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Reader How to: Overclocking the Yikes! G4
By: Todd Eddie
Published: 10/14/99

DISCLAIMER/WARNING: First and foremost I want to stress that modifying the clock speed of PowerMacs will void your Apple warranty. Once the label over the jumper block is removed, no Apple Authorized Service provider will perform warranty work on that system regardless if the problem is motherboard or CPU related!

Recently there was a reported failure from a G4/400 Yikes running 450MHz after 12 hours.

The following information is for reference information only and is not recommended by xlr8yourmac or the author.

Introduction:

Let's start with a small introduction on clocking, just to get the basic idea of what it does to your computer. The clock speed controls how fast data is moved though the computer via chips and data paths, or buses, between them. All busses in a computer run at a certain clock frequency, and all components on that bus must be designed to run correctly at that frequency. All parts are made within tolerances to make sure the device will perform within the range needed. The foundation of overclocking is to push the limits of these tolerances to find an acceptable speed boost, for very little to no cost. Now what is acceptable to me may not be acceptable to you, but most people who dare to overclock computers have the personality type that nothing below the absolute maximum speed attainable is considered acceptable.

Apple offers two flavors of the Power Macintosh G4. The "low-end" of the line is based on the Yikes! motherboard architecture, which is the model this article pertains to. Yikes! is a slightly modified Yosemite motherboard, the one found in Blue & White G3s. The most visible change is the removal of the ADB port on the Yikes! board. The higher-end model is based on the all new "Sawtooth' design. Sawtooth offers many enhancements over the Yikes! model, but there is one area the Yikes! offers an advantage, overclocking. If you own a 400MHz G4, you definitely have a Yikes! model. But since Apple offered a 450MHz BTO option for the Yikes!, the easiest way to tell which you have is by taking a peek at the back of the tower. If your headphone and microphone jack are oriented horizontally, your G4 uses the Yikes! architecture. Vertical ports indicate a Sawtooth board.

So, if you are indeed a lucky Yikes! owner, read on. Why are you lucky? You have the last machine of an era. Apple as put an end to the jumper block used to set CPU and main bus speeds with the introduction of the Sawtooth model. So what does that mean? This means most Yikes! owners can easily boost the speed of their G4 systems in a matter of minutes, without any soldering. However, not all Yikes! owners may be able to boost their speed, it all depends on the chips inside your particular G4. So why do some 400MHz G4s run stable at 450MHz where others don't? There are several factors that attribute to various limitations. The primary factors are the CPU and the back-side cache. What is the difference between a 400MHz G4 and a 450MHz G4? The common idea is nothing, really. It's just that the 450 "came out of the mold" a little cleaner, with fewer imperfections. Motorola tests each CPU for the maximum operating speed then backs it off to a reliable speed, then tosses it in the 400MHz or 450MHz box. The backside cache is just as crucial. High speed cache memory is very expensive, and even though the cache operates at half the frequency of the CPU, Apple is certainly not going to put in any faster cache than they have to. So although your CPU may be capable of higher speeds, your cache may not be able to keep up. Enough details, lets see how it's done!

Yikes! Jumper Block Settings:

Thanks to the "El capitan" enclosure, upgrading the Blue & White G3s and Black & Silver G4's is a snap! In fact, its so easy to open while the power is on please be sure to power down your mac before attemping this modification . Also remember to always touch a metal "ground" source beforemaking contact with any parts inside a computer,

Next, locate the "Void warranty if seal broken" sticker, near the lower right corner of the CPU. Let me repeat myself, doing this modifcation to your Mac will result in no warranty coverage for your brand new G4 system! A possiblity is that your G4 will not even run faster than the stock speed, and therefore you will have no speed gains and no warranty! Once the sticker is out of the way, gently lift on the white block from both ends. It should slide off without much trouble.

Now that you have the jumper off, it is necessary to locate the first jumper. This is indicated by the notch on the upper right corner of the block if you are looking at the underside of the block. The table below provides the jumper settings to change the Multiplier ratio of CPU speed to Bus speed. G4s have a 100MHz bus, so a 4.5x multiple will result in a 450MHz CPU speed and a 225MHz backside cache speed.

CPU Speed
Multiple
Jumpers 1 - 4
400MHz
4x
S x S x
450MHz
4.5x
x x x S
500MHz
5x
x x S x
550MHz
5.5x
x S S x
600MHz
6x
x S x x
650MHz
6.5x
x S x S
700MHz
7x
S x S S
750MHz
7.5x
S x S S
800MHz
8x
S S x x

S:Jumper x:No Jumper


The trick to removing the jumpers without destroying them is to use a push pin as a lever, and gently slide the jumper out. The jumpers are labeled in the picture, and as you can see only four is set, indicating a multiplier of 4.5. After you safely store the extra jumper(from 400 to 450MHz), pop the block back on and start her up!



Results:

The G4/400 I overclocked runs great at 450MHz. At 500MHz it drops into open firmware. I've been running this way for several days now with no crashes. The G4 does seem to run hotter than the G3. Unfortunately I did not get a chance to test the temperature at 400MHz (can a G4/400 Yikes! Reader contribute this?).

Synthetic Benchmark:

Macbench 5.0, extensions off.

Game Performance:

Unreal 2.2.4b7

Mike's unreal.ini file. 800x600 Voodoo3 2000
G4 / 400MHz
50 fps
G4 / 450MHz
54 fps

 

G4TimeDemo

Altivec Enhanced 3d game engine by Altor Systems

Excellent detail, thousands of colors. Rage 128

G4 / 400MHz
45 fps
G4 / 450MHz
53 fps


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