DPCM Letterbox

This NES program demonstrates abusing the NTSC NES's sampled sound
playback hardware as a scanline timer to split the screen twice
without needing to use a mapper-generated IRQ.

== How it works ==

The NES has sample playback hardware that can trigger when it
finishes playing a differential pulse code modulated (DPCM) waveform.
There are eight samples to a byte, and a waveform is 16n + 1 bytes
long.  There are sixteen valid rates for sample playback, numbered 0
to 15, and DPCM rate 15 has 54 CPU cycles per sample, or 54*3*8=1296
PPU dots per byte, or 3.8 scanlines per byte.  But the time between
NMI and the first sample data fetch drifts from frame to frame.
So at the start of the frame, the program has to measure exactly
how far apart the CPU and PPU are, and then the IRQ handler has to
waste a corresponding amount of time before doing raster effects.

This version has a slight visual artifact in the top overscan region
because it uses sprite 0 as a timing reference so that it can be
used even with an NMI handler whose execution time in CPU cycles
varies.  But a game with a cycle-timed NMI handler would not need
sprite 0; it could use the end of the NMI handler as a reference.

Reset handler:
Set up screen data
Enable IRQ
Wait forever

NMI handler:
Set up sprite 0 hit at top of screen
Disable sample playback
Turn on background rendering at a fixed scroll position
Wait for Sprite 0 off and on
Turn off background rendering
Clear number of elapsed IRQs
Enable playback and IRQ
Measure time until IRQ in 8-cycle units
Convert this to an amount of time to waste
Read the controllers
Compute next scroll value
Return

IRQ handler:
Add 1 to elapsed IRQs
Restart sample playback
If elapsed IRQs is at first threshold:
  Waste time in 8-cycle units
  Turn on background rendering
If elapsed IRQs is at second threshold:
  Switch next sample playback to 17-byte mode
If elapsed IRQs is at third threshold:
  Waste time in 8-cycle units
  Turn off background rendering

== Legal ==

The following license applies to the source code, binary code, and
this manual:

Copyright 2010 Damian Yerrick
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.  This file is offered as-is,
without any warranty.