← Back to context

Comment by buredoranna

9 hours ago

Still my all time favorite snippet of code.

    TC    BANKCALL    # TEMPORARY, I HOPE HOPE HOPE
    CADR  STOPRATE    # TEMPORARY, I HOPE HOPE HOPE
    TC    DOWNFLAG    # PERMIT X-AXIS OVERRIDE

https://github.com/chrislgarry/Apollo-11/blob/master/Luminar...

9 comments

buredoranna

Reply
f1shy  6 hours ago

Cadr here has no relation with lisp cadr, right?

  • jasomill  5 hours ago

    Correct.

    CADR is an AGC assembly directive defining a "complete address" including a memory bank, in this case a subroutine to be called by the preceding BANKCALL (TC = transfer control, i.e., store return address and jump to subroutine), which switches to the memory bank specified in the CADR before jumping to the address specified in the CADR.

    For a brief explanation of AGC subroutine calls, see [1].

    CAR and CDR in Lisp come from the original implementation on the IBM 704, where pointers to the two components of a cons cell were stored as the (C)ontents of the (A)ddress and (D)ecrement fields of a (R)egister (memory word).

    (CADR x) is just shorthand for (CAR (CDR x)), i.e., a function that returns the second element of a list (assuming x is a well-formed list).

    [1] https://epizodsspace.airbase.ru/bibl/inostr-yazyki/American_...

foxyv  4 hours ago

I'm having a really bad Mandala effect right now where I remember some XKCD that wrote a poem about this. Maybe I'm thinking of another comic.