Sometimes it can be useful disabling CPU cores for testing purposes, for example, if you want to test an application under single-core use. You can change settings within Open Firmware to make your system believe it has fewer processors than the number that are actually present.

You can create the same cpus setting in NVRAM using the Open Firmware user interface. Once the setting is in NVRAM, the setting persists across system restarts.

To set the number of cpus within Open Firmware, follow these steps:

  1. Boot into Open Firmware (Command + Option + O + F during the boot process).
  2. To view the current boot-args settings, enter the  following command:
    cr boot-args type cr
    printenv boot-args will accomplish the same thing, but will output the hexadecimal value of each character in boot-args rather than readable text.
  3. Enter the number of CPU cores to utilise with the following command:
    setenv boot-args cpus=1
  4. You will need to reenter the current value of boot-args if you want to preserve it. For example, if the result of Step 2 was:
    debug=0x4 ok
    The command would read:
    setenv boot-args debug=0x4 cpus=1
  5. After the ok response, continue booting your system with the mac-boot command.