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
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