Loader

This program is a menu-driven tape loader that presents eight programs for selection and then LOADs the chosen one from cassette. The screen is decorated with a bordered frame built from block graphics: a top row of inverse semicolons, 22 rows of a repeating checkerboard pattern stored in B$, and a bottom row of inverse exclamation marks. Menu item names are displayed in normal video alongside inverse-video equivalents that animate over them to highlight the prompt. Selection is validated by checking that the INKEY$ code falls between 29 and 37 (the digit characters 1–8 on the ZX81 keyboard), then a computed GO TO of VAL A$*1000 dispatches directly to the appropriate LOAD statement.

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Program Analysis

Program Structure

The program is organised into four logical phases:

1. Initialisation (line 1): POKE 16418,0 sets the system variable controlling the display file position, effectively resetting the screen output cursor to the top.
2. Screen decoration (lines 2–8): A bordered frame is drawn using block graphics stored in a string and printed via a loop.
3. Menu display and input loop (lines 9–110): Program names and numbers are printed, then a tight polling loop waits for a valid digit keypress.
4. Dispatch and tape loading (lines 120, 1000–8000): A computed GO TO selects the appropriate LOAD statement.

Screen Border Construction

Three distinct graphic rows form the frame:

• Top row (line 4): A PRINT statement fills the line with inverse semicolons (\;; block graphic = ▄ repeated) flanked by inverse spaces, producing a solid top border.
• Middle rows (lines 2, 5–7): B$ holds a string of \@@ block graphic characters (checkerboard pattern) between inverse spaces. The FOR N=1 TO 22 loop prints this 22 times to fill the body of the screen.
• Bottom row (line 8): Inverse exclamation marks (\!!) produce a contrasting bottom border.

Menu Display Technique

Line 9 prints the title “2/83 PROGRAM LOADER” entirely in inverse video using the %X escape sequences. Line 10 uses a single PRINT statement with multiple AT and TAB clauses to place all eight menu entries and their corresponding digit labels in one operation, minimising execution overhead. Line 20 first prints a normal-video prompt, then immediately overwrites it with an inverse-video version at the same AT position — a common ZX81 technique to render text in inverse without using a separate PRINT statement.

Input Validation

The input loop at lines 100–110 uses INKEY$ for non-blocking polling. Validation checks three conditions in one IF:

• A$="" — no key pressed yet
• CODE A$<29 — key code below the digit “1” (ZX81 codes 29–37 map to digits 1–9)
• CODE A$>37 — key code above digit “9”, rejecting any non-numeric input

This means digits 1–8 are accepted (and implicitly digit 9, though line 9000 is a catch-all LOAD "").

Computed GO TO Dispatch

Line 120 uses GOTO VAL A$*1000 to calculate the target line number at runtime. Since VAL A$ returns the numeric value of the single digit entered (1–8), multiplying by 1000 produces exactly the line numbers 1000–8000 where each LOAD statement resides. This is a compact and efficient dispatch table requiring no IF/THEN chain.

Utility Lines

Line	Purpose
9000	LOAD "" — loads the next program from tape regardless of name (fallback / digit 9)
9998	SAVE "LOADER" — re-saves this loader program to tape for duplication
9999	RUN — restarts the loader after returning from a saved program or after saving

Notable Idioms and Anomalies

• The program names in the LOAD statements (e.g., "ADDITIO%N") contain the %N inverse-video escape, meaning the final letter “N” is stored as an inverse character in the tape header name. This must match exactly with how those programs were originally saved.
• POKE 16418,0 at line 1 directly manipulates the DF_CC system variable (display file current character address low byte), which resets the print position — a lower-level approach than CLS.
• The single long PRINT at line 10 is a memory optimisation; each additional PRINT statement costs extra bytes for the line header overhead.