I think a fundamental aspect of application programming
in C is that devices like disks that are read as blocks
(sectors) are presented to the application as character
streams.

What I think would be good is for the operating system
to have the same benefit, with a BIOS that makes an
entire disk look like a file that can be opened with
fopen("0x80", "r+b");

The BIOS can't return FILE * though, as FILE is defined
independently by the OS's C library. The BIOS needs to
be defined to return void * instead.

Another thing is that stdin should come as setvbuf
no-buffering with the implication that that means keys
are not being echoed, so the OS is responsible for the
echoing.

So what I envisage is something similar to UEFI but done
with C90-like functions.

Am I missing anything?

Thanks. Paul.