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ISAWAIT

(C) 2022-2023 Eric Voirin (oerg866)

ISAWAIT is a DOS program to change and set the ISA I/O Recovery Timing.

This program exists because some boards, such as ECS P5VX-Be have BIOSes which sets this up with an optimistic default and have no option in the BIOS to change this.

As of now, it supports the following chipsets:

Vendor ID Device ID Name
8086 112E Intel PIIX
8086 7000 Intel PIIX3
8086 7110 Intel PIIX4(E)
1039 5113 SiS 5113
1039 0008 SiS 559x

Running

Usage is as follows:

ISAWAIT.EXE <8-bit IO Recovery Timing> <16-Bit IO Recovery Timing>

  • 8-bit IO Recovery Timing:

    This is the amount of extra clock cycles to wait after performing an 8-Bit ISA I/O operation.

    The range is from 1 to 8.

    To disable additional wait states (i.e. default minimum wait of 3.5 clocks), set it to 0.

    If you wish to not change this value, use -1.

  • 16-bit IO Recovery Timing:

    This is the amount of extra clock cycles to wait after performing a 16-Bit ISA I/O operation.

    The range is from 1 to 4.

    To disable additional wait states (i.e. default minimum wait of 3.5 clocks), set it to 0.

    If you wish to not change this value, use -1.

If you just wish to see the current values and change nothing, use:

ISAWAIT.EXE -1 -1

This tool works with EMM386 present, and may be put in AUTOEXEC.BAT. It should play nicely with MS-DOS, IBM PC DOS, DR-DOS, FreeDOS and Windows 95 / 98.

Building

  • Download and install the OpenWatcom compiler.
  • Change to the repository folder
  • Type wmake

That's it!

Difficulties when writing ISAWAIT

I wanted to do this in Real mode using a contemporary toolchain for that. Unfortunately none of those seem to have any concept or any knowledge of Intel's 386 CPU whatsoever.

As such, the required 32-Bit I/O port operations required to access PCI configuration space are not available (the OUT instruction operates on an extended register in this case.)

To fix this, I emulate those instructions in inline assembly using the db keyword, and make sure to prefix those with 0x66 to enable the extended registers (else you'll get missing words ...).

It works, but it is probably a lot more headache-y than it needs to be ... :-P