          Amiga 500 processor accelerator project
                                        by Leslie Ayling
                                        p.o. box 334
                                        GLEBE 2037

     Welcome once again to cheapskates corner, where we try and buy the 
world for a song. This time around we shall increase the speed of the 
processor to just over 14Mhz, double the standard speed,
for under $25 !!!!

     You will need the following -

1. an MC68000p12A 16MHz processor. These are available in Sydney from VSI 
   electronics, 16 DICKSON ave ARTARMON 2064 ph (02)4398622  cost is 
   $18.90 plus tax ($22.68 tax inc.)
2. a 74F74 D type flip flop. Geoff wood at Lane Cove or Rod Irving 
  should keep them.
3. a few short lengths of hook-up wire.
4. a SPDT switch.

Refer to the accompanying circuit diagram
Note: Keep the leads to the switch as SHORT as possible.
The circuit is relatively straight forward. the 74F74 is wired as a 
positive edge triggered divide by two circuit, which provides the alternate 
clock frequency of 14.18Mhz approxiamately. The old 68000 8Mhz processor 
is removed and replaced with the 16Mhz version which has is clock pin (15) 
bent out horizontally so we can connect our new circuitry to it. 
The SPDT switch either connects the normal 7Mhz signal or the new 14Mhz 
signal to the 68000's clock input.

The switch can be changed while the machine is on, however it will simply 
lock up. Just reset as usual from the keyboard once you have changed speed.

I have a revision 5 board with new 1.3 roms which cope with the new speed 
quite happily. Using the program CRAMDEN-SI from MEGADISC 10 to compare 
the difference the mod makes : 

The standard position gives-

performance relative to amiga 1000 : 1.0
performance relative to ibm pc/xt  : 3.2

On high speed the performance is-

performance relative to amiga 1000 : 1.7
performance relative to ibm pc/xt  : 5.2

so roughly a 70% increase in performance.
Even though the clock speed is doubled to the processor, the performance 
dosen't increase to 2.0 because the agnus and the other support chips are 
still running at the same speed as before thus holding the processor up 
for a slightly higher percentage of the total time than before.

This mod basically gives the same capability as the CMI accelerator board 
priced at $399 in sydney, with the exception that it isn't software
controlled. However when it's only costing 1/16th the price one cant 
really complain!!.

If you have any queries or would like the mod installed in your machine, 
either contact me direct or via Megadisc.

The machine should work on the 1000 and 2000 however the pin numbers may 
be different.I suggest you compare the a500 schematics with that of your 
machine and make note of the differences.

Hope this is of some help...........regards   Leslie Ayling.

#addendum#
One machine this mod was carried out on had trouble with the one of the 
disk drives at high speed. It appeared to be trying to step the head of 
the internal drive faster than the drive could physically move and 
refused to boot.When I connected a drive with a 5ms track-track access 
time and booted off of it the problem dissappeared.
I have a patch for this problem for normal DOS disks to slow down the 
step rate which sits in the boot sector.It may be a problem however for 
protected software or non-standard boot sector type programs.
Don't despair however the patch is very small and should fit in 
somewhere anyway.Just drop me a line. If your machine exhibits the 
problem, the patch disk is available DIRECT from me with various usefull 
disk speedup utils and a Trackdisk step-rate alteration program, plus 
loads of other priceless information on gurus, reset program flow, 
diagnostics all on an ARP1.3 system disk for the princely sum of $5 !! 
There should be no problem with HARD disks however if anyone encounters 
any please let me know........thanks.
