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Were there any home computers before 1985, on which you could create a loop (finite or infinite) in direct-mode?

And if it was possible on some machines, what may it have been useful for? For example, some sort of program loader from tape or disk.

For example, by typing

    PRINT "TEXT" : RUN

or

    PRINT "TEXT" : GOTO 0

so that it would loop the printing of "TEXT", just as an example.

EDIT - Since I am completely blocked ( due to some unknown problem ) from commenting and now also from posting any answers, I just want to add that it seems that you can use a pre-user-defined function in direct-mode, by first defining the function in a program using line numbers ( e.g. 10 DEF FNADD(X,Y)=X+y ) and then using it in direct-mode, so then trying to do a recursion trick like FNADD(FNADD(X,Y),Y), to try and cause an infinite loop using recursion, would be interesting, and I wonder why pre-user-defined functions can be accessed through direct-mode .

mnml
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    There was no virtual line number 0 in any BASIC I ever played with in the 80's. GOTO X would always jump to that line in the current program (essentially a RUN without clearing variables or unDIMing arrays)--and this might be something you really wanted to do if you stopped your program (either with BREAK key or STOP statement) and then didn't want to CONT at the breakpoint. Many BASICs allowed a line number 0. One use of FOR/NEXT in direct mode is outputting the contents of an array for debugging. – LawrenceC Jul 20 '20 at 13:06
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    As far as what it would be useful for, I used to write a simple game on any TRS-80 model I/III I found in a shop or school etc as a kid. I could write it in one line but had to compromise on the infinite loop by making it count to a very large number. I don't know that this toy use case is any less valid than other use cases. – hippietrail Jul 21 '20 at 03:14

4 Answers4

25

In the Microsoft BASIC variants (Commodore, Atari, Apple, others) you can specify a FOR/NEXT loop with a STEP size of 0, which never increments the loop.

FOR X = 0 TO 1 STEP 0: PRINT "TEXT": NEXT

is how you'd write it as a one-liner.

Now, as to why it would be useful to do such a thing...

George Phillips
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Joe
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  • Doesn't the STEP parameter merely specify by how much the loop variable is be incremented after each iteration? How does that pertain to the question of whether or not a loop could be executed directly from the command line? – Solomon Slow Jul 20 '20 at 12:23
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    You can write a multiline statement on a single line separated by semicolons as above. Thus you can write a complete FOR loop (with STEP 0) on the command line that never terminates. – Joe Jul 20 '20 at 13:30
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    I believe BASIC uses colons, not semicolons, to separate sentences. – mcleod_ideafix Jul 20 '20 at 13:32
  • Right, but can't you also write a complete FOR loop with STEP 5 or, without specfying the STEP at all? The OP was asking when BASIC became something more like a modern repl in which you could type a complete loop on the command line and have it execute as soon as you hit enter. How does STEP affect the answer to that question? – Solomon Slow Jul 20 '20 at 13:36
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    You need the STEP 0 to make it repeat forever just like OP's examples do. The OP doesn't explicitly state 'endless loop' but their examples do. – Guntram Blohm Jul 20 '20 at 14:13
  • FOR-NEXT-STEP 0 was definitely how we did it on the TRS-80. (And yes, colons were the MS-BASIC standard everywhere I remember: TRS-80, C-64, Apple ][, even IBM PC.) Infinite loops at the REPL weren't terribly useful, no; the 10 PRINT "DISCO SUX! ": GOTO 10 thing that everybody did to watch the scrolling pattern, was almost always done as an "in memory" program and not immediate. – Ross Presser Jul 20 '20 at 15:27
  • @GuntramBlohmsupportsMonica Maybe I'm just thick. I don't see where the OP asked how to make a loop that never terminates. – Solomon Slow Jul 20 '20 at 17:30
  • @SolomonSlow My answer gives examples for both infinite and finite loops. – Chromatix Jul 20 '20 at 18:08
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    @GuntramBlohmsupportsMonica: if I'm not wrong, you don't need STEP 0. You can do FOR X = 0 TO 1: PRINT "TEXT": X = -1: NEXT – Martin Argerami Jul 21 '20 at 13:41
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    @MartinArgerami yours is the only one which can work in TI Extended BASIC as FOR X = 0 TO 1 :: PRINT "TEXT" :: X = -1 :: NEXT X – Jesse C. Slicer Jul 21 '20 at 16:28
  • @MartinArgerami - changing the loop variable would fail in any interpreter providing strict ANSI Minimal BASIC (ANSI X3.60-1978) compliance. Now, whether many or even any interpreters stuck with this, I don't know: MS certainly didn't. – scruss Jul 22 '20 at 20:04
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    As to why it would be useful, the following one-liner reads the floppy status on a C64 from standard BASIC without destroying the stored BASIC program (POKE58,1allows using GET from direct mode, which is normally forbidden): OPEN1,8,15:FORI=0TO1STEP0:POKE58,1:GET#1,A$:PRINTA$;:IFST=0THENNEXT – TeaRex Jul 28 '20 at 13:38
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In any BASIC variant that allows multiple statements on one line, the multiple statements that are required to implement a FOR-NEXT or REPEAT-UNTIL loop can be provided in immediate mode. Many 8-bit micros of circa 1980 could do this:

REPEAT : PRINT "*"; : UNTIL FALSE

Or this:

X=1 : FOR Y=0 TO 30 : PRINT "2^";Y;" = ";X : X=X+X : NEXT Y

Try these examples in jsBeeb.

By contrast, any looping technique that relies on GOTO would not work, because there is no line number associated with the immediate-mode command. RUN also would not work, because its function is to start a stored program, which the immediate-mode command is not.

Chromatix
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    WHILE … WEND would have worked in more early machines. It was certainly present in MBASIC-80. REPEAT…UNTIL was more limited to Acorn machines. – scruss Jul 22 '20 at 20:00
5

Using BASIC+ on RSTS/E you can do this:

Ready

print i%; "Hello!" for i% = 1 to 9
 1 Hello!
 2 Hello!
 3 Hello!
 4 Hello!
 5 Hello!
 6 Hello!
 7 Hello!
 8 Hello!
 9 Hello!


Ready

This has a variety of uses. A trivial one would be to start a long running program then enter print chr$(7) for i%=1 to 20 into the type ahead buffer. The terminal would then beep 20 times when the long-running program completed.

It's also useful for testing code:

Ready

old stuff


Ready


listnh
1000    def fnstuff.happens()
1010        a.number = rnd
1020        print &
         \  print "Working on stuff..."
1030        fnstuff.happens = a.number
1040    fnend
3276    end


Ready


print fnstuff.happens for i = 1 to 3


Working on stuff...
 .204935


Working on stuff...
 .229581


Working on stuff...
 .533074


Ready

Not strictly speaking a "home computer" unless, like me, you used to run a PDP-11 in your spare room. The above output is copied from a RSTS/E V8.0-06 system running under the SIMH PDP-11 emulator.

Terry Ebdon
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    Upvoted - while not a home computer, RSTS is very RC! I used to program professionally on Systime systems in their very similar BASIC 500! – user7761803 Jul 20 '20 at 18:09
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    Ah, BASIC-PLUS! I learned programming on a PDP-11 running RSTS/E. BASIC-PLUS was a great basic dialect. Though I was envious of BASIC-PLUS 2. Also learned MACRO-11 and COBOL on that system. – mannaggia Jul 21 '20 at 14:06
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    BASIC-PLUS-2 allowed this also. I think that they removed if for VAX BASIC though. – RBarryYoung Jul 22 '20 at 00:26
  • I think you're right @RBarryYoung. I don't recall using immediate mode in VAX BASIC and there's no mention of it in the VAX BASIC User Manual. You could also compile BASIC, on later versions of RSTS/E, with the CUSP compiler, CSPCOM. That was very cut down, with no immediate mode. – Terry Ebdon Jul 22 '20 at 16:32
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    For VAX BASIC, you could do immediate mode in the debugger, or by running just the BASIC command (I think, it’s been a long time). What was removed was the control-suffix syntax (IIRC), ie. <statement> For ... and <statement> If <condition>. – RBarryYoung Jul 22 '20 at 22:54
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    Yes, you're right. I dug out the VAX BASIC Reference Manual, it describes immediate mode in great detail. I can't see any mention of implied IF etc. I'm quite surprised, both by their absence and by forgetting such a key change. Though getting rid of nasty stuff like goto 2300 if x% > 27 was surely a good thing. I did like BASIC_PLUS's support of UNLESS, FOR and WHILE as modifiers. – Terry Ebdon Jul 22 '20 at 23:26
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In Microsoft-based BASIC interpreters, the primary limitation with direct mode was the fact that the same buffer is used for inputting BASIC lines as for input within a program (e.g. the INPUT statement and the GET statement). If one were to perform e.g. INPUT A$ in direct mode, then once the input was complete, the buffer would be left holding the text that was input, rather than the immediate-mode line. I don't think there would have been any particular difficulty designing GET to store the new character someplace else (since it would only be a single byte), but I suspect that INPUT and GET share a routine that takes a string stored in the input buffer and copies it to garbage-collected string space; sharing that routine wouldn't avoid the need to have the GET function set the length to a hard-coded value of 1, but would avoid the need have it include its own code to set the source address. Even with that design, GET could probably have been made to support immediate mode if it did something like:

lda InputBufferStart
pha
jsr handleGet
pla
sta InputBufferStart
rts

but that would made the interpreter 12 bytes bigger for relatively minor benefit.

The FOR loop construct that saves the start-of-loop address doesn't need to use the input buffer, and thus has no difficulty with direct mode. Constructs that would need to GOTO the direct-mode line wouldn't work, of course, but otherwise the only constructs that are problematic in direct mode are those that would use the input buffer.

supercat
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