Character Editor

This program is an interactive UDG (User Defined Graphics) character editor that allows users to design custom characters on an 8×8 pixel grid. The editor draws a visual grid using PLOT and DRAW commands, supports cursor movement via the numeric arrow keys (5/6/7/8), and allows pixels to be added with “a” or deleted with “d”. Each pixel change updates a byte array c(8) using bit manipulation (powers of two based on column position), and the computed values are immediately POKEd into UDG RAM via USR CHR$(144+c). The program supports 24 UDGs (numbered 0–23) and displays both the current byte values and a live preview of the character being edited.

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Program Structure

The program is organized into clearly separated routines, navigated by line-number ranges:

Lines	Purpose
1–5	Introduction text and keypress gate
60–180	Initialization: INK/PAPER, cursor position, character selection, grid setup
300–610	Main keyboard input loop and cursor rendering
700–800	Add pixel routine
900–950	Delete pixel routine
1300–1450	Draw grid lines and display byte values
1500–1800	Calculate and update UDG character data
2000–2060	Draw screen border

There is a notable gap between lines 390 and 410: line 392 routes unrecognised keypresses back to the input loop, but the cursor-bounds clamping at lines 410–440 is only reached via a GO TO 400 from the movement keys. This means the GO TO 400 target does not actually exist as a line — lines jump to 400 but execution falls through to 410, which is the intended behavior in Sinclair BASIC (execution continues at the next available line).

Cursor Rendering

The cursor is drawn using PLOT/DRAW with INVERSE 1 to erase the old cursor position by XOR-inverting pixels, then redrawn at the new position without INVERSE. The cursor shape is a cross: a horizontal segment and a vertical segment, each 4 pixels long, offset by 2 pixels from the cell corner. The screen coordinates are derived as xc = x*8 and yc = (21-y)*8, mapping grid coordinates to pixel coordinates.

Pixel Add/Delete and Attribute Detection

When adding a pixel (line 700), the program checks PEEK(q) against attribute values before writing. Attribute byte 10 corresponds to INK 2, PAPER 1 (used to indicate already-set or forbidden cells), and 8 indicates a special state; if either is found, the add is skipped. When deleting, attribute 56 (INK 0, PAPER 7 — the background) is the guard value. Direct attribute manipulation via POKE q, 10 and POKE q, 56 is used rather than drawing characters, relying on the attribute file at address 22528.

The address calculation q = x + 22528 + (32*y) is the standard Spectrum attribute file formula, confirming these are attribute memory addresses, not display file addresses.

UDG Data Calculation

The subroutine at line 1500 computes which bit within the character byte corresponds to the current pixel:

• xv = 7-(x-10) converts screen column (10–17) to bit position (7–0, MSB to LSB)
• z = 2^xv produces the bitmask for that column
• v = y-5 maps screen row (6–13) to array index (1–8)
• The flag p selects addition (p=0) or subtraction (p=1) to set or clear the bit in c(v)

No bitwise AND/OR is available in Sinclair BASIC, so arithmetic addition and subtraction serve as bit-set and bit-clear operations, which is correct only if the bit was not already set/cleared — the attribute pre-checks at lines 715–716 and 915 serve as guards to prevent double-toggling.

UDG RAM Poking

At line 1710, USR CHR$(144+c) returns the address of the first byte of the selected UDG in RAM. The loop at lines 1720–1740 then POKEs all 8 bytes from the array c(). There is a subtle bug here: POKE q+(s-1), c(q) should be POKE s+(q-1), c(q) — using q as both the loop index and as part of the address with an off-by-one adjustment (s-1 instead of s) means when q=1 the POKE goes to address s, which is correct, but the expression is needlessly confusing and would only be correct because q+(s-1) = s+(q-1) algebraically — so the result is actually correct despite the unusual form.

Key BASIC Idioms

• Busy-wait key polling: IF INKEY$="" THEN GO TO 310
• Screen grid drawn with FOR loops using PLOT/DRAW rather than character graphics
• DIM c(8) initializes the byte array to zeros, conveniently representing a blank UDG
• PRINT AT 5,5;CHR$(144+c) provides a live preview of the UDG as it is edited