Machine Characteristics - CMOS and RTC

This is one of a number of machine-characteristics.

CMOS and RTC

This table answers some specific questions about access to CMOS/RTC chip addresses:

• Will the address port, 0x70, report the last address written to it?
• How does the hardware use the bits of CMOS Register D? (In early machines it was read only; later ones allowed it to contain a day-of-month alarm value.)
• How many primary CMOS registers does the machine have. (Early machines had 64; later ones have 128.)

Machine	Read of Port 0x70	Bits of Register D	Number of CMOS Registers
Dell Latitude XP 475D laptop. 80486 75MHz. Phoenix BIOS 1.10	0xFF	rrrr_rrrr	128
AOpen motherboard. Pentium 3 600MHz. Award BIOS 3.60	0xFF	rrww_wwww	128
Viglen MPC-L. Geode 399MHz. ION A503 BIOS	The last value written	rrrr_rrrr	128
Asus EEE 4G netbook. Celeron M ULV 630MHz. AMI ASUS 701 BIOS	0xFF	rrww_wwww	128
Gateway laptop. Dual-core Core 2 T5300 1.7GHz. Phoenix BIOS	0xFF	rrww_wwww	128
Jetway NC91. Dual-core Atom 1.6GHz. Phoenix-AwardBIOS	0xFF	rrww_wwww	128

Notes

• Many settings of CMOS registers are not shown as they relate to how the device was last programmed rather than anything specific to either the hardware or the BIOS.
• Port 0x70 can be read on some machines but not all.
• CMOS register D is traditionally read-only with the top bit being cleared on a read and all other bits fixed as zero but it provides extra usable bits on some machines. The Unitrode bq3285LF datasheet shows the bottom six bits as for a day-of-month alarm in the range 1 to 31 and disabled if zero. When these bits are writable and the top two are not this is shown in the table as rrww_wwww.