A Personal History Porting...

RCA 1802 and the ACE
commonality of systems, EFI maps differed

Z80's and CP/M
BIOS and BDOS standardized
custom I/O addresses for each system

Early'ish Days of UNIX
is it "BSD" or "SYSV"
serial interface issues
HP-UX with BSD guts and SVID Compliance

Emergence of Free OS Community
Almost Entirely on IA-32 Systems
Minix -> 386BSD -> BSDI -> Linux

Debian
ia32 was "home", m68k was the first "port"
but... Debian now runs on something like 11 architectures!
Debian even runs on more than one kernel...