Micro-Pro

This program implements a scrolling text-ticker display using a string buffer. The variable B$ accumulates characters and is printed at screen position AT 0,0, with a cursor marker (▖ followed by a space) appended to show the current insertion point. Four control keys—coded 112, 113, 114, and 115—provide editing functions: character deletion (backspace), forward scroll by one line (32 spaces), space insertion, and cursor movement. The string B$ is constrained to a maximum length of roughly 672 characters (21 screen lines × 32 columns), enforcing a soft upper bound on the text buffer. The program relies on raw INKEY$ polling in a tight loop (lines 100–120) rather than INPUT, giving it immediate single-keypress response.

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Program Analysis

Program Structure

The program is divided into three functional regions:

1. Initialisation (lines 10–20): Clears the buffer B$ and prints the initial cursor graphic.
2. Input loop (lines 100–120): Polls INKEY$ continuously until a key is pressed, then dispatches to the command handler.
3. Command handler (lines 1000–1530): A chain of IF tests on CODE A$ routes execution to one of four editing sub-routines, followed by a display refresh and a return to the input loop.

Key Codes and Their Actions

Code	Key	Action	Handler
114	r	Backspace / delete last character	1200
115	s	Insert a single space	1300
113	q	Scroll forward one line (append 32 spaces)	1400
112	p	Scroll back one line (trim 33 chars from end)	1500

Buffer Management

All text is held in the string B$. Normal printable characters are appended at line 1100 after the dispatch chain finds no matching control code. The buffer’s effective maximum length is enforced by the guard at line 1030: if LEN B$>672 when the scroll-forward key (q) is pressed, the operation is silently skipped by jumping directly to the display refresh at 1110. Similarly, line 1050 prevents the scroll-back operation when LEN B$<32, avoiding an attempt to slice a string shorter than 33 characters.

Cursor Display

The cursor is represented by the block graphic ▖ (ZX Spectrum char 130, zmakebas escape \. ) followed by a space. It is appended to B$ at print time (line 1110: PRINT AT 0,0;B$;"▖ ") but never stored in B$ itself, so the buffer always contains only the actual content.

Backspace Routine (lines 1200–1230)

The delete handler at line 1200 replaces the last character of B$ with a space (B$( TO LEN B$-1)+" "), prints this padded version to erase the on-screen character, then removes the trailing space before falling through to the normal display refresh. This two-step approach avoids leaving a ghost character on screen when the cursor graphic shifts left.

A guard at line 1000 prevents deletion when LEN B$=0, jumping straight to the refresh instead.

Scroll-Back Routine (lines 1500–1530)

The scroll-back (p) handler appends a space, prints the buffer with an extra trailing space to clear any residual cursor graphic, then trims 33 characters from the end with B$( TO LEN B$-33). The asymmetry with scroll-forward (which appends 32 spaces) is intentional: the extra character accounts for the temporary space appended at line 1500 before slicing.

Notable Techniques and Idioms

• Tight INKEY$ polling (lines 100–110) rather than INPUT gives immediate single-keystroke response with no Enter key required.
• The dispatch chain at lines 1000–1060 uses paired guards (check boundary, then check key code) so boundary violations fall through to the no-op path at 1110 without needing explicit ELSE branches.
• PRINT AT 0,0; redraws from the top-left on every keypress, effectively using the screen as a 32-column-wide viewport into the string buffer.
• The cursor graphic is appended purely at print time, keeping B$ clean and simplifying all length arithmetic.