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Hi,

I am not sure about a thing.
Say I have a sata hdd, a ssd, a usb key, a pci-e graphic card, a usb keyboard and a x86_64 architecture.

Apparently, the devices are all memory mapped.

So, is it correct to assume that I will never use in/out instructions on hardware 2011+ old (except on boot with the a20 gate and other legacy things)?

Bye

I believe that that is true, yes. But remember that you will need to write a driver; there is no simple way to do input and output via e.g. pci devices.

I wouldn't go that far. I'd be suprised if you can build a whole kernel with no in/out instructions. I think OHCI USB controllers are I/O based, at least partially, so avoiding it entirely may be out of the question.

I really like to ask extreme questions

So, how may I know if and when I need those instructions?

Rule of thumb or something?

The hardware documentation will tell you when you need to use port-mapped I/O.

In most cases, a flag in the PCI device's configuration space determines if the device uses memory addresses or I/O addresses.

Great,

Thanks !

Unless everything is PCI-e you will still need to use I/O ports.

As far as I recall the ACPI SMI_CMD port can only be an IO port, thus you'd need at least one out instruction to switch to ACPI mode.

Regards,
John.

Some DMA and NICs are based on I/O ports. ACPI is based on I/O ports. You will still need them in your kernel, even if they are rarely used.