aries-mu, the onboard IDE controller on UM8881/8886 chipset boards, like the MB-8433UUD, will not work at 40 MHz. You will need to use a BIOS-controlled 2/3 PCI divisor to run the PCI bus at 27 MHz for the onboard IDE to work. I don't recall if the IDE controllers on SiS 496/497 boards work at 40 MHz.
The simple answer is that double-bank cache allows for less wait states, especially on marginally stable configurations, compared to single-banked cache. Double-bank cache allows for interleaving. Double-bank cache generally uses 9 SRAM chips, while single-banked cache uses 5 chips. The MB-8433UUD natively allows for up to 256K of double-banked cache, or 512K of single-banked cache. The MB-8433UUD can be modified with some effort to allow for up to 512K or 1024K of double-banked cache.
Normally,
9 pieces of 32Kx8 DIP-28 SRAM modules make 256K double-banked
4 pieces of 32Kx8 DIP-28 SRAM modules + 1 piece of 64kx8 makes 256 single-banked
8 pieces of 64Kx8 DIP-32 + 1 piece of 32Kx8 makes 512K double-banked
4 pieces of 128Kx8 DIP-32 + 1 piece of 32kx8 makes 512K single-banked
8 pieces of 128kx8 DIP-32 + 1 piece of 64Kx8 (or sometimes 128kx8) makes 1024K double-banked
Personally, I would search for FPM RAM as it is more versatile and doesn't degrade performance. For example, on eBay, you might search for "SIMM 72 32 MB FPM" 72 is for 72-pin SIMMs. 32 MB is the module size. Every time I search for RAM, I also check the datasheets for the individual IC's on the module, because sometimes you get 3.3V SIMMs being sold as 5 V. All consumer-ended 486 board's I've seen use 5 V.
When searching online for SRAM, it is best to hunt down some part numbers and search from there. E.g. 'IS61C1024' for 128kx8 modules, 'W24512AK' for 64kx8 modules, or 'W24257AK' for 32kx8 modules.
You generally cannot modify the PLL in the computer, however, there are very often undocumented jumper settings which yield 50, 60, and 66 Mhz outputs. Sometimes you need to solder on a missing jumper header, or sometimes the jumper headers are already in place and you just need to try different jumper combinations.
Promise Ultra 100 is an IDE controller which works on my 486's. The drawback, though, is that I cannot get a CD-ROM drive to work with it in DOS. This seems to be a known limitation. The CD-ROM will work in Windows though. For such situations, I tend to use the Promise for the HDD and the onboard IDE for the CD-ROM, but then you run into that 40 MHz FSB issue when using the onboard IDE. So if I am using the FSB at 40 MHz, I tend to use a SCSI controller card, which allows for bootable CD-ROM's, CD-ROM access in DOS, and HDD access. If you like, you can just use an ISA SCSI card, e.g. 1540/1542/1520/1522, just for the CD-ROM and us the Promise for the IDE HDD. SCSI HDD's are getting hard to find, especially the quiet ones. Although, there are SD to SCSI solutions, http://store.inertialcomputing.com/
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Wait states rae adjusted in the BIOS. FSB frequencies are adjusted on the hardware, usually by jumpers. The PCI freq is normally the same as the FSB, unless you go into the BIOS and set the PCI freq. to be 1/2 or 2/3 of the FSB. 2/3 option is very rare and I've only seen it on UM8881/8886 based boards, namely the M919 and MB-8433UUD (it is automatic on the M919).
There is no pipeline burst for 486. you are limited to direct-mapped cache. Not possible to buy cache at the "computer store" in 2018. Online shopping is infinitely larger than the "computer store".
Not possible to run a 486 in a Pentium motherboard. The exception are high-end workstations or servers from around 1992-1995 which used riser cards for their CPUs. You could get 486 or Pentium riser cards. Such systems sometimes allowed for dual 486 risers.
I do not have any experience with VLB or PCI cards at 50 MHz.
Adaptec AHA-2842A is a nice VLB SCSI controller because it has a jumper which allows you to run the CPU's L1 cache in write-back mode, assuming the motherboard supports WB L1 cache.
If anyone knows of PCI Ultra 33/66/100 IDE controller which allows for DOS CD-ROM access and/or CD-ROM bootability, please let me know.
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