Sprite Editor

This program implements a simple 8×8 grid editor where the user moves a cursor (“+”) around the grid and can mark or clear individual cells. The grid is stored as a two-dimensional string array A$(8,8), with each cell holding either “O” (empty) or an inverse space character (marked). Navigation uses keys 5/6/7/8 (up/down/left/right style), while “D” marks a cell, “E” erases it, “C” clears the entire grid, and “I” reinitialises the program. The editor uses single-character cell access via A$(row,col) and restores the previous cell’s display character before drawing the cursor at its new position, implementing a lightweight sprite-style cursor movement. A SAVE command at line 600 stores the program under a filename, though it is never reached by normal program flow.

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Program Analysis

Program Structure

The program is organised into a main initialisation block, an input/display loop, and several subroutine-like sections reached by GOTO:

1. Lines 1–2: Initialise cursor position variables X (row) and Y (column) to 1.
2. Lines 10–100: Declare the 8×8 string array A$, fill every cell with "O", and print the grid.
3. Lines 101–250: Main editor loop — display cursor, poll keyboard, dispatch commands, move cursor with wrapping.
4. Lines 300–320: “Draw” handler — marks the current cell with an inverse space character.
5. Lines 400–420: “Erase” handler — resets the current cell to "O".
6. Lines 500–590: “Clear” handler — reinitialises the whole grid and redraws it.
7. Lines 600–610: A SAVE and restart block that is unreachable from normal program flow.

Grid Representation

The grid is stored in DIM A$(8,8), a two-dimensional string array where each element is a single character. Cells hold either "O" for empty or an inverse-video space ("% " in the source) for marked. Individual cells are accessed with the two-index form A$(Z,M) and A$(X,Y), while entire rows are written using the single-index form A$(A) during initialisation, assigning the string "OOOOOOOO" to fill all eight columns at once.

Cursor Movement and Display

Before moving the cursor, the program saves the current position into Z and M (lines 145–146) and reads both the old cell content D$ and the new cell content C$ from A$ (lines 231–232). It then restores the vacated cell’s original character (line 240) and draws "+" or inverse "%+" at the new position depending on whether that cell is marked (lines 241–242). This ensures the cursor does not corrupt the underlying grid data.

Keyboard Polling Idiom

Lines 105–107 implement a busy-wait for a keypress using a short FOR loop (lines 105–106) followed by a check on INKEY$. The loop target at line 104 does not exist, so GOTO 104 falls through to line 105 — a well-known technique to re-enter a loop efficiently. The key is then read again into B$ at line 110 to capture the value for dispatch.

Wrapping Logic

Cursor wrapping at the grid boundaries is handled by a series of IF statements on lines 200–230. When X or Y reaches 0 it wraps to 8, and when it reaches 9 it wraps to 1, producing a toroidal grid where movement off any edge reappears on the opposite side.

Key Bindings

Key	Action
8	Move cursor right (Y+1)
5	Move cursor left (Y−1)
7	Move cursor up (X−1)
6	Move cursor down (X+1)
D	Mark (draw) current cell
E	Erase current cell
C	Clear entire grid
I	Reinitialise (restart program)

Bugs and Anomalies

• Missing line 104: GOTO 104 at line 107 targets a non-existent line; execution resumes at line 105, which is the intended behaviour for re-entering the polling loop.
• Double INKEY$ read: Lines 107 and 110 both call INKEY$. The first read at line 107 is used only to test for “no key pressed”; the actual key value is re-read at line 110. In theory a very brief keypress could be detected at line 107 but missed at line 110, though this is unlikely in practice.
• Unreachable SAVE: Line 600 (SAVE "1008%8") is never reached during normal execution. It appears to be a developer utility for saving the program state.
• Redundant status messages: Lines 20 and 510 print initialisation/clearing status text but lines 102 and 586 print "ED " in a fixed corner regardless of state, with no semantic distinction between editing modes.