Droppages

This program renders a three-dimensional surface plot of the function z = 3·e^(−0.1·(x²+y+y)) using a perspective projection, then stores successive views to memory pages and plays them back as a simple animation. The projection routine at lines 20–30 applies a full rotation matrix using precomputed sines and cosines of azimuth angle theta and elevation angle phi, projecting 3-D coordinates to a 2-D screen position centered at (cx, cy). Two 12-byte machine code routines are POKEd into RAM at addresses 32000 and 32050; these use the Z80 LDIR instruction (opcode ED B0) to block-copy the screen buffer (6912 bytes from address 16384) to and from off-screen memory pages, enabling multi-frame storage and retrieval. The program iterates theta in steps of PI/4 from PI/8 to PI, drawing four rotated views and cycling through them in a continuous loop via the recall routine.

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Program Analysis

Program Structure

The program is organized into several functional blocks:

1. Initialization (lines 1–8): Clears memory above 31999, defines base addresses for two machine code routines, and POKEs them into RAM.
2. Projection subroutine (lines 20–30): Applies a 3-D rotation and perspective transform.
3. Main drawing loop (lines 35–140): Iterates theta, draws the surface for each viewing angle, and saves the screen to a memory page.
4. Page-save logic (lines 1000–1050): Patches the save machine code routine with the destination address and calls it.
5. Playback loop (lines 2000–2060): Recalls saved screen pages in sequence with a short pause between frames, then repeats indefinitely.
6. Save lines (9000–9050): SAVE statements for backing up the BASIC program and the machine code block.

Machine Code Routines

Two 12-byte Z80 routines are installed at s=32000 (save) and r=32050 (recall). Both use the Z80 block-move instruction LDIR (mnemonic opcodes ED B0) to copy 6912 bytes (a full Spectrum display file). The DATA statements decode as follows:

Address	Bytes	Z80 Mnemonics	Purpose
32000	17, a1, a2	LD DE, <dest>	Destination address (patched at runtime)
32003	33, 0, 64	LD HL, 16384	Source = screen start
32006	1, 0, 27	LD BC, 6912	Byte count (27×256 = 6912)
32009	237, 176	LDIR	Block copy
32011	201	RET	Return to BASIC

The recall routine at 32050 mirrors this with source and destination swapped: DE is loaded with 16384 (the screen) and HL with the page address, so the off-screen data is written back to the display. Addresses are patched dynamically by poking the low and high bytes into 32001/32002 (save) and 32054/32055 (recall) before each USR call.

Perspective Projection

The subroutine at lines 20–30 implements a standard perspective camera transform. The 3-D point (x, y, z) is first rotated using precomputed values s1=SIN(t), c1=COS(t), s2=SIN(phi), c2=COS(phi), producing camera-space coordinates xe, ye, ze. The offset rho=50 shifts the scene along the camera’s z-axis. Screen coordinates are then computed as sx = d*xe/ze + cx and sy = cy + d*ye/ze with focal length d=350 and screen center (cx, cy) = (127, 87).

Surface Function and Drawing

The surface is defined at line 50 as DEF FN z(x) = 3*EXP(-.1*(x*x+y+y)). Note that the exponent argument contains y+y (i.e., 2y) rather than the expected y*y, which is almost certainly a typo — the intended function is the Gaussian bell surface z = 3·e^(−0.1·(x²+y²)). The outer loop iterates x from 10 to −10 in steps of −0.1 and the inner loop iterates integer y from −10 to 10. The variable fl acts as a “pen down” flag: when a valid screen point is found after an out-of-bounds gap, PLOT is used to begin a new stroke; otherwise DRAW connects to the previous point using the stored values sx1, sy1. However, lines 125–130 overwrite sx/sy with themselves before DRAW, meaning the DRAW always has delta arguments of zero — this is a bug; the previous screen coordinates should have been saved before recalculating sx and sy for the new point.

Memory Page Addressing

Each of the four screen pages is stored at address a = p*7000 + 26000, giving addresses 33000, 40000, 47000, and 54000 for pages 1–4. These regions lie well above the BASIC/display area but must fit within a 64 KB address space; page 4 at 54000 + 6912 = 60912 is within bounds. The low and high byte decomposition uses integer arithmetic: a2 = INT(a/256), a1 = a - a2*256, avoiding the BASIC INT/MOD combination.

Notable Techniques and Anomalies

• Commented-out INPUT and IF statements at lines 1000–2050 suggest the program was originally interactive but was simplified to run automatically with p auto-incrementing and q driven by a FOR loop.
• LET l=1 at line 120 appears to set a variable that is never subsequently used — likely a remnant of a color or attribute selection feature.
• The CLEAR 31999 at line 3 sets RAMTOP just below the machine code, protecting it from BASIC’s memory manager.
• The playback loop (lines 2000–2060) runs indefinitely via GO TO 2000 after exhausting the FOR loop, creating a continuous slideshow.
• The REM at the end of line 2050 contains RETURN, suggesting an earlier design where the recall block was a subroutine.
• Two separate SAVE "droppages" LINE 0 statements exist at lines 9000 and 9050, which is redundant.