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  CMOS RAM map, PC/AT:
   offset  contents
    00h    Seconds
    01h    Second Alarm
    02h    Minutes
    03h    Minute Alarm
    04h    Hours
    05h    Hour Alarm
    06h    Day of the Week
    07h    Day of the Month
    08h    Month
    09h    Year
    0Ah    Status Register A
    0Bh    Status Register B
    0Ch    Status Register C
    0Dh    Status Register D
    0Eh    Diagnostic Status Byte
    0Fh    Shutdown Status Byte
    10h    Disk Drive Type for Drives A: and B: The drive-type bytes use bits
	   0:3 for the first drive and 4:7 for the other
           Disk drive types:
	   00h	       no drive present
	   01h	       double sided 360k
	   02h	       high capacity (1.2 meg)
	   03h-0Fh     reserved
    11h    (AT):Reserved
	   (PS/2):drive type for hard disk C:
    12h    (PS/2):drive type for hard disk D:
	   (AT, XT/286):hard disk type for drives C: and D:
	   Format of drive-type entry for AT, XT/286:
	   0  number of cyls in drive (0-1023 allowed)
	   2  number of heads per drive (0-15 allowed)
	   3  starting reduced write compensation (not used on AT)
           5  starting cylinder for write compensation
	   7  max. ECC data burst length, XT only
	   8  control byte
	   Bit 7    disable disk-access retries
	       6    disable ECC retries
	       5-4  reserved, set to zero
	       3    more than 8 heads
	       2-0  drive option on XT (not used by AT)
	   9  timeout value for XT (not used by AT)
	   12 landing zone cylinder number
	   14 number of sectors per track (default 17, 0-17 allowed)
   13h     Reserved
   14h	   Equipment Byte (corresponds to sw. 1 on PC and XT)
   15h-16h Base Memory Size	 (low,high)
   17h-18h Expansion Memory Size (low,high)
   19h-20h Reserved (PS/2) POS information Model 50 (60 and 80 use a 2k CMOS
	   RAM that is not accessible through software)
   21h-2Dh Reserved (not checksumed)
   2Eh-2Fh Checksum of Bytes 10 Through 20  (low,high)
   30h-31h Exp. Memory Size as Det. by POST (low,high)
   32h	   Date Century Byte
   33h	   Information Flags (set during power-on)
   34h-3Fh Reserved

  1. The alarm function is used to drive the BIOS WAIT function Int\15f\90.
  2. To access the configuration RAM write the byte address (00-3Fh) you need to access to I/O port -0070, then access the data via I/O port -0071.
  3. CMOS RAM chip is a Motorola 146818
  4. The equipment byte is used to determine the configuration for the POST power-on diagnostics.
  5. Bytes 00-0Dh are defined by the chip for timing functions, bytes 0Eh-3Fh are defined by IBM.
  6. Compaq 386 uses same CMOS chip as IBM AT.
    Extra functions:

See also:

file: /Techref/BIOS/CMOSRAM.HTM, 3KB, , updated: 2005/10/21 15:32, local time: 2019/7/21 08:31,


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