Vertical Write

Vertical Write is a BASIC program that renders text as large, vertically-scrolling graphics drawn with PLOT and DRAW commands on a black background. The program first displays a built-in demo string (“ENTER A STRING NO SPECIAL CHARACTERS OR NUMBERS”) read from DATA at line 9500, then loops to accept user input via INPUT and render each letter in the same style. Each uppercase letter A–Z is encoded as a series of line-drawing commands stored in DATA lines 9000–9250, with each letter’s DATA block located by computing an offset from the character’s ASCII code: L=(CODE A$(N)-65)*10+9000, then RESTOREing to that line. The numeric constants 8 and 48 are pre-computed into variables L8 and LS, and NOT PI evaluates to 0 to avoid storing explicit zeros in the DATA statements. Colors are randomized per letter, and horizontal column position is randomized per space character, creating a kinetic display effect.

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Program Structure

The program divides into three phases:

1. Initialization (lines 1–20): Sets up constants, dimensions the array, configures display attributes, and RESTOREs to the demo message DATA at line 9500.
2. Demo loop (lines 25–200): Reads ASCII codes from DATA into array A() and renders the built-in prompt string vertically.
3. Interactive loop (lines 300–430): Accepts user INPUT, then renders each character of the entered string using the same PLOT/DRAW engine, repeating indefinitely via GO TO 300.

Note that line 300 does not exist in the listing — control jumps there from line 430 via GO TO 300, which is a well-known technique that causes execution to resume at the next line after 300, which is line 310.

Letter Shape Encoding

Each uppercase letter A–Z has its own DATA block at lines 9000–9250 (spaced 10 lines apart). The block begins with a count CT of line segments, followed by CT groups of four values: A, B, E, F representing a PLOT offset (C+A, R+B) and a DRAW delta (E, F). The correct DATA block for a character is located by computing:

L = (CODE A$(N) - 65) * 10 + 9000

then executing RESTORE L to seek directly to that line’s DATA. This avoids any lookup table and makes adding or modifying letters straightforward.

Notable Techniques

• NOT PI as zero: Since PI is non-zero, NOT PI evaluates to 0. This is used extensively throughout the DATA lines to encode zero offsets without the character overhead of the digit 0 — a byte-saving idiom for DATA-heavy programs.
• Pre-computed constants L8 and LS: L8 is set to 8 (via VAL "8") and LS to 48, both stored as variables so that their use in DATA statements is evaluated at READ time, reducing the number of integer literals in DATA and saving memory.
• Vertical scroll via PRINT with quote-images: Lines 105 and 405 use a chain of apostrophes in a PRINT statement (PRINT '''''''') to advance the display scroll by multiple lines, pushing previously plotted graphics upward to simulate vertical movement.
• POKE 23692,0: This resets the scroll counter (SCRCT in the system variables) to prevent the “scroll?” prompt from interrupting the display during the PRINT-driven scrolling.
• Randomized color and column: INK is randomized to a value 1–7 (avoiding black) for each letter. On encountering a space character, both the ink color and horizontal column position C are re-randomized, creating visual variety between words.
• Array pre-loading for demo: The demo message is read from DATA into A(100) before display, allowing RESTORE to be reused for letter shape lookups during the render loop without losing the message data.

PLOT/DRAW Attribute Usage

The PLOT and DRAW commands specify both INK and PAPER attributes inline. Notably, PLOT uses INK INK; PAPER INK; — setting both ink and paper to the same color — while DRAW alternates the ink/paper ordering between the demo loop (line 100) and the interactive loop (line 360), though the effect is the same since both colors are identical. This makes each character’s cell block appear as a solid color rather than a two-tone line on a contrasting background.

Input Limitations

The program only handles uppercase letters A–Z. The demo message warns “NO SPECIAL CHARACTERS OR NUMBERS.” Any character outside the A–Z range will cause the offset calculation at line 355 to produce an incorrect RESTORE target, potentially RESTOREing to an unintended DATA line or causing an error. No input validation or error handling is implemented.

Potential Anomalies

• Line 200 uses INK 9 before CLS — ink value 9 is a transparent/contrast attribute. This is used to reset the ink setting to a neutral state before clearing the screen and entering the interactive phase.
• The interactive loop starts at GO TO 300 but line 300 is absent; execution falls through to line 310 as intended.
• The variable R is initialized to 0 at line 8 and never changed, so all letters are plotted with a fixed vertical base offset of 0 — vertical movement is achieved entirely through PRINT-driven scrolling, not by incrementing R.