DOS/4G and DOS/4GW FAQ:
Hardware I/O

1. Can I virtualize I/O ports under DOS/4G?
2. Is there any restriction on using the I/O address space access functions, such as inp(), with DOS/4G?

1. Can I virtualize I/O ports under DOS/4G?

We've been approached about this several times, so we have a pretty good idea about what's involved and it isn't pretty.

You would need to set up a TSS that had an IOPL that caused trapping for accesses to your I/O ports. Unfortunately, DOS/4G doesn't normally use a TSS, although it has at various times. What's worse, we're not sure how you would switch from the 4G TSS (if it existed) to yours. This is complicated by the fact that DOS/4G programs run in ring 0 under native DOS or a VCPI host, and at ring 3 under a DPMI host like Windows or Win 95 or NT or OS/2. Also, DPMI doesn't allow for more than one extender, so you can't even set up something to trap the references before 4G comes into play.

Seriously, doing what you want is definitely possible, but it is also definitely not easy. You would need to replace a bit of the extender, which isn't too hard but makes the packaging issues a bit sticky.

So far, we have not decided to do I/O port virtualization.

2. Is there any restriction on using the I/O address space access functions, such as inp(), with DOS/4G?

No. We neither add nor subtract from the I/O address space that can be addressed. This is controlled by the operating system that you are using.

For example, using plain DOS will let you address all 65K I/O ports, while in a Windows/NT DOS box, you will probably not have access to very many ports (although NT does simulate some)!