http://emulator101.com/ then 8080 reference in the top right http://www.emulator101.com/reference/8080-by-opcode.html

Space Invaders, (C) Taito 1978, Midway 1979

CPU: Intel 8080 @ 2MHz (CPU similar to the (newer) Zilog Z80)

Interrupts: $cf (RST 8) at the start of vblank, $d7 (RST $10) at the end of vblank.

Video: 256(x)*224(y) @ 60Hz, vertical monitor. Colours are simulated with a plastic transparent overlay and a background picture. Video hardware is very simple: 7168 bytes 1bpp bitmap (32 bytes per scanline).

Sound: SN76477 and samples.

Memory map: ROM $0000-$07ff: invaders.h $0800-$0fff: invaders.g $1000-$17ff: invaders.f $1800-$1fff: invaders.e

RAM
$2000-$23ff:	work RAM
$2400-$3fff:	video RAM

$4000-:		RAM mirror

Ports: Read 1 BIT 0 coin (0 when active) 1 P2 start button 2 P1 start button 3 ? 4 P1 shoot button 5 P1 joystick left 6 P1 joystick right 7 ?

Read 2
BIT	0,1	dipswitch number of lives (0:3,1:4,2:5,3:6)
	2	tilt 'button'
	3	dipswitch bonus life at 1:1000,0:1500
	4	P2 shoot button
	5	P2 joystick left
	6	P2 joystick right
	7	dipswitch coin info 1:off,0:on

Read 3		shift register result

Write 2		shift register result offset (bits 0,1,2)
Write 3		sound related
Write 4		fill shift register
Write 5		sound related
Write 6		strange 'debug' port? eg. it writes to this port when
		it writes text to the screen (0=a,1=b,2=c, etc)

(write ports 3,5,6 can be left unemulated, read port 1=$01 and 2=$00
will make the game run, but but only in attract mode)

I haven't looked into sound details.

16 bit shift register: f 0 bit xxxxxxxxyyyyyyyy

Writing to port 4 shifts x into y, and the new value into x, eg.
$0000,
write $aa -> $aa00,
write $ff -> $ffaa,
write $12 -> $12ff, ..

Writing to port 2 (bits 0,1,2) sets the offset for the 8 bit result, eg.
offset 0:
rrrrrrrr		result=xxxxxxxx
xxxxxxxxyyyyyyyy

offset 2:
  rrrrrrrr	result=xxxxxxyy
xxxxxxxxyyyyyyyy

offset 7:
       rrrrrrrr	result=xyyyyyyy
xxxxxxxxyyyyyyyy

Reading from port 3 returns said result.

Overlay dimensions (screen rotated 90 degrees anti-clockwise): ,___________________________. |WHITE ^ | | 32 | | v | |-------------------------------| |RED ^ | | 32 | | v | |-------------------------------| |WHITE | | < 224 > | | | | ^ | | 120 | | v | | | | | | | |-------------------------------| |GREEN | | ^ ^ | |56 ^ 56 | | v 72 v | | v ______________| | ^ | | ^ | |<16> | < 118 > |16 < 122 > | | v | | v | |WHITE| | WHITE| `-------------------------------'

Way of out of proportion :P