Music Maker

Music-Maker is a ZX81/TS1000 melody composition and playback program that lets the user define a tune measure by measure, specifying pitch and duration values for each note. Playback is driven by machine code accessed via USR 16514, with timing and pitch values written directly to memory locations 16516, 16518, 16520, and 16528 using POKE statements. The tempo is stored as a single-element array Z(1), calculated as 960 divided by the user-supplied tempo value, and passed to the machine code routine. Notes are stored in parallel arrays N() for pitch and D() for duration, with pitch value 100 used as a special rest code that triggers POKE 16528,255 instead of 254. After entry, the melody can be replayed, have its tempo adjusted, or be saved to tape under the user-supplied melody name.

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Program Analysis

Program Structure

The program is divided into several functional sections linked by GOTO jumps rather than subroutines:

1. Lines 20, 180–210: Title screen and initialisation — displays the “MUSIC-MAKER” and “MOUNTAINEER SOFTWARE 1984” banners, then clears the screen.
2. Lines 220–350: Session setup — prompts for melody name (X$), number of measures (M), and tempo (T); computes Z(1)=960/T.
3. Lines 360–670: Note entry — iterates over each measure (J) and each note within it (K), collecting pitch into N(I) and duration into D(I).
4. Lines 30–170: Playback loop — executes the machine code routine for each note, then presents a post-play menu (replay, tempo change, or tape save).
5. Lines 680–850: Tape save — counts down and issues SAVE X$ to record the program under the melody name.
6. Lines 860–940: Tempo change — re-prompts for T and recomputes Z(1) before returning to the menu.
7. Lines 950–960: Program self-save at line 950, followed by RUN.

Machine Code Sound Routine

All audio output is delegated to a machine code routine whose entry point is address 16514. Four memory locations are configured via POKE before each USR call:

Address	Variable	Purpose
16516	Z(1)	Tempo/timing constant (960÷T), set once per play
16520	N(I)	Pitch value for current note
16518	D(I)*1000/N(I)	Duration scaled by pitch
16528	254 or 255	Rest flag: 255 = rest (N=100), 254 = normal note

The machine code is not defined within this BASIC listing; it is presumed to reside in memory already (likely POKEd in elsewhere or present in a companion loader). The result of USR 16514 is assigned to A at line 100 but the value is never used — this is simply the idiomatic way to call a machine code routine that returns a value.

Data Storage

Three arrays hold the session data:

• Z(1) — single-element array holding the tempo constant passed to machine code.
• N(X) — pitch values; pitch 100 is the special rest code.
• D(X) — duration values; duration for note I is passed as D(I)*1000/N(I), scaling duration relative to pitch.
• A(M) — note counts per measure; summed during entry to compute total note count X.

The total note count X is accumulated at line 410 inside the measure-count loop, and the N() and D() arrays are dimensioned only after all measure sizes are known (line 460–470), which is a correct two-pass approach.

Key BASIC Idioms

• Busy-wait loops: FOR I=1 TO 50: NEXT I (and similar) are used as crude delays throughout, a common ZX81 technique in the absence of a PAUSE command in FAST mode.
• FAST/SLOW switching: Playback runs inside FAST (line 30) to maximise the timing accuracy available to the machine code, with SLOW (line 120) restored for display output.
• Centred text: AT 2,16-(INT (LEN X$/2));X$ at lines 380 and 510 centres the melody name by computing its half-length offset — a standard ZX81 centring idiom.
• INKEY$ polling: Lines 140 and 700 use the IF INKEY$="" THEN GOTO spin-wait pattern to wait for any keypress.

Notable Techniques

The tempo formula Z(1)=960/T converts a musical tempo number into a machine-code-friendly period constant. The duration scaling D(I)*1000/N(I) means that for a given user duration value, higher-pitched (higher-frequency) notes have proportionally shorter machine code duration counts, which compensates for the relationship between pitch period and perceived note length.

The post-play menu (lines 130–170) is implemented without a PAUSE — it waits for a non-empty INKEY$ at line 140, then tests the same INKEY$ again at lines 150 and 160. Because INKEY$ is re-evaluated on each reference, there is a race condition: by the time lines 150 or 160 are reached, the key may have been released, causing the test to fail and falling through to line 170 (replay) even if S or T was pressed. A held key press is needed to reliably reach those branches.

Bugs and Anomalies

• INKEY$ race (lines 140–160): As noted, each reference to INKEY$ is a fresh read; a brief keypress detected at line 140 may not still be held when tested at lines 150 and 160.
• Missing line 630: The note-entry loop ends with NEXT J at line 620, but line numbers jump from 620 to 640 — line 630 is absent. This is harmless as it causes no execution gap.
• Unused variable A: LET A=USR 16514 at line 100 stores the machine code return value in scalar A, which conflicts with the array A(M) dimensioned at line 360. On the ZX81, assigning to scalar A after DIM A(M) would erase the array. However, by the time playback reaches line 100, note entry is complete and A() is no longer needed, so no data is lost in practice.
• Tape save countdown: The inner loop FOR J=1 TO 10: NEXT J in the countdown (lines 730–740, 770–780) provides very short delays between countdown digit updates — the countdown will appear nearly instantaneous rather than one second per digit.