[CodeWriter23]

Well, it’s own doc says the OS occupies 2 sectors, IOW > 512 bytes. Doing all those system calls in less than 1K is still impressive though.

[WesolyKubeczek]

Says 512 bytes of machine code, not "fits in the MBR". You can only cram 355 or so bytes of code into MBR area designated for that.

[FreeFull]

If you're fine with overwriting the partition table, you can go up to 510 bytes of code, followed by 2 bytes for the mandatory MBR boot signature

[CodeWriter23]

No partition table on floppy disks.