Dtyjlui

Terminal escape codes can be used for various functions such as positioning the cursor or querying the size of the screen. What was the largest resolution (in columns and rows of characters) supported by the encoding of escape codes?

The question may be answered in the context of a standard (such as ANSI), or for a particular terminal model; either is acceptable.

There is no limit, at least not by definition. And more so, not something across all terminals ever made.

Most prominent nowadays (*1) is the so-called ANSI sequences standard - understood by next to all terminals built after ~1978. Trying to be as device agnostic as possible, it implies no upper limits by itself. The structure is built (*2) around an initiating sequence (ESC+[; *3), one (or more, separated by ;) value(s) expressed in ASCII numerals, and a command letter.

For example:

• 
  <ESC>[<Value>A moves the cursor <Value> lines up.

or

• 
  <ESC>[<Value1>;<Value2>H moves the cursor to line <Value1> and column <Value2>, counted from the top left position (as 0;0).

There is no definition how many valid digits a value may have, or what's their upper limit. While the standard defines a default value of Zero for a left out parameter, it doesn't always apply, as these examples conveniently display. Where Cursor-Up assumes 1 as default, if no value is given, Cursor-Positioning assumes 0;0 as default (*4). Similarly, upper limits are defined by each command and terminal depending on their capabilities (and how intelligent or lazy the programming was done).

*1 - Well, since the late 70s at least that is.

*2 - And based around the command sequences defined for the popular DEC VT-52 of 1975.

*3 - Or CSI (Control Sequence Indicator), X'9B' - that's ESC set with high bit set. A trick to reduce overhead after 8 bit lines became standard. Not that it meant a lot with the verbose encoding anyway.

*4 - It is simply a left over from ANSI-encoding being an extension of the VT52 command set - here ESC+ A moved the cursor up one line, so for compatibility it had to do the same in the later standard.

Tektronix (and emulated Tektronix) graphics mode

Not exactly what you are looking for (because not "characters"), but Tektronix 4010/4014 mode as supported by Wyse 99GT and Wyse 370 (and I assume by actual Tektronix terminals, though I haven't used those since ~ 1986), supported Escape Codes for positioning on a 4,096 x 4,096 virtual display. According to the Wyse 370 manual, the actual display is a (simulated, of course, not actually that high resolution) 4,096 x 3,128.

The Wyse 370 could actually display up to 161 columns of text, though I never used it in that mode.

In the early 1980s practically every terminal had its own particular set of escape codes, to the point where the software we were selling then had a separate module which we could quickly reprogram as each customer sent us a copy of their terminal's manual. This continues at least as far as 1984.

Some "grown-up" terminals did indeed use escape sequences that gave X and Y coordinates as straightforward decimal numbers, sequences of characters between '0' and '9'. But this demanded exceptionally high computing capacity in the terminal itself, and of course also higher bandwidth in the connection from computer to terminal. Thus our "terminal adaptor" layer maintained two memory maps, one of the screen as it ought to be and one of the screen as it currently was, and its main job was to work out the "optimum" way of turning "current" into "ought to be". For instance, if characters 1 and 5 on a line have changed, is it better to send "character 1, escape sequence, character 5", or "character 1, character 2, character 3, character 4, character 5"? Life was made more interesting by the fact that some terminals had short ways of moving from one position to another, other than a full X-Y positioning (think of the LF character, for an example).

So the answer to the question is that in the minority of terminals that thought of themselves as cut-down supercomputers and actually did all the multiplications and additions required to convert "1,0,0" into the number 100, there was no intrinsic limit to the addressable positions on the screen. But most terminals would try to encode each coordinate in one character, which meant a maximum of 95 or so if you confined yourself to printable characters or a maximum of 128 if you allowed any character at all. What encoding was used, and whether X came before Y or Y came before X, was down to the manufacturer's whim.