Certificate

This program generates a decorative certificate on screen, prompting the user to enter between one and eight lines of text, which are then centered automatically using the expression `(32-LEN c$)/2` as the column argument to `PRINT AT`. Before displaying the certificate, a subroutine at line 9000 defines fourteen User Defined Graphics (UDGs “A” through “N”) by POKEing 8-byte bitmap data into their reserved memory locations via `POKE USR “\x”+a,b`. The user is then shown a menu of the available UDG symbols and chooses one to form the certificate’s border: two solid rows across the top and bottom, and a double-character column on each side using `OVER 1` for the vertical edges. Line 9999 provides a tape SAVE/VERIFY routine with visual feedback using `INVERSE` and `FLASH` attributes.

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Program Analysis

Program Structure

The program is divided into three logical sections:

1. Initialization (line 10): Sets border color, resets the DATA pointer, and calls the UDG setup subroutine.
2. Certificate builder (lines 200–8999): Collects user text, centers and prints it, then prompts for a border symbol and draws the frame.
3. UDG definition subroutine (lines 9000–9700): Loads 14 custom 8×8 pixel bitmaps into UDG memory.
4. Save/Verify utility (line 9999): A standalone tape management line, executed only if directly run or GOTOed.

Text Centering

Lines 225–232 implement an if-ladder to compute a starting row y based on the number of requested lines l, ensuring the text block is vertically centered on the 22-row display. Each line of user input is horizontally centered with the idiom:

PRINT AT y,(32-LEN c$)/2;c$

This works because the display is 32 characters wide; the expression yields the left margin needed to center the string. Lines are spaced two rows apart (y=y+2 at line 235), leaving a blank row between each entry for visual breathing room.

UDG Definition Subroutine

Lines 9000–9700 define 14 UDGs (labeled “A” through “N”) using a repetitive pattern: 8 DATA values are READ and POKEd into USR "\x"+a for a=0 TO 7. Each UDG occupies 8 bytes of bitmap data. The fourteen symbols defined are:

UDG	Data (hex)	Visual description
\a	FF FF C3 C3 C3 C3 FF FF	Solid border rectangle
\b	81 42 3C 3C 3C 3C 42 81	Diamond / hourglass
\c	00 FF FF 00 00 FF FF 00	Horizontal double bar
\d	00 18 18 7E 7E 18 18 00	Plus / cross
\e	00 3C 7E 66 66 7E 3C 00	Open circle/oval
\f	66 66 66 66 66 66 66 66	Vertical double stripe
\g	3C 7E C3 C3 C3 C3 7E 3C	Filled circle
\h	CC CC 33 33 CC CC 33 33	Checkerboard 2×2
\i	18 18 00 DB DB 00 18 18	Dotted cross
\j	C3 C3 3C 3C 3C 3C C3 C3	Corners only
\k	00 EE EE 00 00 EE EE 00	Horizontal double bar (wide)
\l	E7 E7 E7 00 00 E7 E7 E7	Top/bottom thick bands
\m	00 7E 42 42 42 42 7E 00	Hollow square
\n	FF 81 81 81 81 81 81 FF	Thin hollow square

Border Drawing

After the user selects a graphic symbol a$, the border is drawn in three passes:

• Lines 8100–8110: Fill rows 0 and 1 completely with 32 copies of a$ for the top border.
• Line 8120: A loop of 18 iterations prints a$;a$ at column 0 and column 30 with OVER 1 for the left and right edges (rows 2–19).
• Line 8130: Prints 64 copies of a$ to fill the bottom two rows (rows 20–21).

The use of OVER 1 (XOR pixel mode) at line 8120 means the border symbol is drawn over whatever text already exists in those columns without fully overwriting it — a deliberate technique to preserve any text that might fall near the edge, though in practice the text centering keeps content away from columns 0–1 and 30–31.

Notable Techniques and Idioms

• The RESTORE at line 10 ensures the DATA pointer is reset before the UDG subroutine reads its values, making the program safely re-runnable.
• The vertical centering if-ladder (lines 225–232) is a straightforward but verbose substitute for a single arithmetic formula; it could equivalently be written as LET y = 12 - l.
• The UDG selection menu at lines 8080–8090 displays letters A–N alongside their rendered symbols, giving the user a visual preview before choosing.
• Line 8999 is a STOP, preventing accidental fall-through into the UDG subroutine or the save utility.

Bugs and Anomalies

• Line 200 only validates l>8 but does not guard against l<1 or non-integer inputs, so entering 0 or a negative number would proceed and produce an off-screen or incorrect y value since the if-ladder only covers l=1 through l=8.
• The PRINT at line 240 prints an empty line, but because a prior PRINT AT was used, the cursor position is indeterminate — this may scroll the screen unexpectedly if the cursor is at the bottom.
• The side-border loop at line 8120 uses TAB 30 for the right edge, placing the double symbol at columns 30–31, while the top/bottom fills all 32 columns (0–31). This is consistent and intentional but means the corners are only single-width on the left and right.