TED

TED is a ZX Spectrum screen-transition demo that showcases a suite of machine code routines for manipulating the SCREEN$ display in real time. Seven distinct effects — panning, zoom in, zoom out, corner zoom, fading, flip horizontal, and flip vertical — are driven by seven DEF FN wrappers (lines 10–70), each calling a USR entry point spaced five bytes apart starting at address 60000. Two picture files (“ted D” and “ted E”) are loaded from tape to addresses 46000 and 53000 respectively, serving as the source images for the transitions. The 900-byte machine code block at address 60000 (saved as “ted C”) provides all the manipulation routines, and RANDOMIZE USR is used throughout to invoke them without disturbing program flow. The demo loops continuously from line 130, cycling through all effects and finishing with a combined sequence before restarting.

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Program Structure

The program is divided into two logical phases. Lines 540–610 form the loader/introduction sequence: they set up memory with CLEAR 38999, load the 900-byte machine code block (“ted C”) to address 60000, display explanatory text, and wait for a keypress before jumping to RUN 10. Lines 10–530 are the actual demo, defining the function wrappers, loading the two picture files, and executing the transition loop.

The main demo loop runs from line 130 and ends with GO TO 130 at line 450, making the demo repeat indefinitely. A subroutine at line 520 handles on-screen effect labeling, printing the current effect name centered on the lower status line using PRINT #0 with padding spaces, then waiting for a keypress via PAUSE 0 before returning.

Lines 460–510 appear to be unreachable dead code — they are never called by a GO SUB or jumped to by any GO TO in the active loop, suggesting they are leftover development fragments or an abandoned alternate sequence.

Machine Code Interface

The 900-byte machine code block is loaded to address 60000 (“ted C”). Seven entry points are exposed, each five bytes apart:

DEF FN	Address	Effect
FN S(A)	60000	Unknown (used once, with S3)
FN L(A)	60005	Panning
FN H(A)	60010	Unknown (used for initial load and reset)
FN F(A)	60015	Fading
FN I(A)	60020	Zoom In
FN O(A)	60025	Zoom Out
FN T(A)	60030	Corner Zoom

Two additional addresses are stored as numeric variables: FH (60035) for flip horizontal and FV (60040) for flip vertical. These are called directly via RANDOMIZE USR FH and RANDOMIZE USR FV rather than through DEF FN wrappers, likely because they do not take a source-buffer argument.

Memory Layout

Three memory addresses are used as image buffers, stored in S1, S2, and S3:

• S1 = 46000 — “ted D” picture (first image)
• S2 = 53000 — “ted E” picture (second image)
• S3 = 39000 — third buffer (used only in the combined sequence at lines 420–500)

The CLEAR 38999 at line 540 protects the machine code at 60000 and places the BASIC stack safely below the S3 buffer region.

Key BASIC Idioms

• RANDOMIZE USR / RANDOMIZE FN — used throughout to call machine code routines and discard the return value without triggering errors, a standard Spectrum technique.
• DEF FN wrappers around USR — allow the machine code entry points to accept a parameter (the source buffer address A) via the function argument, even though the dummy parameter is formally required by BASIC syntax. The actual value of A passed is the buffer address.
• PAUSE 0 at line 530 — used as a keypress gate before each effect transition, giving the user control over pacing.
• Centering formula at line 520: (32 - LEN A$) / 2 calculates the correct column for a 32-column display.

Tape File Structure

The program saves and loads four separate tape files:

1. “TED” — the BASIC program itself, saved with LINE 540 to auto-run from the introduction.
2. “ted C” — 900 bytes of machine code at address 60000.
3. “ted D” — first screen image loaded to 46000.
4. “ted E” — second screen image loaded to 53000.

Anomalies and Notes

• Line 430 contains RANDOMIZE FV (without USR), which simply randomizes the random number seed to the value stored in FV (60040) rather than calling the machine code. This is almost certainly a bug — it should be RANDOMIZE USR FV as used elsewhere.
• Lines 460–510 are unreachable from the main demo loop and appear to be dead code, possibly a discarded alternate sequence.
• The DEF FN parameter A in lines 10–70 is syntactically required but its role as the actual buffer address passed to machine code depends entirely on the Z80 routine reading the BC or HL register set up by the USR call — a common convention for parameterized USR routines on this platform.