My 486 (nominally 40MHz) had one of those jumper blocks, and it was right near the front corner of the motherboard. Right behind the floppy-drive opening in the case, which had the drives mounted in these little removable sleds. And I didn't use my floppy drive much, and besides, the floppy cable didn't seem to mind being hot-plugged as long as you weren't accessing the disk at the time and unplugged the power connector first.
So during a long download, I didn't need all 40 MHz screaming along (and heating up the chip to the point that it needed a cooling fan -- a COOLING FAN, can you imagine a CPU running so fast it couldn't cool itself on ambient air?), so I decided to see if the clock generator jumpers were hot-pluggable.
Lo and behold, they were! I could reach in and seamlessly downclock the CPU to 8MHz (which was just one jumper-cap different than the 40MHz setting), which was still plenty to service the UART FIFO interrupt. Unplug the CPU fan too, which made the machine silent. Turn the monitor off, kick back in my chair, and take a catnap. The Telemate terminal software would play a little tune when a download finished, which would wake me up, I'd turn the monitor back on, open a DOS prompt, start unzipping the file, and then reach in and clock the CPU back up so the pkunzip process would finish in a timely manner.
It would do 50MHz but the upper half of RAM would disappear, so there weren't a lot of workloads appropriate for that configuration....
and heating up the chip to the point that it needed a cooling fan -- a COOLING FAN, can you imagine a CPU running so fast it couldn't cool itself on ambient air?
This feels super nitpicky but I'm curious about your setup and if you're either remembering the clock speed wrong or if the fan was actually completely extraneous, because in fact neither of the common 40 MHz 486 parts, the Cyrix Cx486DX40 or the AMD Am486DX40, required a fan. The Cyrix one came with a heatsink, which was a rarity at the time.
The first 486-class CPU that pretty much always ran with active cooling was the DX4/100. Even the DX2/66 could run fanless if you had half decent airflow.
I'm certain of the clock speed, but I think you're right that the manufacturers alleged they were fine without additional cooling. (They considered it a bad look, and actually said that chips sold with fans were likely overclocked chips intended for a lower speed bin, or otherwise graymarket.)
But consensus among everyone _but_ the manufacturers was that additional cooling couldn't hurt. (A representative opinion can be found in Upgrading And Repairing PCs, whatever edition was current at the time.) Running right at the top of Tcasemax wasn't good for longevity in terms of electromigration within the chip itself, nor for the capacitors and other components in the neighborhood. Thermal goop wasn't commonplace yet, but the little heatsinks and fans sold like hotcakes (har!) at the local computer shows. Plain aluminum heatsink, clear (polystyrene?) fan, with a holographic "CRYSTAL COOLER" sticker on top. I still see the fans around, but without the shiny sticker.
The Am486DX-40 was my favorite chip. With a VLB video card (Trident 9400CXi) that worked well on the 40MHz bus, its pure pixel-pushing power ran rings around 33MHz-bus systems regardless of their core clock, and that included the P-75. I later got the impression that I lucked out with that Trident card, as almost everyone else with a 40 or 50MHz VLB machine had tales of woe and flakiness.
Thanks for the reply! That is interesting and makes a lot of sense.
Yes, running a 40 or 50 MHz bus made a huge difference, especially if you could get VLB graphics running reliably on it. I'm into collecting and tinkering with 486-era machines for nostalgia's sake and often i see things like DX2 or DX4 systems with plain cheapo 16-bit ISA graphics cards and think such wasted potential...
I was pretty sure none of the 486 CPUs absolutely required a cooling fan (though I only had a DX2/66MHz): I remember Pentium II being the first CPU I've seen that absolutely required it (came with one integrated on the second computer I assembled for myself), and I distinctly remember having trouble testing an AMD CPU a year of so later on a computer I was assembling for someone else because... well, it wouldn't even POST without a CPU cooler.
Luckily I did not fry it and after adding a cooler it worked just fine.
I don't remember if AMD or Cyrix CPUs were worse though.
There were cards that had a pot on the back. You could just dial up more speed, until your machine ran poorly, then back off just a little. Kind of crazy to think about!
The speed on my Apple FastChip is adjustable in real time too. It's neat to just dial a speed appropriate for the application at hand.
If you are interested in programming your Apple in assembly, you can ask nicely for your FastChip to include a 65816 processor. It's going to act like a 65802, due to hardware limitations, but otherwise yeah. You get the 16 bit instructions to use.
I've not had any compatibility trouble with mine, which is a 65816.
I am pretty sure I've had this on at least my 80286, and maybe even 80386 and 486DX2 (33/66MHz I think) too (though highly uncertain on the latter). Perhaps you only needed a case that would connect the turbo switch to the motherboard.
It was fun to turn it on for games that used timing-loops for frame rendering to make games twice as fast :)
Do I recall correctly that those turbo buttons would, counterintuitively, actually down-clock the CPU? For compatibility with software that had hard-coded timings or something?
The correct way to wire them is so that turbo "on" means full speed and turbo "off" means slowed down. Different motherboards implemented it differently. Usually downclocking the FSB or inserting waitstates for memory access.
They originated with "Turbo XT" class machines which ran an 8088 but at 8, 10 or 12 MHz -- faster than a real IBM PC/XT. Turbo on meant a faster machine, and turbo off meant 4.77 MHz -- fully compatible with timing-sensitive PC software.
Later, in the 386/486 whitebox PC era, some machines had the buttons wired wrong and now it's a meme that turbo made the computer go slower, but that was never true for systems built correctly.
Yes. Old software had timing loops and other delay constructs. For a while, that button was meaningful when running games intended for the original clock rates.
I discovered this a few years ago playing Ultima IV on my Amiga 3000, which is 16Mhz. It was impossible to play (and funny to look at) because the game was so sped up. All of the NPCs in the game, which normally just stand in place and move their arms about, were moving so fast it was bananas. You could barely see their arms they were like hummingbirds.
The game was intended to be on a 7Mhz stock machine pre-1990. And it was perfectly timed to that speed.
Seriously. The guy put in a good amount of time and delivered an Altima with many modern sensibilities baked in. I've been playing it on my Apple it's a lot of fun.
So during a long download, I didn't need all 40 MHz screaming along (and heating up the chip to the point that it needed a cooling fan -- a COOLING FAN, can you imagine a CPU running so fast it couldn't cool itself on ambient air?), so I decided to see if the clock generator jumpers were hot-pluggable.
Lo and behold, they were! I could reach in and seamlessly downclock the CPU to 8MHz (which was just one jumper-cap different than the 40MHz setting), which was still plenty to service the UART FIFO interrupt. Unplug the CPU fan too, which made the machine silent. Turn the monitor off, kick back in my chair, and take a catnap. The Telemate terminal software would play a little tune when a download finished, which would wake me up, I'd turn the monitor back on, open a DOS prompt, start unzipping the file, and then reach in and clock the CPU back up so the pkunzip process would finish in a timely manner.
It would do 50MHz but the upper half of RAM would disappear, so there weren't a lot of workloads appropriate for that configuration....