Casio PV-1000/ASIC registers

The µPD65010G031 ULA has 10 total ports. They are fully decoded, available via Z80 I/O instructions at $F8-$FF.

There are eight write-only functions and two read-only functions.

Square 1 ($F8) > write

7  bit  0
..PP PPPP
  || ||||
  ++-++++--- Period for first square wave.

f_square = f_ULA ÷ 1024 ÷ (63-P) ≈ 8739 Hz ÷ (63-P)

Where f_ULA is the 17.897727MHz crystal divided by 2.

If P is specified to be 63, it instead stops clocking the square wave. This can cause it to stay outputting a "1"

Square 2 ($F9) > write

Same as Square 1, but 3dB louder.

Square 3 ($FA) > write

Same as Square 1, but 6dB louder.

Sound control ($FB) > write

7  bit  0
.... ..ES
       ||
       |+--- Sound effect mode (??)
       +---- 1: Enable

Writes to ports F8-FA are silent unless enabled. Not yet known if disabling sound clears the current phase, but it does drive all three outputs to "0"

Interrupt controller? ($FC) > write

Existing games write 0, 1, or 3.

7  bit  0
.... ..BC
       ||
       |+--- IRQ 1 enable?
       +---- IRQ 2 enable?

Interrupt status? ($FC) < read

7  bit  0
ZAAA AABC
|||| ||||
|||| |||+-- Unclear. Counts 2 scanlines?
|||| ||+--- Usually 1?
|+++-++---- Always 0?
+---------- Always 1 (Probably pulled up by resistor next to Z80)

GPO ($FD) > write

7  bit  0
.AAA BBBB
 ||| ||||
 ||| ++++-- 4 general purpose outputs, accessible via both controller ports
 +++------- Strongly implied to exist on the ULA, but not accessible externally

GPI ($FD) < read

7  bit  0
ZAAA BBCC
|||| ||||
|||| ||++-- 2 general purpose inputs for reading the player 1 joystick
|||| ++---- 2 general purpose inputs for reading the player 2 joystick
|+++------- Always 0. (Strongly implied to exist on the ULA, but grounded there)
+---------- Always 1. (Probably pulled up by resistor next to Z80)

Tilemap and pattern RAM address ($FE) > write

7  bit  0
BBBB AA..
|||| ||
++++-++---- Address A15-A10 from where to fetch the tilemap. 
||||        Almost all software write $B8, meaning the ULA fetches tilemap from $B802 through $BAFD.
++++------- Address A15-A12 from where to fetch pattern RAM tiles (A11-A10 always 1)
            The common value of $B8 means that ULA fetches data for tiles $E0-$FF from $BC08-$BFFF

Display properties ($FF) > write

7  bit  0
AAAB CDDD
|||| ||||
|||| |+++-- Border color. (0=black, 1=red, 2=green, 3=yellow, 4=blue, &c).
|||| +----- Display disable. (Screen solid border color, faster Z80)
|||+------- Pattern RAM disable 
|||          (0: Tiles $E0-$FF are fetched from address specified by port $FE
|||           1: Tiles $E0-$FF are fetched from address specified by port $FF)
+++-------- Address A15-A13 from where to fetch patterns
             (0: Patterns are fetched from $0008-$1BFF or -$1FFF
              1: starting at $2008
              2: starting at $4008 &c

It's not yet confirmed that the pattern fetch is a full three bits wide. At least Dig Dug uses both B and the lsbit of AAA.

Other sources (e.g. Enri) refer to the pattern memory as "PCG"