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|
- 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.
page revision: 1, last edited: 08 Nov 2014 23:32