Maybe I'm misunderstanding what you mean by a card, but the 7090 used 80-column punch cards of which 72 columns could be read into the computer. (Which is why Fortran used 72-character lines.) The 7090 had a plugboard so you could select which 72 columns you wanted, but normally it would be the first 72. Why 72? Because cards were read "sideways" and 72 columns fit into two 36-bit words.
That is, cards were read row by row, not column-(character)-by-column — one ‘bit’ from every character at once. And this is the case for practically all card equipment. That surprised me a bit when I first learned it, as someone used to sequential character streams, but it makes sense in the context of pre-computer card operations, where cards and holes started and stopped counting wheels for each digit (column) at once to calculate totals.
We (they) have this mostly under control, but for one single function, called "LETTER." that we cannot find the code for, and are having trouble divining the function of.
The code of concern is here:
If you search for "LETTER." there is a single call to it on card 00386. (MAD functions all end with "."; There are several occurrences of LETTER as labels, but only one function call.) This is in the function(entry) LISTRD. (000096) [This was in the days when functions could have multiple entry points in order to conserve memory.] and is used to load an array called KNOW (indexed by I). LISTRD is an s-expression reader (but into SLIP not LISP -- see below for LISTRD documentation).Some potentially important factoids: The 7090 had 36 bit words and packed 6, 6-bit BCD characters into each. (You can see this in line 000356 where it appears to be packing 6 close parens (BCD 34k) into a word: CARD(I)=343434343434K (K for octal). Notice cards are (were) 84 characters in width and 84/6 is 14 -- the number 14 is used in multiple places as a loop limiter, so it's apparently scanning across cards (and the minimal comments and some variable names suggest this as well).
[Unfortunately, comments were used quite sparingly in those days because you had to punch them into cards!]
Again, the only function for which we don't have code is LETTER. So, okay, Sherlock Hackers...What exactly does LETTER. do and what exactly is it doing here?
Here's a SLIP manual:
LISTRD is documented on pg. 24 of the above manual.(Possibly important is that lists had to start in column 1 with an open paren.)