No BIOS necessary when we're talking about bare metal systems. printf() will just resolve to a low-level UART-based routine that writes to a FIFO to be played out to the UART when it's not busy. Hell, I've seen systems that forego the FIFO and just write to the UART blocking while writing.
I hope nobody was confused into thinking I thought a BIOS was required, I was pointing out the evolution from this to a monitor. I've written some code[1] that runs on the STM32 series that uses the newlib printf(). I created the UART code [2] that is interrupt driven[3] which gives you the fun feature that you can hit ^C and have it reset the program. (useful when your code goes into an expected place :-)).
Yup, I recall Atari ST (68000) and BBC Micro (6502) having unbuffered
and interrupt access to 6402 UART - which I used to C/ASM to fire MIDI
bytes to and from.
