Hi Tom
 I have a reply from xxxxxxxxx, the person who found the 
 information to change the Crescendo extention.
 He kindly gives me a permit to translate it.
 However he will not gurantee the end result and will not take any 
 responsibility at all.
 If anyone wants to alter the Crescendo extention, he/she has to do it 
 with own risk.
 Also because it will alter the Crescendo extention, it might be a 
 copyright infringement. He will not take any blame or responsibility 
 either. He will open this information just as reference purpose.
 ----------------------------------------------
 How to alter the Crescendo extention to clock it up
 1 Copy the Crescendo extention
 2 Open it with the Resedit
 3 Open the pref.
 4 Open the resource ID=128
 5 The line you have to alter depends on the original speed of your card. 
 Rewrite it according to the target speed that you want to get
 
 The line you have to change
 Your original Crescendo speed
 220MHz card：0x0088, 0x00A0, 0x0108*
 (I am not sure about this card. In the case of it won't work, change all 
 address)
                   0x0088 : 0DE34C40
                   0x0090 : 0EE6B280
                   0x0098 : 0FDAD680
                   0x00A0 : 0DE34C40
                   0x00A8 : 0EE6B280
                   0x00B0 : 11E1A300
                   0x00B8 : 11E1A300
                   0x00C0 : 0FDAD680
                   0x0108 : 0E4E1C00
                   0x0110 : 11E1A300
                   0x0120 : 0EE6B280
                   0x0130 : 0FDAD680
 
 240MHz card：0x0090, 0x00A8, 0x0120
 260MHz card：0x0098, 0x00C0, 0x0130
 300MHz card：0x00B0, 0x00B8, 0x0110
 
 The figure you write into 
 Target speed
 250MHz：0EE6B280 *
 266MHz：0FDAD680 *
 280MHz：10B07600
 300MHz：11E1A300
 320MHz：1312D000
 340MHz：1443FD00
 360MHz：15752A00
 380MHz：16A65700
 400MHz：17D78400
 (* Actually it will run on 240MHz and 260MHz)
 
 So if the original speed of your card is 240MHz and you want to clock it 
 up to 280MHz, change the contents of 0x0090, 0x00A8, 0x0120 into 10B07600
 
 Note: This alteration will not work if the busspeed of your computer is 
 33MHz and currently running 260MHz. As already the card is using limit 
 speed. 33 x 8 = 264MHZ
 
 The computer I am using is Power Mac 8100 and the Crescendo G3 250/512K.
 I clock it up to 320MHz. I tried it to 400MHz but as I expected it didn't 
 work.
 However if the card is 300MHz and using x 10 chip (750LF?), it might work.
 
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 I didn't translate it word by word.
 So if you have any question, let me know.