Screen Routines

This program is a demonstration of four screen-manipulation effects on a ZX81/TS1000, driven by machine code routines embedded in a REM statement at line 1. The four effects are: a border-drawing routine (using a user-supplied character), a line-scroll routine that repeatedly scrolls a typed message up the screen, a screen-fill routine, and a screen-invert routine. Machine code is stored starting at the address of the REM statement’s data bytes (around address 16514–16598) and called via RAND USR to invoke each effect. A POKE at address 16515 or 16546 is used to pass parameters—such as the fill character code or the screen row—directly into the machine code before execution.

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Program Analysis

Program Structure

The program is organized as a menu-driven demo with five logical sections:

1. Line 1 (REM): Holds the raw machine code bytes for all routines.
2. Lines 10–120: Main menu — displays four options and polls INKEY$ after calling a wait/scan routine via RAND USR 16545.
3. Lines 1000–1070: Option A — Border effect using a user-chosen character.
4. Lines 2000–2070: Option B — Line-scroll effect for a user-typed message.
5. Lines 3000–3030: Option C — Screen-fill effect.
6. Lines 4000–4030: Option D — Screen-invert effect, then returns to the menu.
7. Line 5000–5010: SAVE block, then restarts via GOTO 1.

Machine Code Routines in the REM Statement

All machine code is packed into the data portion of the REM at line 1. On the ZX81/TS1000, a REM statement’s text bytes are stored directly in memory beginning two bytes after the line’s start, making them executable as Z80 code. The system variable E_LINE (address 16404/0x400C) holds the address of the edit line, but the REM data itself starts at the fixed address 16514 (the program loads at 16509, with the REM token at offset 4 and its length bytes occupying two more, placing data at 16514).

The identified entry points and their approximate roles are:

USR Address	Role
16514	Border draw — fills screen border region with character from POKE at 16515
16545	Keyboard scan / wait routine (called from main menu loop)
16580	Screen fill routine (Option C)
16598	Screen invert routine (Option D)

Key BASIC Idioms and Techniques

• RAND USR as a subroutine call: RAND USR nnnnn is the standard ZX81 idiom for invoking machine code; the return value of USR is passed to RANDOMIZE and discarded.
• POKE for parameter passing: Option A uses POKE 16515, CODE A$(1) to write the desired character code directly into the machine code (one byte past the entry point), effectively setting a parameter before calling RAND USR 16514. Similarly, POKE 16546, 21 and POKE 16546, 10 set a row parameter used by the scroll routine.
• INKEY$ polling loop: Lines 80–120 form a tight polling loop — RAND USR 16545 is called to perform a scan, then each INKEY$ is tested. This avoids a slow FOR/NEXT delay and instead uses machine code for timing.
• Wait-for-keypress idiom: Line 1060 uses IF INKEY$="" THEN GOTO 1060 to spin until any key is held, then returns to the main menu via RUN.
• RUN to restart: Each option ends with RUN rather than GOTO 10, which fully reinitialises BASIC variables — a common ZX81 pattern to avoid variable accumulation.

Machine Code Disassembly Notes

Examining the hex bytes in the REM:

• 3E 26 — LD A, 0x26 (load character/value)
• 2A 0C 40 — LD HL, (0x400C) — loads the D_FILE pointer (display file address) from system variable at 0x400C
• 06 20 — LD B, 32 — sets column count (32 characters wide)
• 23 77 — INC HL / LD (HL), A — writes character to display, repeated in loops
• 10 FC — DJNZ loop back — tight inner loop for filling rows
• C9 — RET — return to BASIC at routine end
• ED B0 — LDIR — block copy instruction, used in the scroll/copy routine
• 76 — HALT — used as the ZX81 display newline sentinel (0x76 = HALT opcode = ZX81 newline character)

Notable Techniques

• The screen invert loop (Option D) calls RAND USR 16598 ten times inside a FOR loop, creating a flicker/animation effect rather than a single inversion.
• The scroll demo (Option B) calls RAND USR 16545 64 times in a FOR loop — using the machine code routine as a per-step delay/scroll engine.
• The SAVE at line 5000 followed by GOTO 1 means the program can save itself and immediately continue running.

Potential Anomalies

• Line 1060 waits for a key to be pressed before returning to menu, but the check is INKEY$="" (wait while no key), then falls through to CLS and RUN — this is correct but means any key press exits immediately without identifying which key was pressed.
• The POKE targets (16515, 16546) are hardcoded absolute addresses; if the program is loaded at a different address (e.g., with a different REM length), these will be incorrect. The program is therefore position-dependent.