85 lines
3.9 KiB
Plaintext
85 lines
3.9 KiB
Plaintext
Amiga 500 processor accelerator project
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by Leslie Ayling
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p.o. box 334
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GLEBE 2037
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Welcome once again to cheapskates corner, where we try and buy the
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world for a song. This time around we shall increase the speed of the
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processor to just over 14Mhz, double the standard speed,
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for under $25 !!!!
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You will need the following -
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1. an MC68000p12A 16MHz processor. These are available in Sydney from VSI
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electronics, 16 DICKSON ave ARTARMON 2064 ph (02)4398622 cost is
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$18.90 plus tax ($22.68 tax inc.)
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2. a 74F74 D type flip flop. Geoff wood at Lane Cove or Rod Irving
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should keep them.
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3. a few short lengths of hook-up wire.
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4. a SPDT switch.
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Refer to the accompanying circuit diagram
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Note: Keep the leads to the switch as SHORT as possible.
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The circuit is relatively straight forward. the 74F74 is wired as a
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positive edge triggered divide by two circuit, which provides the alternate
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clock frequency of 14.18Mhz approxiamately. The old 68000 8Mhz processor
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is removed and replaced with the 16Mhz version which has is clock pin (15)
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bent out horizontally so we can connect our new circuitry to it.
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The SPDT switch either connects the normal 7Mhz signal or the new 14Mhz
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signal to the 68000's clock input.
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The switch can be changed while the machine is on, however it will simply
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lock up. Just reset as usual from the keyboard once you have changed speed.
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I have a revision 5 board with new 1.3 roms which cope with the new speed
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quite happily. Using the program CRAMDEN-SI from MEGADISC 10 to compare
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the difference the mod makes :
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The standard position gives-
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performance relative to amiga 1000 : 1.0
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performance relative to ibm pc/xt : 3.2
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On high speed the performance is-
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performance relative to amiga 1000 : 1.7
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performance relative to ibm pc/xt : 5.2
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so roughly a 70% increase in performance.
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Even though the clock speed is doubled to the processor, the performance
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dosen't increase to 2.0 because the agnus and the other support chips are
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still running at the same speed as before thus holding the processor up
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for a slightly higher percentage of the total time than before.
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This mod basically gives the same capability as the CMI accelerator board
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priced at $399 in sydney, with the exception that it isn't software
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controlled. However when it's only costing 1/16th the price one cant
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really complain!!.
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If you have any queries or would like the mod installed in your machine,
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either contact me direct or via Megadisc.
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The machine should work on the 1000 and 2000 however the pin numbers may
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be different.I suggest you compare the a500 schematics with that of your
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machine and make note of the differences.
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Hope this is of some help...........regards Leslie Ayling.
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#addendum#
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One machine this mod was carried out on had trouble with the one of the
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disk drives at high speed. It appeared to be trying to step the head of
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the internal drive faster than the drive could physically move and
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refused to boot.When I connected a drive with a 5ms track-track access
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time and booted off of it the problem dissappeared.
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I have a patch for this problem for normal DOS disks to slow down the
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step rate which sits in the boot sector.It may be a problem however for
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protected software or non-standard boot sector type programs.
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Don't despair however the patch is very small and should fit in
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somewhere anyway.Just drop me a line. If your machine exhibits the
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problem, the patch disk is available DIRECT from me with various usefull
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disk speedup utils and a Trackdisk step-rate alteration program, plus
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loads of other priceless information on gurus, reset program flow,
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diagnostics all on an ARP1.3 system disk for the princely sum of $5 !!
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There should be no problem with HARD disks however if anyone encounters
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any please let me know........thanks.
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