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Below is a typical CMOS RAM memory map for an AT PC.  Under a 128byte ISA compatible CMOS, 16 bytes (00h-0fh) is the real time clock, 32 bytes (10h-2Fh) is the ISA configuration data, 16 bytes (30h-3Fh) is the BIOS specific configuration data and 64 bytes (40h-7Fh) is the ESCD (Extended System Configuration Data).

 

Offset Hex Offset Dec Field Size Function

00h

0 1 byte RTC seconds.  Contains the seconds value of current time
01h 1 1 byte RTC seconds alarm.  Contains the seconds value for the RTC alarm
02h 2 1 byte RTC minutes.  Contains the minutes value of the current time
03h 3 1 byte RTC minutes alarm.  Contains the minutes value for the RTC alarm
04h 4 1 byte RTC hours.  Contains the hours value of the current time
05h 5 1 byte RTC hours alarm.  Contains the hours value for the RTC alarm
06h 6 1 byte RTC day of week.  Contains the current day of the week
07h 7 1 byte RTC date day.  Contains day value of current date
08h 8 1 byte RTC date month.  Contains the month value of current date
09h 9 1 byte RTC date year.  Contains the year value of current date
0Ah 10 1 byte Status Register A 
      Bit 7 = Update in progress (0 = Date and time can be read, 1 = Time update in progress)
      Bits 6-4 = Time frequency divider (010 = 32.768KHz
      Bits 3-0 = Rate selection frequency (0110 = 1.024KHz square wave frequency)
0Bh 11 1 byte Status Register B
      Bit 7 = Clock update cycle (0 = Update normally, 1 = Abort update in progress)
      Bit 6 = Periodic interrupt (0 = Disable interrupt (default), 1 = Enable interrupt)
      Bit 5 = Alarm interrupt (0 = Disable interrupt (default), 1 = Enable interrupt)
      Bit 4 = Update ended interrupt (0 = Disable interrupt (default), 1 = Enable interrupt)
      Bit 3 = Status register A square wave frequency (0 = Disable square wave (default), 1 = Enable square wave)
      Bit 2 = 24 hour clock (0 = 24 hour mode (default), 1 = 12 hour mode)
      Bit 1 = Daylight savings time (0 = Disable daylight savings (default), 1 = Enable daylight savings)
0Ch 12 1 byte Status Register C - Read only flags indicating system status conditions
      Bit 7 = IRQF flag
      Bit 6 = PF flag
      Bit 5 = AF flag
      Bit 4 UF flag
      Bits 3-0 = Reserved
0Dh 13 1 byte Status Register D - Valid CMOS RAM flag on bit 7 (battery condition flag)
      Bit 7 = Valid CMOS RAM flag (0 = CMOS battery dead, 1 = CMOS battery power good)
      Bit 6-0 = Reserved
0Eh 14 1 byte Diagnostic Status
      Bit 7 = Real time clock power status (0 = CMOS has not lost power, 1 = CMOS has lost power)
      Bit 6 = CMOS checksum status (0 = Checksum is good, 1 = Checksum is bad)
      Bit 5 = POST configuration information status (0 = Configuration information is valid, 1 = Configuration information in invalid)
      Bit 4 = Memory size compare during POST (0 = POST memory equals configuration, 1 = POST memory not equal to configuration)
      Bit 3 = Fixed disk/adapter initialization (0 = Initialization good, 1 = Initialization bad)
      Bit 2 = CMOS time status indicator (0 = Time is valid, 1 = Time is invalid)
      Bit 1-0 = Reserved
0Fh 15 1 byte CMOS Shutdown Status
      00h = Power on or soft reset
      01h = Memory size pass
      02h = Memory test pass
      03h = Memory test fail
      04h = POST complete; boot system
      05h = JMP double word pointer with EOI
      06h = Protected mode tests pass
      07h = protected mode tests fail
      08h = Memory size fail
      09h = Int 15h block move
      0Ah = JMP double word pointer without EOI
      0Bh = Used by 80386
10h 16 1 byte Floppy Disk Drive Types
      Bits 7-4 = Drive 0 type
      Bits 3-0 = Drive 1 type
      0000 = None
      0001 = 360KB
      0010 = 1.2MB
      0011 = 720KB
      0100 = 1.44MB
11h 17 1 byte System Configuration Settings
      Bit 7 = Mouse support disable/enable
      Bit 6 = Memory test above 1MB disable/enable
      Bit 5 = Memory test tick sound disable/enable
      Bit 4 = Memory parity error check disable/enable
      Bit 3 = Setup utility trigger display disable/enable
      Bit 2 = Hard disk type 47 RAM area (0:300h or upper 1KB of DOS area)
      Bit 1 = Wait for <F1> if any error message disable/enable
      Bit 0 = System boot up with Numlock (off/on)
12h 18 1 byte Hard Disk Types
      Bits 7-4 = Hard disk 0 type
      Bits 3-0 = Hard disk 1 type
      0000 = No drive installed
      0001 = Type 1 installed
      1110 = Type 14 installed
      1111 = Type 16-47 (defined later in 19h)
13h 19 1 byte Typematic Parameters
      Bit 7 = typematic rate programming disable/enabled
      Bit 6-5 = typematic rate delay
      Bit 4-2 = Typematic rate
14h 20 1 byte Installed Equipment
      Bits 7-6 = Number of floppy disks (00 = 1 floppy disk, 01 = 2 floppy disks)
      Bits 5-4 = Primary display (00 = Use display adapter BIOS, 01 = CGA 40 column, 10 = CGA 80 column, 11 = Monochrome Display Adapter)
      Bit 3 = Display adapter installed/not installed
      Bit 2 = Keyboard installed/not installed
      Bit 1 = math coprocessor installed/not installed
      Bit 0 = Always set to 1
15h 21 1 byte Base Memory Low Order Byte - Least significant byte
16h 22 1 byte Base Memory High Order Byte - Most significant byte
17h 23 1 byte Extended Memory Low Order Byte - Least significant byte
18h 24 1 byte Extended Memory High Order Byte - Most significant byte
19h 25 1 byte Hard Disk 0 Extended Type - (10h to 2Eh = Type 16 to 46 respectively)
1Ah 26 1 byte Hard Disk 1 Extended Type - (10h to 2Eh = Type 16 to 46 respectively)
1Bh 27 1 byte User Defined Drive C: - Number of cylinders least significant byte
1Ch 28 1 byte User Defined Drive C: - Number of cylinders most significant byte
1Dh 29 1 byte User Defined Drive C: - Number of heads
1Eh 30 1 byte User Defined Drive C: - Write precomp cylinder least significant byte
1Fh 31 1 byte User Defined Drive C: - Write precomp cylinder most significant byte
20h 32 1 byte User Defined Drive C: - Control byte
21h 33 1 byte User Defined Drive C: - Landing zone least significant byte
22h 34 1 byte User Defined Drive C: - Landing zone most significant byte
23h 35 1 byte User Defined Drive C: - Number of sectors
24h 36 1 byte User Defined Drive D: - Number of cylinders least significant byte
25h 37 1 byte User defined Drive D: - Number of cylinders most significant byte
26h 38 1 byte User Defined Drive D: - Number of heads
27h 39 1 byte User Defined Drive D: - Write precomp cylinder least significant byte
28h 40 1 byte User Defined Drive D: - Write precomp cylinder most significant byte
29h 41 1 byte User Defined Drive D: - Control byte
2Ah 42 1 byte User Defined Drive D: - Landing zone least significant byte
2Bh 43 1 byte User Defined Drive D: - Landing zone most significant byte
2Ch 44 1 byte User Defined Drive D: - Number of sectors
2Dh 45 1 byte System Operational Flags
      Bit 7 = Weitek processor present/absent
      Bit 6 = Floppy drive seek at boot enable/disable
      Bit 5 = System boot sequence
      Bit 4 = System boot CPU speed high/low
      Bit 3 = External cache enable/disable
      Bit 2 = Internal cache enable/disable
      Bit 1 = Fast gate A20 operation enable/disable
      Bit 0 = Turbo switch function enable/disable
2Eh 46 1 byte CMOS Checksum High Order Byte - Most significant byte
2Fh 47 1 byte CMOS Checksum Low Order Byte - Least significant byte
30h 48 1 byte Actual Extended Memory Low Order Byte - Least significant byte
31h 49 1 byte Actual Extended Memory High Order Byte - Most significant byte
32h 50 1 byte Century Date BCD - Value for century of current date
33h 51 1 byte POST Information Flags
      Bit 7 = BIOS length (64KB/128KB)
      Bit 6-1 = reserved
      Bit 0 = POST cache test passed/failed
34h 52 1 byte BIOS and Shadow Option Flags
      Bit 7 = Boot sector virus protection disabled/enabled
      Bit 6 = Password checking option disabled/enabled
      Bit 5 = Adapter ROM shadow C800h (16KB) disabled/enabled
      Bit 4 = Adapter ROM shadow CC00h (16KB) disabled/enabled
      Bit 3 = Adapter ROM shadow D000h (16KB) disabled/enabled
      Bit 2 = Adapter ROM shadow D400h (16KB) disabled/enabled
      Bit 1 = Adapter ROM shadow D800h (16KB) disabled/enabled
      Bit 0 = Adapter ROM shadow DC00h (16KB) disabled/enabled
35h 53 1 byte BIOS and Shadow Option Flags
      Bit 7 = Adapter ROM shadow E000h (16KB) disabled/enabled
      Bit 6 = Adapter ROM shadow E400h (16KB) disabled/enabled
      Bit 5 = Adapter ROM shadow E800h (16KB) disabled/enabled
      Bit 4 = Adapter ROM shadow EC00h (16KB) disabled/enabled
      Bit 3 = System ROM shadow F000h (16KB) disabled/enabled
      Bit 2 = Video ROM shadow C000h (16KB) disabled/enabled
      Bit 1 = Video ROM shadow C400h (16KB) disabled/enabled
      Bit 0 = Numeric processor test disabled/enabled
36h 54 1 byte Chipset Specific Information
37h 55 1 byte Password Seed and Color Option
      Bit 7-4 = Password seed (do not change)
      Bit 3-0 = Setup screen color palette
      07h = White on black
      70h = Black on white
      17h = White on blue
      20h = Black on green
      30h = Black on turquoise
      47h = White on red
      57h = White on magenta
      60h = Black on brown
38h-3dh 56-61 6 bytes Encrypted Password - (do not change)
3Eh 62 1 byte Extended CMOS Checksum - Most significant byte
3Fh 63 1 byte Extended CMOS Checksum - Least significant byte
40h 64 1 byte Model Number Byte
41h 65 1 byte 1st Serial Number Byte
42h 66 1 byte 2nd Serial Number Byte
43h 67 1 byte 3rd Serial Number Byte
44h 68 1 byte 4th Serial Number Byte
45h 69 1 byte 5th Serial Number Byte
46h 70 1 byte 6th Serial Number Byte
47h 71 1 byte CRC Byte
48h 72 1 byte Century Byte
49h 73 1 byte Date Alarm
4Ah 74 1 byte Extended Control Register 4A
4Bh 75 1 byte Extended Control register 4B
4Ch 76 1 byte Reserved
4Dh 77 1 byte Reserved
4Eh 78 1 byte Real Time Clock - Address 2
4Fh 79 1 byte Real Time Clock - Address 3
50h 80 1 byte Extended RAM Address - Least significant byte
51h 81 1 byte Extended RAM Address - Most significant byte
52h 82 1 byte Reserved
53h 83 1 byte Extended RAM Data Port
54h 84 1 byte Reserved
55h 85 1 byte Reserved
56h 86 1 byte Reserved
57h 87 1 byte Reserved
58h 88 1 byte Reserved
59h 89 1 byte Reserved
5Ah 90 1 byte Reserved
5Bh 91 1 byte Reserved
5Ch 92 1 byte Reserved
5Dh 93 1 byte Reserved

 


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