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. These first fourteen addresses do not address RAM, they address registers in the Real Time Clock chip.
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 for the clock
(the default value)
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 ???
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
0Fh Reset Code (IBM PS/2 "Shutdown Status Byte")
The next 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 BIOS's will generate a CMOS Checksum error if this value is invalid however many programs ignore the checksum and report the apparent value. 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.
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)
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)
Bit 3-0 Second Hard Disk Drive Type
(same as above except extended type will be found in 1Bh).
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 and 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)
15h - Base Memory in K, Low Byte
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
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-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.
1Ah - Second Extended Hard Disk Drive Type (see 19h above)
1Bh - (AMI) First Hard Disk (type 47d) user defined: # of Cylinders, LSB
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 - (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
20h - (AMI) First Hard Disk user defined: Control Byte
(80h if # of heads is = > 8)
20h - (PHOENIX) First user defined hard disk (type 48) Cylinders LSB
21h - (AMI) First Hard Disk user defined: Landing Zone, Low Byte
21h - (PHOENIX) First user defined hard disk (type 48) Cylinders MSB
22h - (AMI) First Hard Disk user defined: Landing Zone, High Byte
22h - (PHOENIX) First user defined hard disk (type 48) of Heads
23h - (AMI) First Hard Disk user defined: # of Sectors per track
23h - (PHOENIX) First user defined hard disk (type 48) Write Precomp. LSB
24h - (AMI) Second Hard Disk user defined: # of Cylinders, Low Byte
24h - (PHOENIX) First user defined hard disk (type 48) Write Precomp. MSB
25h - (AMI) Second Hard Disk user defined: # of Cylinders, High Byte
25h - (PHOENIX) First user defined hard disk (type 48) Parking zone LSB
26h - (AMI) Second Hard Disk user defined: Number of Heads
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 - (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
29h - (AMI) Second Hard Disk user defined:
Control Byte (80h if # of heads is = > 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 (believed to indicate the presence of the special 128k memory
expansion board for the IBM 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 and 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.
38h-3Fh (PHOENIX) - used only when PS/2 style password is NOT in effect
38h - Second user defined hard disk (type 48) Write Precomp. LSB
39h - Second user defined hard disk (type 48) Write Precomp. MSB
3Ah - Second user defined hard disk (type 48) Parking Zone LSB
3Bh - Second user defined hard disk (type 48) Parking Zone MSB
3Ch - Second user defined hard disk (type 48) Sectors per track
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 Don't know 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 Don't know 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 Don't know 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 Don't know 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 Don't know