (PHOENIX) LSB word to Intel 82335 CC1 compare register
2Ch - (AMI) Second Hard Disk user defined: # of Sectors per track
2Ch - (COMPAQ) bit 6: 0 - numlock OFF on boot, 1 - numlock ON at boot
29h - (PHOENIX) MSB word to Intel 82335 CC1 compare register
2Dh - (AMI) Configuration Options
Bit 7 Weitek Installed(1 = On)
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 - (PHOENIX) Checks for values AAh or CCh
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.
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 19)
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
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)
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 - (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.
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.
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.
3Ch - (PHOENIX) Second user defined hard disk (type 48) Sectors per track
NOTE: used only when PS/2 style password is NOT in effect.
3Eh - (AMI) Extended CMOS Checksum, High Byte (includes 34h - 3Dh)
3Fh - (AMI) Extended CMOS Checksum, Low Byte (includes 34h - 3Dh)
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 ???
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 ???
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
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
47h - 50h ???
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
52h ???
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
54h-7Fh ???
Revision History
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 (hjh@gwd.dst.gov.au)
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