darry wrote on 2020-06-20, 19:29:

_UV_ wrote on 2020-06-20, 19:09:

One thing many may be missing - IRQs, PS/2 mouse need IRQ 12, which can be used for some PCI devices such as USB/Ethernet/SCSI/etc. Serial mice need IRQ too, but idk about bad consequences of enabling COM1 interface for using with mice.

COM ports take IRQs too . On a at least one Pentium 1 board I have had, IRQ12 was used for PS/2 port whether PS/2 mouse was connected or not . There was no way to disable it and re-use IRQ 12 for anything else . Since then, I have just used the PS/2 port whenever available .

With decent BIOS you could disable IRQ for PS/2 devices. IRQ 12 could be used for PCI devices, also IRQ 15 (disabled second IDE channel). And i know at least 😀 two devices which can use it Adaptec SCSI and Intel NIC. And this especially good for old systems prior to i815 (example, first intel chipset that mooved ATA away from shared PCI bus, like AGP), because SCSI (or more modern SATA which identify yourself as "unknown SCSI" device) saves you from DMA errors on some chipsets or free roam implementations of ATA devices (including interface cards...), i have a very bad experience UDMA 66/100 HDDs on PIIX4 (BX/TX) and enabled DMA, which working well in pure DOS due too using just plain PIO4.

Now back to COM interface, devices past 386 era tend not use this IRQs by default, except maybe some 3Com NICs. So if you have serial mouse, you probably have free IRQs for other important things not fighting each other. Thats all about older systems up to PIII and W98, where it really matters. If you building something like P3/GeForce/Live and thats all, you will probably never have any issues besides AGP.