3 ms·

How is the PDP-10 big endian if it has no byte addressable memory? I would agree that culturally it's in the big endian camp because bits are numbered MSB to LS

• gumby 4y ago
  Bytes are very much addressable, they are simply not limited to only being 8 bits. The 8-bit convention was uncommon when these machines were designed -- I think it was only on some IBM machines at the time. Remember the 10s, like several contemporary machines, had 36-bit words with an 18-bit address space (yes Gordon Bell specifically designed them with Lisp in mind) As for the technical reason, consider wiring up the arithmetic unit by hand (work it out on a piece of paper). If you don't have an architecture book handy, look at Ken Sheriff's walkthrough of the Z80 die and consider some of the extra complexity (less of an issue of course for an MPU that was an IC) to handle little endian arithmetic ops (routing carry and such).
  • aap_ 4y ago
    Not sure I get your point. The PDP-10 has no notion of bytes in the arithmetic unit, it's all words (and hence there's no endianness). The byte instructions are handy ways to load/store parts of words, sure, but I feel like they're more similar to modern bitfield instructions and feel rather tacked on (they were optional in the KA10 even) rather than being a central part of the architecture. Come to think of it though, byte pointers move right in the word as they're incremented. Guess you could count that as big endian.