This overlaps with experience with the first Fortran.

About 1957 at Livermore I was rummaging around the filing cabinets for punched cards. I came upon a drawer mostly full of cards with a small deck of binary cards at the front wrapped in a rubber band. The deck was in binary and started with a familiar loader and was less than an inch thick; the rest of the cards were symbolic (Hollerith.) There were a few pages of description of a new assembler format for the IBM 704 that differed considerably from the NYAP from IBM that we were using. I guessed that the binary was the assembler proper and that the rest was the source for it. That turned out to be the case.

There are three programmer’s whose assembler code I have found especially rewarding to read:

The 704 assembler, originally called SAP, was written in its own language by Roy Nutt. The source was just 2000 lines, for it fit in one card drawer. The above three programmers all wrote code that was easy to read from the bottom up. It was clear what the subroutines did if you had first read the deeper routines; there was no recursion. That dispelled my illusion that system routines were ugly, boring and difficult to write. Actually most are, but there are a few wizards who seem able to make them small, fast and elegant, and incidentally, easy to read. Alan Perlis said:
Fools ignore complexity. Pragmatists suffer it. Some can avoid it. Geniuses remove it.

I was soon able to adapt the assembler for our use; we did not have the source for NYAP. Indeed this was perhaps the first large program I had seen where the output of the assembler would actually run correctly. Debugged programs had generally been patched versions of assembler output. The programmer would patch by punching a few binary commands on a card and adding that to the end of the binary deck from the assembler. He would hopefully annotate the assembler listing so that the same change would be made to the source. The debugging cycle did not generally afford an assemble step. The cost of reading 2000 cards thru a card reader at 150 cards/min and then printing at 75 lines/min made that infeasible. In one (just one) extreme case the programmer threw out the source assuming that he would never be able to afford to assemble it again. Here is a bit more detail on the assembler’s function.

We developed an alternative debug cycle for the Stretch which was to manually annotate the assembler listing, make a corresponding card deck defining the symbolic change, or add changes to the change deck used in the previous debug cycle, bring the symbolic source tape and change deck to the machine and assemble or compile without a new listing. The assembler and compiler listing included line numbers to make preparation of change decks easy. When the annotations became burdensome, a new listing and magnetic tape was made and the change deck was retired and a new one began to grow. The Fortran compiler worked the same way.

For each instruction of the program an assembler listing would include the instruction in binary (octal) and its symbolic source precursor on one line. Some assemblers provided symbolic cross references. Recent assemblers do not produce listings. Debugging was done at a desk away from the computer with an assembler listing and a binary (octal) printout of memory (dump) at the time of the program failure. This was feasible when memory cost one dollar per bit.