Hardware

https://github.com/mamedev/historic-mame/blob/master/src/mame/video/phoenix.c

Each memory bank:

0000 - 033F  - 32x26 foreground tiles
0340 - 07FF
0800 - 0B3F  - 32x26 background tiles
0B40 - 0FFF

The lower bit of video register 50xx selects the memory bank and the screen flipping for cocktail mode
(if cocktail mode is available via dipswitch). When the 2nd bank is selected (and switch) then the screen
is flipped.

The next-to-lowest bit control the color palette for both foreground and background.

From phoenix.cpp:

   map(0x0000, 0x3fff).rom();
	map(0x4000, 0x4fff).bankr("bank1").w(FUNC(phoenix_state::phoenix_videoram_w)); /* 2 pages selected by bit 0 of the video register */
	map(0x5000, 0x53ff).w(FUNC(phoenix_state::phoenix_videoreg_w));
	map(0x5800, 0x5bff).w(FUNC(phoenix_state::phoenix_scroll_w));
	map(0x6000, 0x63ff).w("cust", FUNC(phoenix_sound_device::control_a_w));
	map(0x6800, 0x6bff).w("cust", FUNC(phoenix_sound_device::control_b_w));
	map(0x7000, 0x73ff).portr("IN0");                            /* IN0 or IN1 */
	map(0x7800, 0x7bff).portr("DSW0");                           /* DSW */

Upper bit of DSW (0x78xx) is a screen-blanking flag. 0=in_VBLANK 1=not_in_VBLANK

0x7000
Bit goes from 1 to 0 when a button is pressed:
  0xBF..1011_1111 = button 'left'
  0xDF..1101_1111 = button 'right'
  0xEF..1110_1111 = button 'fire'
  0x7F..0111_1111 = button 'shield'
  0xFD..1111_1101 = button '1 player start'
  0xFB..1111_1011 = button '2 players start'
  0xFE..1111_1110 = coin is inserted
  0xFF..1111_1111 = no button pressed

Music: Melody chip used is a 18 pin DIL IC: Matsushita MN6221AA (and rare MN6221AB playing different tunes). For technical details and the differences with tunes, see: https://eleken.y-lab.org/melody_ic/MN6221.shtml

Tune1 -- Alarm sound
Tune2 -- Fuer Elise, Beethoven          0x6800 = 1000_1111
Tune3 -- ESTUDIO (Phoenix theme song)   0x6800 = 1100_1111
Tune4 --  single notes

Switch Settings

LL_BB_C_xx_M

LL: Number of lives
  11 = three
  01 = four
  10 = five
  00 = six

BB: Bonus lives at
  11 = 3K/30K
  01 = 4K/40K
  10 = 5K/50K
  00 = 6K/60K

C: Coinage
  1 = 1 coin for 1 credit
  0 = 2 coins for 1 credit

M: Cabinet mode
  1 = cocktail
  0 = upright

5000	videoRegister	Lower bit selects the RAM bank
5800	scrollRegister	Screen scrolling
6000	soundControlA	Sound control A
6800	soundControlB	Sound control B
7000	IN0	Player inputs
7800	DSW0	DIP switch settings