How To: Installing OS X Leopard on Unsupported Macs

Many of us have an old Mac lying around the house that are capable of running Leopard but are not officially supported.

Every Mac with PCI architecture supports Open Firmware, meaning this technique will work with any Mac that could possibly run Leopard. However be sure your firmware is up to date before attempting to install Leopard. (If you are unsure then run Software Update)

The easiest way to install Leopard on unsupported G4s is to spoof the clock speed in Open Firmware before installing. All you're doing is making Open Firmware tell OS X that your G4 is running at a different speed until you reboot. This will allow you to install without hacking an install disc or working in Target Disk Mode. Note however that you will still need to meet the 512 MB of RAM requirement.

DO NOT install Leopard on:

-A G3 Macintosh

-A 400 MHz system with a sub-200 MHz bus and less than 32 MB of VRAM

-Any machine with less than 512MB of RAM

To install Leopard on an unsupported G4 Macintosh clocked under 867 MHz:

1. Remove any disc from your optical drive. 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. If the command is properly typed and understood, Open Firmware will display "ok" at the end of each line after you hit the return key. What these lines do is set the CPU speed that is reported by Open Firmware to the installer as that of an 867 MHz G4 processor system. The last line then continues the boot from the DVD drive.

For single CPUs

dev /cpus/PowerPC,G4@0

d# 867000000 encode-int " clock-frequency" property

boot cd:,:tbxi

For dual CPUs

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 Leopard installation

5. This CPU setting is only in effect until the Mac reboots. Once Leopard has been completely installed and your Mac has rebooted, the actual CPU speed should be displayed.

Not working?

If you have problems booting the Leopard install disc using the above commands, substitute "mac-boot" (without the quote marks) for the last line in the above Open Firmware commands (this applies to single and dual core machines).

If your Mac locks up hard during this process, resetting the PRAM should fix any problem. This is done by rebooting your Mac and holding down Cmd-Opt-P-R until it restarts.