Why were TECO variables called Q-registers?

Question

The TECO editor, (originally Tape Editor and COrrector, later Text Editor...) found in some form on just about every computer and operating system DEC ever made, provides variables in which you can store numbers and/or text.

These variables are called "Q-registers".

Anyone know why that name was used? Is this related to anything on PDP-1 (which had the original TECO implementation)?

Answer 1

It's really more of a conjecture, but if it is considered suitable as an answer:

Already in early TECO, the letter q ("quantity") was used to retrieve the value of a register. That might have been the reason to call them Q-registers.

Trying to trace reasons for why things are named a certain way is always difficult; they way this usually goes is that someone comes up with a name, other people start using it, and the name then sticks. This process doesn't leave a lot of written records, and even if you ask people involved, they are probably not aware of how this happened.

Answer 2

TLDR: TECO uses single-character commands. When extending TECO to add registers to store numeric values, there was a limited set of unused characters left for the commands associated with this functionality. q was chosen as one of these command characters because it could serve as a mnemonic for "quantity." This led to the numeric registers being called "Q-registers" and, though an unfortunate misunderstanding, eventually this was applied to the completely separate set (albeit sharing the same names) of text-valued registers as well.

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dirkt's answer gives us the paper describing the timeline of how registers storing text and numeric values came about: "The Beginnings of TECO" by the original author of TECO, Dan Murphy.

On page 113 (PDF page 4), in the section "TECO becomes a programming language," Murphy describes the appearance of storage registers:

By late 1962, TECO had about the level of capabilities that simple editors would have for years to come, but TECO’s development didn’t stop there....

I added the concept of text variables—that is, a variable that would hold an arbitrary section of text. The command to move a range of text into a text variable was x. (I was running low on unused letters by then.) [emph. added] ...g (get) was used to copy the contents of a text variable into the main buffer at the editing point.... Despite the lack of a mnemonic association for x, it quickly came to seem natural, almost intuitive. To x something became as easily spoken and understood by TECO users as "store" would have been.

He goes on to discuss a separate set of registers for numeric values, sharing the same names:

Integer variables were also added around this time, again from the single-letter set a–z, 0–9. u (use) would store its argument into the variable, and q (quantity) would stand as the stored value in an expression. The same variable name could have both a text and numeric value; the command determined which was being referenced.

"Q-register" was a natural name for the registers retrieved by the q command. The name "Q-register" had been codified by October 1964: Daniel J. Edwards, in PDP-6 Memo No.2 / Memorandum MAC-M-191 ("TECO 6") describes them and the five commands that operate on them (p.4):

Q Registers

     Q registers are general storage registers for

         1) numbers
         2) buffer pointers
         3) text
         4) macro definitions

There are 36 Q registers and they are referred to by Qn where n is a
single letter or digit. The following commands pertain to Q registers.

xUQn                The value of the arithmetic expression x is placed
                    in Qn.

mXQn                m lines of text starting at the current buffer pointer
                    are copied into Qn.

b,cXQn              Text between buffer pointers b and c is copied into Qn.

QnG                 Get text stored in Qn and insert at the current text
                    buffer pointer. The contents of Qn are unchanged.

QnM                 Macro: text stored in Qn is executed as a TECO com-
                    mand string....

MIT was using what appears to be a separate line of TECO development (probably starting with the PDP-1) that merged the Q and G registers together, as you can see above. (Peter Samson's July 1965 AIM-81 "PDP-6 TECO" also describes merged registers, though mysteriously it seems to have separate commands again for moving numeric and textual values out of the registers; using the wrong one "does not make sense" (p.5), presumably resulting in rubbish output.)

The original version with separate "Q" and "G" registers either continued to be developed or the split was restored, but MIT's usage seems to have stuck even for these versions. The Standard TECO User's Guide and Language Reference Manual (May, 1985, describing TECO-11 version 40, TECO-10 version 3 and TECO-8 version 7) uses "Q-register" to refer to both the text and integer registers though these are clearly two separate sets of storage:

3.3.2 Q-registers

TECO provides data storage registers, called Q-registers, which may be used to store single integers and/or ASCII character strings. Each Q-register is divided into two storage areas. In its numeric storage area, each Q-register can store one signed integer. In its text storage area, each Q-register can store an ASCII character string (which can be any text, including the important case of a TECO command string).

So, to summarize, the numeric registers were reasonably referred to as "Q-registers" because they were retrieved with the q command, the merge of "Q" and "G" registers caused MIT writers naturally to use just one name for the merged registers, and the split-register versions of TECO picked up this usage probably because it's simpler to say, and the two sets could also be conceived as a single set due to the shared names.