SuperScan

SuperScan is a demonstration program that uses an embedded machine language routine to scroll and display text within a sub-region of the screen, allowing simultaneous output on different screen areas. The machine code is stored beginning at address 16514 (just after the system variables area) and is invoked repeatedly via RAND USR 16528 and RAND USR 16525 throughout the program. POKEs at lines 5, 6, and 7 write parameters to addresses 16514–16516, configuring the scroll routine’s operating region, and are zeroed out at the end (lines 620–630) to restore normal display behavior. The demonstration sequence introduces itself, accepts user input (a name and two numbers whose sum it displays), prints decorative patterns, and cycles through multiple screen segments, concluding with a full CLS and a GOTO loop back to line 1. SLOW mode is engaged at line 9 to ensure the machine code routine and display operations remain in sync.

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Program Analysis

Program Structure

The program is organized into a linear demonstration sequence with no subroutines in BASIC. The flow is:

1. Lines 1–9: Machine code storage in REM and initial POKEs to configure the scroll routine; SLOW mode enabled.
2. Lines 20–199: Introductory conversation — greets the user, accepts a name via INPUT N$, and displays a series of teaser messages.
3. Line 200–205: Waits for a keypress using PAUSE 4E4 followed by a IF INKEY$ <>"" branch to continue.
4. Lines 209–390: Demonstrates the scrolling sub-region by printing decorative strings (A$, D$) in a loop, interspersed with RAND USR 16528 calls.
5. Lines 395–500: Accepts two numbers, adds them, and displays the result, all punctuated by scroll calls.
6. Lines 505–610: Closing messages and farewell sequence.
7. Lines 620–640: Teardown — zeroes the POKE parameters, CLS, and loops back to line 1 via GOTO 1.
8. Lines 697–699: STOP, SAVE, and another GOTO 1 — utility tail used during development/saving.

Machine Code Routine

The machine language payload lives inside the REM statement at line 1. This is a standard technique for storing executable code: the Z80 bytes are embedded as the text of a REM, keeping them safe from BASIC’s memory management. Two entry points are referenced:

Address	Role
16525	Secondary entry point — called to reset or reinitialize the scroll region (used after pauses between sections).
16528	Primary entry point — called before every PRINT to advance or manage the sub-region scroll.

Addresses 16514, 16515, and 16516 are POKEd at program start with values 6, 3, and 17 respectively. These fall just beyond the system variables and serve as parameters (likely defining the scroll window’s top row, height, or column boundaries) read by the machine code. They are zeroed at lines 620–630 to restore normal PRINT behavior before the program restarts.

Key BASIC Idioms

• PAUSE 4E4 at line 200 uses floating-point scientific notation to express a large pause value (40,000 frames), a common trick to avoid typing many zeros.
• IF INKEY$ <>"" THEN GOTO 209 at line 205 polls for a keypress immediately after the long pause, providing early-exit capability without halting execution.
• RAND USR is used purely as a mechanism to call machine code and discard the return value — RANDOMIZE USR does not affect subsequent random number generation in a meaningful way here.
• GOTO 1 at line 640 loops back to the start of the program including the REM line, effectively restarting the demo without re-executing the POKEs independently — however, lines 5–7 will re-POKE the parameters on each cycle.

Notable Techniques

The core novelty is the sub-region scroll: by calling RAND USR 16528 before each PRINT, the machine code presumably shifts previously printed lines upward within a defined window, preventing the normal full-screen scroll. This allows text to appear to accumulate and scroll within a portion of the display while the rest of the screen remains static — the effect advertised in the demo messages about “printing on one side while you read the other.”

The RAND USR 16525 calls, used at section boundaries (lines 272, 356, 395, 505, 535, 595), likely clear or reset the scroll window, analogous to a localized CLS for the sub-region.

Block graphic characters (the ▞ pattern in A$ at line 210) are used as a decorative fill string, demonstrating the machine code’s scrolling visually with a high-contrast repeating pattern.

Bugs and Anomalies

• Line 380 reads PRINT "ON THIS SIDE." while line 370 already printed PRINT "OTHER THING ON" — the word “ON” is repeated across two consecutive lines, which appears to be a typo (“OTHER THINGS ON / THIS SIDE” was likely intended).
• Line 640 does GOTO 1, which jumps to the REM line. Execution then falls through to line 5 and re-POKEs the machine code parameters, which is harmless but slightly redundant.
• Lines 697–699 form an unreachable tail: STOP at 697 prevents SAVE at 698 from ever being reached in normal flow, suggesting these lines were used manually during development to save the program.