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A Peek at the Apple Sawtooth (G4/450 AGP System) CPU Module
Published: 11/9/99

The Apple G4/AGP systems introduced in Fall 1999 (code named "Sawtooth") have a different motherboard and CPU module design than the previous B&W G3 and PCI based G4 ("Yikes") models. The Sawtooth design includes an AGP graphics card slot, internal Firewire port, dual USB controllers (independent of each other), Airport wireless networking card slot/antenna and a non-ZIF socketed CPU module. (I'm pretty positive there will be Sawtooth CPU upgrades however.)

Many readers had expressed interest in seeing what's under that huge heatsink so I'm posting this to answer those requests. It's humbling to compare the B&W G3 heatsink to the G4 version as the photo below shows. Don't worry - the G4 heatsink may be 4 times as large, but the chip is only about twice as hot according to the chip specs (shown below). Based on what I've seen with G4 CPU upgrades, I think the G3 spec must be low, as I don't think the G4 is actually twice the wattage of the G3.

G3 vs G4 heatsinks
B&W G3 vs G4 Heatsink

As you can see in the image above - the G4's heatsink is massive, however it never seems to get the least bit warm. I can only guess that Apple provided for the day when there would be two G4s on the module (there appears to be room, if the board had a relayout of components). The existing heatsink would need to be modified for two G4s however, as there is currently a single raised contact 'block' on the back of the heatsink for the CPU.

Future G4s will eventually be lower voltage (dissipating less heat), perhaps enough that by next summer there may be the possibility of a G4 Powerbook. The specs on the current G4/450 shows an 8.9W typical, 16.5W maximum rating (a 400MHz Motorola G3 rates 4.8W/8.0W respectively). Again, I think the Motorola G3 chip may be nearer to 8W typical in my opinion.


After removing the two retaining straps, lifting off the heatsink shows the Sawtooth CPU module.

Sawtooth CPU module
Sawtooth CPU Module

Note the single cache chip on this side (another one is on the bottom side of the module). With a relayout, I think there is space on this module for two G4 CPUs.

Technical Details/Speed Control Configuration: For a reference only (don't try this at home) page outlining the Bus ratio (Bus/CPU multiplier), Core Voltage, L2 signal level and Bus Clock (PCI/AGP) timing info, see Michiro Isobe's Sawtooth CPU module details article published here.

PowerPC 7400/G4 Specs: Based on Motorola info as of late August. 500MHz and faster versions due (in mass) early next year. My sources indicate it may have a 10x or 12X bus/CPU ratio. All specs subject to change of course.

PowerPC 7400/G4
based on 8/99 specs

CPU speed - Internal

350 MHz
400 MHz
450 MHz
(500MHz soon)

CPU/Bus Ratios

x3, x3.5, x4, x4.5, x5, x5.5,
x6, x6.5, x7, x7.5, x8, x9
(my sources mentioned 10x or 12X in next rev)

Bus Interface

64-bit

Bus Protocol
MPX/60x
Instructions per Clock

3 (2+branch)

L1 Cache Size

32 Kbyte inst
32 Kbyte data

Backside L2 Cache
Support

512 Kbyte, 1 Mbyte,
or 2 Mbyte

Backside Cache Ratios
(CPU speed/cache speed ratio)

1:1,1.5:1, 2:1, 2.5:1,
3:1, 3.5:1, 4:1

Typical/Max.
Power Dissipation

400 MHz:  7.9W/15.5W
450 MHz:  8.9W/16.3W
[Updated]

Die Size

83 sq mm

Package

360 CBGA

Process

0.15 micron 6LM CMOS

Transistors

6.5 million

Voltage

1.8 / 2.15V internal,
1.8 / 2.5 / 3.3V I/O

SPECint95 (est.)

21.4 @ 450 MHz

SPECfp95 (est.) 20.4 @ 450 MHz
Other Performance 825 MIPS @ 450 MHz
Execution Units

Integer (2)
Floating-Point
Vector

Branch
Load/Store
System


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