E clock speedup mod
2 Attachment(s)
Hello my fellow EABers!
As you probably know, I've spent way WAY too much time overclocking and hacking my A2630. But all of this time and effort was spent on the 32 bit "Fast" bus of the A2630. So now, I have given some time and effort to the "Slow" bus on the A2000. I mean the snail SLOW bus on the A2000. The E clock bus! So why bother to do a mod which tweaks the performance on the E clock bus? Because all a fast CPU can do is clock off wait states on the snail slow E clock bus. So I want the E clock cycle to be optimized for less wait states... and this is exactly what the new A2630 U506 GAL does (see below): WARNING: Just in case you did not RTFM, this mod assumes no real 6800 E clock devices exist in the system and the 8520 devices can handle some performance tweaks. So you should keep the original U506 PAL just in case you ever get a very rare Zorro II E clock Board! NOTES: A GAL16V8 or equivalent PLD is required and must be programmed with the .jed file provided! Speed is not important but a 15ns device is nominal. This can also be done for the A2620, but I don't have one to test it on. The A2620 PAL U309 is nearly identical to the A2630 PAL U506. So, A2620 owners can just rename the GAL to U309! EDIT: It appears no fix is needed for A3000/A4000 owners. See post #10 below. DISCLAIMER: Use at your own risk! No warranty expressed or implied, etc. Code:
Name U506 ; |
I'm curious how compatible this is with "regular" Z2 cards that don't touch the external clock.
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Hmm... I thought this was very well explained. I have assumed that 99.9% of all Z2 cards use the 7 MHz clock and only .1 % of Z2 cards use the E clock. Therefore, 99.9% of the Z2 card users should have no problem.
However, if you believe my assumption is inaccurate then please provide me with the more accurate information. Thanks. ;) BTW, E means Enable not External! |
This is a very interesting idea ... Do you have some benchmarks for comparison? With only Chip RAM present?
For newer accelerators which generate an E Clock (mine inclusive) based on the original timing (E Clock active -> /DTACK assertion) this would not be too hard to also tweak in the CPLD. |
Sorry, no benchmarks. Some benchmark programs just report the E clock frequency but never bothered with performance results for the E clock bus. Chip RAM is not relevant since it's timing is on the 7 MHz bus.
Some Approx. 7 MHz wait state calculations are as follows: Old U506 PAL --------------- Best case 8 clocks Worst case 17 clocks New U506 GAL ---------------- Best case 5 clocks Worst case 14 clocks Notes: The number of CPU wait states varies with the CPU clock speed and the efficiency of the 68000 state machine logic. Also, the average case performance is much better for the new U506 GAL since it runs the case 3 cycle and the old U506 PAL skips it. Yes, it could be tweaked in a CPLD based design too. |
Are there any benchmarks for before and after?
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Since the 8520's are the only chips in the system that use the E bus, it only affects accesses to the following things:
* Joystick fire buttons * Filter LED * Floppy disk control lines * Parallel port * Serial port The only items on that list that you would hit more than once in a blue moon is the serial, parallel, and floppy ports. And the floppy port is already constrained by the speed of the floppy drive. Time spent fiddlefarting with the floppy control lines is MINISCULE compared to the time to read in a track. Say you're in a game and its reading the fire button 60x a second. Lets give it an absolute best case scenario, and every time you read it, the mod saves 12 clock cycles. So your game saves 720 clock cycles every second, or about 0.001% of the available clock cycles. You might not even be able to measure the speedup with a synthetic test. The only real chance of a measurable improvement would be serial port bit-banging; it *might* allow you to use a faster baud rate (but I doubt it). |
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Regarding the speedup, a logic analyzer would be the best option to REALLY see what's going on here. ;) |
I did some tests with mp3 decoding and didn't get any speedup :(
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Thanks to patrik a benchmark tool is now available: :great
https://eab.abime.net/showthread.php?p=1523681 Unfortunately, I now have my GVP G-Force 030 installed in my A2000 (waiting support for another long delayed project). So I can't provide any immediate benchmark results. |
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