The following values affect how debugging support functions on the system. It can be set with the "nvram" command or in the "additional args" window of SystemDisk. Note that for later G3 and G4 systems, you modify the "boot-args" variable; otherwise, the "boot-command" variable, if you use either the "nvram" command or the OpenFirmware console at boot time.
bit 0 - 1 early breakpoint
bit 1 - 1 enables debug print
bit 2 - 1 enables NMI (cmd-power)
bit 3 - 1 enables kprintfs
These bits are set in the variable "debug", for example as follows (as root):
nvram boot-command='0 bootr debug=VVV'
assuming that boot-command was initially 0 bootr.
This will halt the system after the kernel is loaded, and as soon after the initial operation as the network device can support the debugger.
This enables debugger output to overwrite the window-manager's screen. Normally, output from the kernel is not displayed when the window manager is active. Note that this does not affect the output of "printf()", which is only visible in "console mode".
This key sequence will invoke the debugger, causing it to await a debugger connection, or signal the debugger if already connected. As will all kernel debugging, there can be circumstances where this is ineffective.
Normally, these are not enabled. When enabled, the output is sent to the serial line (modem port), at a speed of 38400 baud.