SuperCalc

SuperCalc is a joystick-controlled on-screen calculator that displays a graphical keypad layout and allows the user to navigate it using a joystick, selecting digits, operators, parentheses, and a square root function. The program uses PLOT and DRAW commands to draw a bordered calculator frame, then prints labeled buttons at fixed screen positions. Button selection is handled by reading STICK input to move a cursor (displayed as an arrow character) around the keypad grid, with the fire button (STICK second parameter) triggering the selected key. Expressions are accumulated in a string variable B$ and evaluated using VAL, with STR$ used to clear trailing display characters after the result is printed. A simple memory function (MEM) stores a previously computed result in variable V for later recall.

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Program Structure

The program divides into four logical phases: initialization and screen drawing (lines 1–68), cursor control loop (lines 70–135), button dispatch subroutine (lines 140–390), and result/memory handling (lines 400–510). The main loop at line 80 redraws the cursor arrow, pauses briefly, erases it, reads the joystick, clamps the cursor position, then checks for a fire button press before looping back.

Screen Layout and Drawing

The calculator border is drawn using PLOT and DRAW commands to form a rectangle, with two additional horizontal rules drawn at lines 20 and 22 (pixel rows 155 and 134) to separate the display area from the keypad rows. Button labels are printed at fixed AT row/column positions: digits on row 9, operators on row 11, and function keys on row 13. The display area occupies row 4.

Joystick Navigation

Cursor position is tracked by two variables: A (row, ranging 8–14 in steps of 2) and B (column, ranging 1–27 in steps of 2). Lines 100–110 update these using STICK direction codes: direction 2 moves down, 1 moves up, 8 moves right, 4 moves left. Each STICK call reads the joystick twice per axis within a single line, which is slightly redundant but functions correctly. Lines 120–126 clamp both coordinates to valid bounds.

STICK value	Direction	Effect
1	Up	A decreases by 2
2	Down	A increases by 2
4	Left	B decreases by 2
8	Right	B increases by 2

Button Dispatch

When the fire button is detected (STICK (2,1)=1 at line 128), control transfers to the dispatch block starting at line 140. Each button is identified by a unique (A, B) coordinate pair. Digit and operator buttons append their character to the accumulator string B$ and jump to line 400, which beeps, prints the current expression at row 4, and returns to the main loop. The equals button (A=12, B=9) branches to line 430 for evaluation.

Expression Evaluation

Evaluation at lines 430–490 uses VAL B$ to parse and compute the accumulated string expression, storing the numeric result in X. This allows full arithmetic expressions including operator precedence and nested parentheses, since VAL evaluates arbitrary numeric expressions. The square root button appends the string "SQR " to B$, so it integrates naturally into expression building rather than requiring special numeric handling.

After printing the result, STR$ X is used to determine the length of the result string, and a FOR loop at line 475 prints spaces from that length to column 30 to erase any leftover characters from a longer previous expression — a practical display-cleanup technique.

Memory Function

The MEM button (A=14, B=13, line 380) branches to line 500. If variable V is non-zero, it prints the stored value and resets V to 0 (recall mode). If V is zero, it stores the current result X into V (store mode). This single-register memory toggle is simple but functional, though it cannot store zero as a meaningful value.

Notable Techniques and Idioms

• The cursor blink is achieved by printing "↑", pausing with PAUSE 3, then overprinting with a space — a common sprite-erasure idiom.
• Boolean arithmetic in lines 100–110 uses the fact that logical comparisons evaluate to 1 (true) or 0 (false), multiplying by 2 to get the step size — avoiding conditional branches for movement.
• The CLR button (A=14, B=9) at line 140 clears the display line and falls through to line 480, which resets B$ and Z$ — a clean reuse of the post-evaluation reset code.
• The ON/OFF label at screen position AT 6,24 corresponds to the cursor position A=8, B=27 (the top-right area), where line 180 issues a STOP command to end the program.

Bugs and Anomalies

• Variable C is set to 1 at line 75 and used in PRINT AT 4,C; at line 410, but C is never changed, making it functionally equivalent to the literal 1. It appears to be a vestigial variable, possibly intended for a scrollable display column.
• If the equals button has never been pressed, X and Z$ are uninitialized, so pressing MEM or CLR before any evaluation could cause an error depending on interpreter behavior.
• Line 500’s memory recall prints V but does not clear the rest of the display row, which could leave stale characters if the previous display was longer than the recalled value.
• The STICK calls within a single line read the joystick multiple times; diagonal inputs could cause unexpected behavior since each direction check is independent.