INT 21 is DOS, not BIOS.

You can still use INT 10 for BIOS video services.

Or just write into the text framebuffer directly; it's at B800:0000 in colour modes and B000:0000 in monochrome modes (it's surprising how much I still remember from when I exclusively did x86 PC Asm, ~3 decades ago.)

Err I don't know... Didn't the bios set the video controller up for you? Not bare metal enough, I'm afraid. /s

On a random, but interesting note though, manuals from some of the early IBM PC's have the BIOs source listing in them

I had a giant pink book of PC BIOS that was my bible from 1988 to 1994 before the internet took off. I can't for the life of me remember who published it, but it had everything you needed to know about PC BIOS and an old IBM bios source listing. It was over 600 pages.

It look liked this 1200 page book, but this isn't it (also a book I used):

https://www.amazon.com/PC-Interrupts-Programmers-Reference-T...

I also had the Apple ][+ ROM listing in one of those spiral-bound books from apple circa 1980 but that too has been lost, sadly.

Sounds like the The Programmer's PC Sourcebook (which is on my shelf, and yes, one could say it has a pink cover).

YES! That is exactly it. I wish I saved mine for nostalgia.

We had something like this but it was a four-inch thick printout on green-bar paper and nobody knew where it came from.

e.g. https://archive.org/details/bitsavers_ibmpcpc602renceAug81_1...

https://sites.google.com/site/pcdosretro/ibmpcbios has the source code for various models typed.