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 Int15円f90円.
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:
   • byte 45 (2Dh) store aditional info not maintained by AT
     • bit 0 indicates is Compaq dual-mode monitor installed
     • 1 indicates whether keyclick is enabled
     • 2 not used
     • 3 if non-Compaq graphics adapter installed

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