Faking Out the Leopard Installer with Open Firmware
To install Leopard on an “unsupported” G4 clocked under 867 MHz:
1. Reboot your Mac and hold down the Cmd-Opt-O-F keys until you get a white screen with black text. This is the Open Firmware prompt.
2. Insert the Mac OS X Leopard Install DVD.
3. Type the following lines exactly as shown below into the Open Firmware prompt. Be mindful of capitalization, spaces, zeros, etc. If the command is properly typed and understood, Open Firmware will display “ok” at the end of each line after you hit “return”. What these lines do is set the CPU speed reported by Open Firmware to OS X as an 867 MHz G4 processor system. They then continue the boot from the DVD drive.
For single CPUs, use the following three lines:
dev /cpus/PowerPC,G4@0 d# 867000000 encode-int " clock-frequency" property boot cd:,\\:tbxi
For dual CPUs, use the following five lines:
dev /cpus/PowerPC,G4@0 d# 867000000 encode-int " clock-frequency" property dev /cpus/PowerPC,G4@1 d# 867000000 encode-int " clock-frequency" property boot cd:,\\:tbxi
4. Continue the install normally.
5. This CPU setting is only in effect until the Mac reboots. Once OS X Leopard is installed and your Mac has rebooted, the proper CPU speed should once again be displayed when you select About This Mac under the Apple menu.
Be sure to enter every line above exactly as written. The system will prompt you with an “ok” after each line if you have
The text above was lifted from the link below… :)