Machine Characteristics - CMOS and RTC
Table of Contents

This is one of a number of machine-characteristics.


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.
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