Gran Marquee

Gran-Marquee is a scrolling marquee display program that smoothly shifts characters across the screen one pixel column at a time using a machine-code routine embedded in a REM statement. The machine code, stored at line 10 from address 16384 onwards, reads a character code from a fixed memory location, uses the ZX81’s character ROM data, and shifts pixel columns through display memory to create a hardware-level horizontal scroll effect. The user can set scroll speed (0–9), enter a custom message via INPUT, and control behaviour with single-key commands: C clears the credit overlay, S and F toggle display options via direct POKEs, and L adjusts display memory settings. The program uses USR to call the machine code, passing data via POKEs to address 16514, and the REM at line 11 contains inverse-video text forming the program’s title screen credit block.

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Program Analysis

Program Structure

The program is organised into several functional sections:

1. Lines 10–11: Machine code payload in a REM statement; inverse-video title credit block in a second REM.
2. Lines 15–70: Initialisation — default message, speed prompt, speed input loop.
3. Lines 90–190: Main scroll loop, calling machine code via USR, overlaying credits, speed delay, and key dispatch.
4. Lines 200–240: Custom message INPUT handler.
5. Lines 300–310: In-flight speed change handler.
6. Lines 400–520: Display option toggle handlers via direct POKEs.
7. Lines 600–610: Enable continuous scroll (skip credit overlay) mode.
8. Lines 9998–9999: SAVE with auto-run flag, then RUN.

Machine Code Routine (Line 10 REM)

The REM statement at line 10 contains a Z80 machine code routine beginning at address 16516 (the first byte after the REM token and length bytes at the start of line 10’s data area). The routine is invoked by USR 16516. Before each call, POKE 16514, CODE A$(I) places the current character code into a fixed memory cell that the routine reads.

Disassembly of the hex payload reveals the routine’s logic:

• Loads the character code from address 16514 ($40AF), subtracts 8 per pass to index into the character ROM at $1E00 (for ZX81 character data).
• Uses LD HL,(16396) (2A 0C 40) to fetch the display file address from the system variable D_FILE.
• Iterates 8 times (one per pixel column of the character), shifting each of 6 display rows one pixel left and inserting the new column on the right edge.
• The outer loop (DJNZ at $10E6) repeats for all 22 display rows worth of data.
• Returns via RET (C9); the BASIC system receives a dummy USR return value in X.

Key BASIC Idioms

LET X=USR 16516 at line 120 discards the return value but is required because USR in an expression context is the only way to call machine code from ZX81 BASIC. The result is assigned to X purely to satisfy the syntax.

Speed is encoded as S=30-3*VAL S$, mapping digit keys 0–9 to delay loop counts 30 down to 3. The delay at lines 150–160 is a simple FOR/NEXT busy-wait, with S=0 (speed 0 entered) giving a count of 30 and maximum wait, reversed from intuitive expectation — speed “9” gives the fastest scroll (smallest S).

The input filter at line 50 uses CODE S$ comparison against the ZX81 character codes for digits: codes 28–37 correspond to characters ‘0’–’9′, so only digit keys pass through to set speed.

Continuous Scroll Flag

The variable CS (initialised at line 15, settable via key F at line 600 or by pressing a key with code 43 at startup — the + key) acts as a “skip credit overlay” flag. When CS=1, line 125 executes NEXT I immediately, bypassing the PRINT at line 130 and the delay/key-check block, producing a faster uninterrupted scroll. Key C at line 122 resets CS to 0, re-enabling the credit overlay.

Display Memory POKEs (Lines 400–520)

Lines 400 and 500 write directly to addresses 16556 and 16566. On the ZX81, these addresses fall within the system variable area or early display file, and the specific values (0 vs. 9 and 0 vs. 128) suggest toggling between normal and inverted or alternate display modes, though their exact effect depends on the precise system variable layout at runtime.

Key	Action	Lines
0–9	Change scroll speed	167, 300
C	Clear/disable continuous mode	122, 163
S	Set display POKEs to 0/0	164, 400–420
L	Set display POKEs to 9/128	165, 500–520
F	Enable continuous scroll	166, 600–610
Any other	Enter custom message	170, 200–240

Notable Techniques

• Embedding Z80 machine code in a REM statement is the standard ZX81 technique for persistent machine code that survives RUN and CLEAR.
• The credit display at line 130 uses PRINT AT with inline inverse-video characters (encoded as %letter in the source) to render a title overlay on the scrolling display without clearing the screen.
• The tail GOTO 100 at line 190 re-appends spaces (A$=A$+" " at line 90 runs only once per message entry) so the loop continues to add padding each time through — actually A$ grows by 5 spaces every time line 90 is reached via GOTO 100, which is a slow memory leak but unlikely to matter for a typical message length during a demo session.
• The SAVE "GRAN MARQUE%E" at line 9998 uses an inverse-E in the filename combined with RUN at 9999 as the auto-run mechanism.

Potential Anomalies

Line 90 (LET A$=A$+" ") is re-executed every time GOTO 100 is reached from line 190, causing the string to grow by 5 characters each full pass. Over many iterations this will gradually increase memory use and slow performance, though this is likely an oversight rather than intentional behaviour.

The speed-input loop at line 40–50 accepts code 43 (‘+’ key) to set CS=1, but line 50’s range check will then GOTO 40 again waiting for a valid digit — so ‘+’ enables continuous mode but still requires a speed digit to proceed, which is the intended startup behaviour.