eharmon
Well-known member
Oh using the jumper that way is very cute. I'll probably give that a go next time my board is out to add some flexibility and easily swap between stock and whatever customized configs I have in the other configuration.
For now I've been hacking the LC 475 ROM directly to futz with even more customizing timings. A few discoveries:
1) Booting the machine with (non-bump) 40MHz timings seems to pull RAM performance down by 11-13%, per Norton SysInfo. Overall system performance is about the same otherwise. Not the worst impact, and really isn't bad for a 21% clock increase.
2) The LC 475 ROM doesn't seem to have the bump config tables at all, only the regular configs, or at least not in the same places. Which might explain why it applies non-bump (faster) timings than a stock ROM would on 80ns machines (C650/Q650). So simply swapping in the LC 475 ROM serves as a minor overclock for memory.
2) The custom ROM I'm using has 55ns chips, which means you can push the ROM Speed a little, even at faster clocks. Lower is better. Stock Q800 runs at a speed of 3, stock Q650 at 4, and the respective 40MHz configs at 4 and 5. Even running at 44MHz I can push that down to 2 and, presumably, get faster ROM reads, so toolbox calls should run faster. At 1 the machine gets very unstable, and I'm guessing at 50MHz you're gonna need 3+, even with fast ROM chips. But that adds another dimension to overclocking. Not sure there's any ROM read test out there, might be worth creating to benchmark/verify checksums to ensure it's running stable.
For now I've been hacking the LC 475 ROM directly to futz with even more customizing timings. A few discoveries:
1) Booting the machine with (non-bump) 40MHz timings seems to pull RAM performance down by 11-13%, per Norton SysInfo. Overall system performance is about the same otherwise. Not the worst impact, and really isn't bad for a 21% clock increase.
2) The LC 475 ROM doesn't seem to have the bump config tables at all, only the regular configs, or at least not in the same places. Which might explain why it applies non-bump (faster) timings than a stock ROM would on 80ns machines (C650/Q650). So simply swapping in the LC 475 ROM serves as a minor overclock for memory.
2) The custom ROM I'm using has 55ns chips, which means you can push the ROM Speed a little, even at faster clocks. Lower is better. Stock Q800 runs at a speed of 3, stock Q650 at 4, and the respective 40MHz configs at 4 and 5. Even running at 44MHz I can push that down to 2 and, presumably, get faster ROM reads, so toolbox calls should run faster. At 1 the machine gets very unstable, and I'm guessing at 50MHz you're gonna need 3+, even with fast ROM chips. But that adds another dimension to overclocking. Not sure there's any ROM read test out there, might be worth creating to benchmark/verify checksums to ensure it's running stable.
