Minicalc

Spreadsheet program that provides a 7-row by 3-column grid of cells, each capable of holding either a formula (F type) or a numeric value (V type). Cells are navigated using cursor keys (mapped to codes 112–115) and can contain arithmetic expressions that reference other cells using column letters A, B, C followed by a row digit. The program uses a 7×3×32 three-dimensional string array A$ to store cell contents, with the first character acting as a type tag (“%F” for formula, “%V” for value). Formula evaluation (subroutine at line 5000) performs a text-substitution pass, replacing cell references like “A1” with VAL A$(row,col,2 TO) subexpressions before calling VAL on the expanded string. A POKE to address 16418 temporarily suppresses the ZX81 scroll prompt when displaying cell contents in the status line.

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Program Analysis

Program Structure

The program is organized into several distinct functional blocks:

1. Initialization (lines 0–100): Title screen display and setup of the 3-D string array, cursor position variables A (row, 1–7) and B (column, 1–3).
2. Main loop (lines 110–400): Reads a keypress via INKEY$, animates a cursor at the current cell position, and dispatches to the appropriate handler.
3. Cursor movement (lines 500–830): Four routines handle up/down/left/right movement with boundary checking.
4. Screen draw subroutine (lines 1000–1060): Redraws the grid using block graphics, labels columns A–C, and numbers rows 1–7.
5. Cell entry: Formula (lines 2000–2060): Accepts a string formula via INPUT, prefixes it with %F, and stores it in A$(A,B).
6. Formula evaluation (lines 3000–3100 + 5000–5230): Expands cell references and evaluates the formula using VAL.
7. Cell entry: Value (lines 4000–4060): Accepts a numeric value via INPUT, prefixes with %V, and displays it immediately.
8. Cell reference substitution (lines 5000–5230): Iterates through the formula string replacing symbolic cell references with VAL A$(...) sub-expressions.
9. Redisplay / recalculate (lines 6000–6240): Redraws all cells, triggered by pressing R.
10. Formula text display (lines 7000–7060): Displays the raw formula text of the current cell in the bottom status line.
11. Home/End navigation (lines 8000–8120): Jump cursor to top-left (B) or bottom-right (N).
12. Save (lines 9000–9060): Prompts for filename and saves the program+variables to tape.

Data Storage

All cell data is stored in A$(7,3,32), a three-dimensional string array with dimensions row × column × 32 characters. The first character of each cell string acts as a type tag: %F (inverse F) denotes a formula cell, %V (inverse V) denotes a numeric value cell. A blank first character means the cell is empty. The formula or numeric text occupies characters 2–32. This tag scheme allows a single array to serve both purposes without separate arrays.

Cursor Animation

The main loop at lines 120–130 uses a busy-wait on INKEY$ combined with a three-step PRINT AT sequence to animate a flashing cursor. The sequence prints \.' (▖), then \'.​ (▘), then a space, all at the same screen position before checking for a keypress. This rapid overwrite creates a simple flicker effect at the current cell position. Line 140 loops back if no key is pressed, keeping the animation running.

Key Dispatch

Key codes 112–115 correspond to the ZX81 cursor keys (shifted 5/6/7/8). These are checked via CODE B$ comparisons. Function keys F, C, V, D, R, S, B, N are checked as plain character comparisons.

Key	Action	Target line
Code 112 (↑)	Cursor up	500
Code 113 (↓)	Cursor down	600
Code 114 (←)	Cursor left	700
Code 115 (→)	Cursor right	800
F	Enter formula	2000
C	Calculate/display formula result	3000
V	Enter numeric value	4000
D	Display raw formula text	7000
R	Redraw/recalculate all cells	6000
S	Save to tape	9000
B	Home (A1)	8000
N	End (G3, bottom-right)	8100

Formula Evaluation via String Substitution

The most technically notable aspect of this program is how it evaluates formulas. When a formula cell is calculated (lines 3000–3100), the raw formula text is copied into B$. The substitution routine at line 5000 then iterates character by character through B$. When it encounters the letters A, B, or C (lines 5100–5120), it treats the following character as a row digit and replaces the two-character cell reference (e.g., A3) with a full BASIC expression VAL A$(3,1,2 TO ). After substitution, the string is longer, so N is adjusted by the length difference (line 5130) and the remaining formula length R is updated (line 5220) to correctly bound the iteration. Finally, the expanded string is passed to VAL at line 3070 for evaluation.

This means formulas are essentially BASIC arithmetic expressions with cell references as operands. For example, entering A1+B2 as a formula would be expanded to VAL A$(1,1,2 TO )+VAL A$(2,2,2 TO ) before evaluation.

Notable Techniques

• POKE 16418: Lines 7010 and 7050 POKE system variable CDFLAG (address 16418) to 0 and back to 2. On the ZX81, this suppresses the automatic scroll/pause behaviour when printing to the bottom of the screen, allowing line 23 to be used as a status bar without triggering a scroll prompt.
• FAST/SLOW toggling: The program switches between FAST (no display) and SLOW modes strategically — screen drawing and computation use FAST, while the interactive keypress loop uses SLOW for visible output.
• Grid drawing with block graphics: Line 1030 uses a long PRINT AT statement containing ZX81 block graphic characters (▌, ▘, ▀, ▖, ▄ etc.) to draw the cell borders in a single print call per row, repeating for rows 1, 4, 7, 10, 13, 16, 19.
• Self-referential SAVE: Line 9998 SAVE "MINICAL%C" (with inverse C) saves the program itself, and line 9050 in the user-invoked save path uses INPUT Y$ for a user-chosen filename — line 9998 is only reached if execution falls through from line 9997 STOP, which would not happen in normal use; it exists as a developer convenience for saving the program source.