In 1959, when Doug Eastwood and I, at the suggestion of George Mealy, set out to add macro capability to SAP (Share assembly program), the word "macro"--short for "macroinstruction"--was in the air, though none of us had ever seen a macroprocessor. We were particularly aware that GE had a macro-capable assembler. I still don't know where or when the term was coined. Does anybody know?

We never considered anything but recursive expansion, where macro definitions can contain macro calls; thus the TX-0 model comes as quite a surprise. We kept a modest stack of the state of each active macro expansion. We certainly did not foresee that within a few years some applications would need a 70-level stack!

General stack-based programming was not common practice (and the term "stack" did not yet exist). This caused disaster the first time we wrote a macro that generated a macro definition, because a data-packing subroutine with remembered state, which was used during both definition and expansion, was not reentrant. To overcome the bug we had in effect to introduce another small stack to keep the two uses out of each other's way. Luckily there were no more collisions between expansion and definition. Moreover, this stack needed to hold only one suspended state because expansion could trigger definition but not vice versa.

Interestingly, the problem in the previous paragraph is still with us 65 years later in many programming languages. To handle it gracefully, one needs coroutines or higher-order functions.

Doug