CAL

This program prints a complete 12-month calendar for any user-specified year on a single ZX81/TS1000 screen, laid out in a 4×3 grid of months. It calculates the day-of-week for January 1st of the requested year using a cumulative day-count formula anchored to 1900, then advances the starting day through each month using a packed string (“312831303130313130313031”) to store the days-per-month. Leap year detection adjusts February to 29 days and corrects the day-count accumulation. The introductory subroutine at line 1030 draws a decorative box using PLOT commands and displays a title screen with a 200-frame pause before clearing to the calendar.

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Program Analysis

Program Structure

The program is divided into three logical phases:

1. Title screen (lines 5, 1030–1210): A decorative PLOT-drawn border with a “CAL” title and UAS attribution, displayed for about 3 seconds before clearing.
2. Setup and layout (lines 6–200): Warns about the 6th-week limitation, prompts for a year, draws the static calendar grid (day headers, month labels, year), and stores days-per-month in a packed string.
3. Date filling (lines 200–600): Decodes days-per-month into array A(), calculates the starting day-of-week for January 1st, and iterates through all 12 months printing dates into the correct screen positions.

Days-per-Month Encoding

Rather than using a DATA/READ approach, the program stores month lengths as the string literal "312831303130313130313031" in A$. Lines 210–230 parse this two characters at a time using VAL (A$(N))+VAL (A$(N+1))*10 to populate the numeric array A(1..12). Note that the formula at line 220 reverses the expected digit order: it computes 10*VAL(A$(N)) + VAL(A$(N+1)), so the leading digit is the tens place. The packed string encodes months as: 31,28,31,30,31,30,31,31,30,31,30,31.

Day-of-Week Calculation

The starting day-of-week for January 1st is derived in lines 240–265:

• Line 240: W = 366 + ((YEAR-1900)*365) — a cumulative day count from a reference point.
• Line 250: Adds one day per leap year elapsed since 1900: INT((YEAR-1900)/4).
• Line 255: If the target year itself is a leap year, subtracts 1 (January 1st precedes the leap day).
• Lines 260–265: Reduces modulo 7; if the result is 0 it is set to 7, mapping Sunday=7 in the SM-TW-TF-S column layout.

The guard at line 245 skips the leap-year correction for years ≤ 1904, avoiding an off-by-one around the 1900 non-leap-year boundary (1900 is divisible by 4 but not a leap year; this is a partial workaround).

Screen Layout

The 12 months are arranged in a 4-column, 3-row grid. Month labels are printed at fixed AT positions (rows 1, 8, 15) with 8-column spacing per column. The “SMTWTFS” header row is printed in inverse video. Each month occupies 8 character columns. Dates are filled starting at screen rows 3, 10, 17 (variable L steps by 7) and columns offset by K*8.

Date-Filling Logic

The subroutine at line 470 fills one month’s dates into the grid:

• The first partial row (lines 470–520) prints day 1 starting at column W1 (the day-of-week) through column 7.
• Four subsequent rows (lines 530–570) fill the remaining days, wrapping columns 1–7.
• Variable C tracks days elapsed in the month; when C = A(M+1)+1 the month is exhausted and the subroutine exits early via GOTO 580.
• A tens-digit suppression trick at line 546 resets D to 0 when it reaches 10, so PRINT D outputs only the units digit — effectively displaying 10 as “0”. This is a ZX81 width-saving technique but means the tens digit of dates 10–19+ is never printed; only the units digit appears for all dates.

Carry-Forward Between Months

After each month, the starting column for the next month is computed by accumulating total days elapsed (line 585: W = W + A(M)) and taking the fractional day-of-week from the running total (lines 591–593). The INT(W1 + 0.5) rounding at line 592 guards against floating-point remainder errors.

Notable Bugs and Anomalies

Line(s)	Issue
405–406	W1=W is set at line 405, then immediately overwritten by M=0 at line 406 (a separate statement). However, W1 is correctly initialised here for January; M being reset to 0 is intentional so the first M+1 reference inside the subroutine indexes A(1).
546	The tens-digit suppression (IF D=10 THEN LET D=0) causes dates 10–19, 20–29, 30–31 to display only their units digit (e.g., “10” prints as “0”). This is a deliberate space-saving measure but makes the calendar hard to read.
232	Leap year detection uses only the divide-by-4 rule; century years (e.g., 1900, 2100) that are not leap years are not handled beyond the partial guard at line 245.
6	The note about “6th week in month will not print” is correct — the grid only allocates 5 date rows per month (one partial + four full), so months starting late in the week can have dates beyond row 5 silently dropped.

Subroutine at Lines 1030–1210

This title-screen subroutine uses PLOT in two loops to draw a rectangular border: horizontal lines at Y=2 and Y=43 (lines 1030–1060), and vertical lines at X=0 and X=63 (lines 1070–1100). It then uses PRINT AT to place asterisk-bordered text and the program title inside the box. A PAUSE 200 (approximately 3.3 seconds at 60 Hz) gives the user time to read the title before CLS clears to the calendar.