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Emulating CMOS Memory

Description

When a conflict between AppleRTC and BIOS occurs, use RTCMemoryFixup.kext to emulate CMOS memory to fix it.

About CMOS Memory

  • CMOS memory holds important data such as date, time, hardware configuration information, auxiliary setup information, boot settings, hibernation information, etc.

  • Some CMOS memory space definitions are:

    • Date, time, hardware configuration: 00-0D
    • Hibernation information storage interval: 80-AB
    • Power management: B0-B4
    • Other

View CMOS memory

  • Add RtcRw.efi (included in the OpenCore package) to EFI\OC\Tools and your config.plist
  • Add OpenShell.efi as well
  • Save and reboot
  • Run the Shell from the BootPicker
  • Go to the directory tools
  • Type rtcrw read XX and hit Enter. Where XX is the CMOS memory address. For example, rtcrw read 08 can view the current month. If this month is May, check the result as 0x05 (BCD code).

Emulating CMOS memory

  • Install RTCMemoryFixup to OC\Kexts
  • Add it to the list of UEFI/Drivers in the config.plist.
  • Add rtcfx_exclude=… to boot-args.

Examples:

  • rtcfx_exclude=0D,
  • rtcfx_exclude=40-AF,
  • rtcfx_exclude=2A,2D,80-AB, etc.

Caution

Emulating CMOS memory will erase the originally defined functions, please use with caution. For example: rtcfx_exclude=00-0D will cause the date and time of the machine to stop updating during sleep.

Appendix: CMOS Memory 00-3F Definition

Address Description
0 Seconds
1 Second Alarm
2 Minutes
3 Minute Alarm
4 Hour
5 Hour Alarm
Day of the week
Day
Month
Year
Status Register A
Status Register B
Status Register C
D Status Register D (0: battery disabled; 80:battery active)
E Diagnostic Status Byte
F Shutdown status byte (power-up diagnostic definition)
10 Floppy drive type (bits 7-4: A drive, bits 3-0: B drive 1-360KB; 2-1.2MB; 6-1.44MB; 7-720KB)
11 Reserved
12 Hard drive type (bits 7-4: C drive, bits 3-0: D drive)
13 Reserved
14 Device bytes (number of floppy drives, display type, coprocessor)
15 Basic memory low byte
16 Base memory high byte
17 Extended memory low byte
18 Extended Memory High Byte
19 First Master Drive Type
1A First slave drive type
1B-1C Reserved
1D-24 First master drive's pillar, head, sector, etc.
25-2C Columns, heads, sectors, etc. of the first slave drive
2D Reserved
2E-2F CMOS checksum (10-2D byte sum)
30 Expanded Memory Low Byte
31 Expanded Memory High Byte
32 Date century byte (19H: 19th Century)
33 Message Flag
34-3F Reserved