Economy Trio

The first two programs implement a software clock: they accept a starting time (with optional AM/PM selection) and use PAUSE 3550/3551 as a one-minute timer, updating the display each iteration and supporting both count-up (mode 1/2) and count-down (mode 3) operation.

Anypoint, the third program, is a bar-chart plotter that accepts a data range and up to 20 values, scaling the X axis dynamically to fit 22–34 columns and using PLOT to draw points on screen.

Day calculator is a day-of-week calculator that converts a Gregorian date into a day number using the formula D + INT(365.25×Y), applies a leap-year correction for January/February, then finds the remainder mod 7 to print the abbreviated day name.

Software Clock with AM/PM and Countdown

This program simulates a real-time clock by relying on PAUSE 3550 (approximately 60 seconds at 50 Hz on a ZX81) as its timing loop. The user selects a mode via C and enters a starting hour H and minute M.

• C=1: 12-hour AM/PM display. The user is also asked for Q (1=AM, 2=PM); if PM, H is incremented by 12.
• C=2 (implied): 24-hour count-up with no AM/PM suffix.
• C=3: Countdown mode — each iteration does M=M+1 then immediately M=M-2, net effect M=M-1; when M=-1 the hour is decremented and M reset to 59.

The AM/PM logic at lines 19–20 contains a subtle anomaly: line 20 uses IF C=1 AND H<=11 OR H=24 without proper parenthesisation. On the ZX81, AND binds tighter than OR, so this evaluates as (C=1 AND H<=11) OR (H=24), meaning midnight (H=24) would always print ” AM” regardless of C. Midnight wrapping is handled at line 32 by resetting H from 25 to 1, skipping 0 entirely, which means H=24 is a transient state only briefly seen before the next iteration.

The leading-zero idiom for minutes (lines 17–18) uses paired IF M<10 / IF M>=10 rather than an IF…THEN…ELSE construct, which is typical ZX81 BASIC.

Simplified 24-Hour Clock

This is a stripped-down variant of Program 1 with AM/PM display removed. PAUSE 3551 is used (one frame longer). The hour-wrap logic differs: line 27 resets H to 0 when H=24 AND M=1, which is slightly off — it triggers one minute into the new hour rather than immediately. Line 31 lacks the IF H=25 THEN LET H=1 reset present in Program 1, so the clock rolls through H=0 correctly for a 24-hour cycle but would count past 23 indefinitely without the countdown termination at line 28.

Anypoint – Bar Chart / Scatter Plotter

This program collects N data values in the range [L, H] and plots them using the ZX81’s PLOT command. The X axis is drawn using a row of block-graphic characters (\: = ▌, repeated 20 times) followed by a ruler string with tick labels at 1, 10, 20, 30, 40, 50, 60.

Axis Scaling Logic

The scaling loop (lines 21–28) attempts to normalise the range X=H-L to fall within 22–34 units, suitable for the ZX81’s 64-column graphics resolution. It multiplies or divides both X and D (the adjusted data value) by 1.5 repeatedly until the range fits. This is a simple iterative rescaler with no termination guard — if the range cannot converge it would loop indefinitely (though in practice, repeated ×1.5 or ÷1.5 will eventually satisfy the condition).

PLOT I+1, INT(D+2) at line 30 maps the entry index to the X coordinate and the scaled, offset data value to Y. The +2 offset shifts the plot slightly off the axis baseline. The running sum and mean are displayed at line 20 with trailing spaces to clear previous longer values.

Day-of-Week Calculator

This implements a compact Gregorian day-of-week algorithm. The cumulative day-of-year offset D is set by a series of IF M=n THEN LET D=… statements for months 2–12 (January has no statement, defaulting to D=0).

The day-of-year is completed by adding E (day of month), then the year contribution INT(365.25*Y) is added. A leap-year correction at line 23 subtracts 1 when the year is divisible by 4 and the month is before March, correctly handling the case where the extra leap day has not yet occurred in the current year. The epoch anchor (X=0 → Friday) implies the formula is anchored to a reference date of approximately 1 January 1 AD using a proleptic Gregorian convention, though no documentation is provided in the listing. The program loops continuously via GOTO 1 after each result, allowing repeated queries.

General Idioms Across All Programs

• FAST mode used in Programs 1 and 2 to suppress display refresh overhead during timing-sensitive PAUSE loops.
• Inverse-video characters (e.g. %S%T%A%R%T%I%N%G) used for prompts — each %X represents the inverse video version of the letter in zmakebas notation.
• Paired IF cond THEN / IF NOT cond THEN used throughout in lieu of IF…THEN…ELSE, which ZX81 BASIC does not support.
• Line numbers are non-contiguous (gaps at 3, 4, 11–13, etc.), suggesting the programs were edited after initial entry.

Content

Appears On

Related Products

The Economy Trio

Three programs available on separate cassettes combined onto one cassette: Clocks and Timers, Your Special Day and Anypoint Plotter. 1K.

Related Articles

Related Content

Image Gallery

Source Code

   1 LET Q=0
   2 FAST 
   3 PRINT "SELECT"
   5 INPUT C
   6 PRINT "%S%T%A%R%T%I%N%G% %T%I%M%E"
   7 INPUT H
   8 INPUT M
   9 IF C=1 THEN PRINT "1:AM 2:PM"
  10 IF C=1 THEN INPUT Q
  11 IF Q=2 THEN LET H=H+12
  14 PRINT AT 8,8;"TIME-";
  15 IF H>12 THEN PRINT H-12;":";
  16 IF H<=12 THEN PRINT H;":";
  17 IF M<10 THEN PRINT "0";M;
  18 IF M>=10 THEN PRINT M;
  19 IF C=1 AND H>11 AND H<=23 THEN PRINT " PM"
  20 IF C=1 AND H<=11 OR H=24 THEN PRINT " AM"
  21 PAUSE 3550
  23 LET M=M+1
  24 IF C=3 THEN LET M=M-2
  25 CLS 
  26 IF C=3 AND M=-1 THEN LET H=H-1
  27 IF C=3 AND M=-1 THEN LET M=59
  28 IF C=3 AND H=0 AND M=0 THEN GOTO 40
  29 IF M<60 THEN GOTO 14
  30 LET M=0
  31 LET H=H+1
  32 IF H=25 THEN LET H=1
  33 GOTO 14
  40 PRINT AT 8,8;"%D%O%N%E"
  41 PAUSE 20
  42 GOTO 40

   1 LET Q=0
   2 FAST 
   3 PRINT "SELECT"
   5 INPUT C
   6 PRINT "%S%T%A%R%T%I%N%G% %T%I%M%E"
   7 INPUT H
   8 INPUT M
  14 PRINT AT 8,8;"TIME-";
  15 PRINT H;":";
  17 IF M<10 THEN PRINT "0";M;
  18 IF M>=10 THEN PRINT M;
  21 PAUSE 3551
  23 LET M=M+1
  24 IF C=3 THEN LET M=M-2
  25 CLS 
  26 IF C=3 AND M=-1 THEN LET H=H-1
  27 IF H=24 AND M=1 THEN LET H=0
  28 IF C=3 AND H=0 AND M=0 THEN GOTO 40
  29 IF M<60 THEN GOTO 14
  30 LET M=0
  31 LET H=H+1
  33 GOTO 14
  40 PRINT AT 8,8;"%D%O%N%E"
  41 PAUSE 15
  42 GOTO 40

   1 LET V=0
   2 PRINT "NO."
   3 INPUT N
   4 PRINT "LOW"
   5 INPUT L
   6 PRINT "HIGH"
   7 INPUT H
   8 CLS 
   9 PRINT L;" TO ";H
  10 FOR Q=1 TO 20
  11 PRINT "\: "
  12 NEXT Q
  13 PRINT "\''1\''\''\''10\''\''\''20\''\''\''30\''\''\''40\''\''\''50\''\''\''60"
  14 FOR I=1 TO N
  15 PRINT AT 2,2;"ENTER ";I
  16 INPUT D
  17 LET X=H-L
  18 LET V=V+D
  19 LET D=D-L
  20 PRINT AT 1,0;"SUM=";V;" MEAN=";V/I;"       "
  21 IF X>34 THEN GOTO 26
  22 IF X>=22 AND X<=34 THEN GOTO 30
  23 LET X=X*1.5
  24 LET D=D*1.5
  25 GOTO 21
  26 LET X=X/1.5
  27 LET D=D/1.5
  28 GOTO 21
  30 PLOT I+1,INT (D+2)
  31 NEXT I

   1 PRINT AT 8,3;"%M%O%N%T%H %D%A%Y %Y%E%A%R"
   2 LET D=0
   3 INPUT M
   4 IF M=2 THEN LET D=31
   5 IF M=3 THEN LET D=59
   6 IF M=4 THEN LET D=90
   7 IF M=5 THEN LET D=120
   8 IF M=6 THEN LET D=151
   9 IF M=7 THEN LET D=181
  10 IF M=8 THEN LET D=212
  11 IF M=9 THEN LET D=243
  12 IF M=10 THEN LET D=273
  13 IF M=11 THEN LET D=304
  14 IF M=12 THEN LET D=334
  17 INPUT E
  18 LET D=D+E
  20 INPUT Y
  22 LET D=D+INT (365.25*Y)
  23 IF INT (Y/4)=Y/4 AND M<3 THEN LET D=D-1
  27 LET W=INT (D/7)
  28 LET X=D-(7*W)
  29 PRINT M;"/";E;"/";Y
  30 IF X=0 THEN PRINT "%F%R%I"
  31 IF X=1 THEN PRINT "%S%A%T"
  32 IF X=2 THEN PRINT "%S%U%N"
  33 IF X=3 THEN PRINT "%M%O%N"
  34 IF X=4 THEN PRINT "%T%U%E%S"
  35 IF X=5 THEN PRINT "%W%E%D"
  36 IF X=6 THEN PRINT "%T%H%U%R"
  50 INPUT K$
  51 CLS 
  53 GOTO 1

Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.