Mini-Pro

This program implements a simple full-screen text editor, using a blinking cursor rendered by rapidly printing and erasing a block-graphic character (▖) at the current position. The cursor position is tracked by variables X (row, 0–21) and Y (column, 0–31), matching the Spectrum’s 22-row × 32-column display. Key mappings handle character entry, backspace (r/code 114), forward movement (s/code 115), line-down (v/code 118), cursor up (q/code 113), home (code 23), end (code 19), clear screen (y/code 121), and a SAVE routine triggered by code 225. Typed characters are written directly to the screen with PRINT AT, and the cursor advances automatically, wrapping from column 31 to the next row.

---

Program Structure

The program is divided into clearly demarcated functional blocks:

1. Initialisation (lines 10–20): Sets cursor position X (row) and Y (column) to 0.
2. Main loop (lines 100–1130): Reads INKEY$, flashes the cursor, dispatches on key code, or prints the character and advances the cursor.
3. Key dispatch (lines 1010–1090): A chain of IF CODE A$=n THEN GOTO tests routing to handlers.
4. Command handlers (lines 1200–2020): Separate subroutine-like blocks for each function, all returning via GOTO 100.

Cursor Rendering

The blinking cursor is produced in the tight loop at lines 105–110. Line 105 prints the block graphic ▖ (lower-left quadrant, char 130) at position (X,Y), line 106 immediately overwrites it with a space, and line 110 loops back if no key has been pressed. Because the Spectrum’s display refresh is fast relative to BASIC execution, this creates a visible flicker that serves as a cursor indicator.

Key Mappings

Code	Key	Action	Handler
114	r	Backspace / move left	1200
115	s	Move right / forward	1300
121	y	CLS	1900
113	q	Move up one row	1400
225	Symbol Shift+A (or extended)	INPUT filename and SAVE	2000
112	p	Move up (same as q?)	1500
118	v	Move to start of next row	1600
23	Home (control code)	Go to top-left (0,0)	1700
19	End (control code)	Go to bottom-right (21,31)	1800

All other key presses fall through to line 1100 where the character is printed and the cursor advances.

Cursor Movement Logic

The backspace handler (1200) decrements Y, wrapping to column 31 on the previous row when Y=0. It guards against going before position (0,0). The forward handler (1300) increments Y, wrapping to column 0 on the next row when Y=31, and guards against going past row 21. The up-row handler (1400) simply increments X (confusingly named “up” but moves down in screen terms), capped at 21. The handler at 1500 decrements X, floored at 0. The newline handler (1600) sets Y=0 and increments X, equivalent to a carriage return/line feed.

Bugs and Dead Code

• Lines 1530–1660 (GOTO 100, NEXT N, SLOW, GOTO 100) are unreachable dead code. NEXT N without a corresponding FOR would cause an error if reached. SLOW is a ZX81 keyword and is not valid on the Spectrum/TS2068, suggesting copied or ported code fragments.
• The label “up” assigned to key q (code 113) dispatches to handler 1400 which moves down a row (X=X+1), while key p (code 112) at handler 1500 moves up (X=X-1). The naming is inverted relative to expected behaviour.
• The maximum row is hard-coded as 21.
• There is no cursor boundary check when printing characters in the normal typing path (line 1100); the automatic wrap at Y=32 (line 1120) uses 32 rather than 31, which means column 31 is valid but column 32 is never reached — this is correct since columns are 0-indexed to 31.