Mucho Macs 'n stuff

Monday, March 24, 2008

Modifying the Frontside Bus Speed on a G4 using Open Firmware

Well I've done CPU overclocking so now it's on to other types, like front side bus overclocking!! I've found various articles on and other places on how to change the bus speed on older 100mhz systems (like the Power Mac G4 Yikes!, PCI Graphics, Gigabit Ethernet) and I've really been wondering if that truly works. Basically you boot into open firmware (Apple-Option-O-F) and you type in a bunch of code. (I'll have to posts the links later as they're on a different computer). The code basically tells the computer to run a script to bypass the ROM and read from it to adjust the bus speed. Since I really wanted to try this but didn't want to mess up the Gigabit Dual 500mhz that my wife always uses I decided to give it a shot on my already modified Digital Audio 800mhz. But since this system is already running a 133mhz front side bus I decided to go to 167mhz, like what's found in later Mirror Door models. So I opened things in openfirmware, typed the code in, rebooted and crossed my fingers. It started fine, everything looked good. I opened system profiler for the moment of truth still read 133mhz. HMMM. I ran Geekbench and Xbench to be sure it just wasn't registering. Sure enough, no difference from previous runs. But I have to say, and maybe it's just a mental thing, it did feel a little faster. But that might also be the difference from the 733mhz to 800mhz overclocking. I haven't had much use with it since I modified it. But we'll see. Maybe later this week I'll change the bus speed on the Dual 500mhz Gigabit. I think I might have better luck with that. I have a feeling the modified Digital Audio doesn't register the changed bus speed because the memory isn't fast enough (SDRAM 133mhz). Problem is, the next step up is DDR RAM which won't physically fit in the slots. Maybe, maybe not.

update: I talked with the apple repair guys and they believe this might be try. They think it's plausible.
Helpful links if you want to try this (for 100mhz-133mhz):

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