pcjs/devices/pc8080/machine/invaders

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page	Space Invaders (Prototype)	/devices/pc8080/machine/invaders/	type id pc8080 invaders

Space Invaders (Prototype)

This is a work-in-progress, with Development Notes below.

{% include machine.html id="invaders" %}

Development Notes

From emutalk.net:

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