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Space Invaders (1978) |
/machines/arcade/invaders/ |
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This 8080-based PCjs machine emulates the original Space Invaders arcade machine.
Hard-coded key mappings include:
- 1: One Player
- 2: Two Player
- 3: Insert Coin
- A: Move Left
- D: Move Right
- L: Fire Missile
The PCjs debugger, along with a few Notes, are provided below.
{% include machine.html id="invaders" %}
For touch-screen devices like the iPhone and iPad, there are "touch regions" across the top of the virtual monitor that correspond to first three buttons:
- Top Left: One Player
- Top Right: Two Players
- Top Center: Insert Coin
and regions across the bottom that correspond to the last three buttons:
- Left Side: Move Left and Move Right
- Right Side: Fire
This is purely experimental and may only work in portrait mode; landscape and full-screen modes will probably need more work to make them usable.
0000-1FFF r ROM code
2000-23FF rw RAM
2400-3FFF rw bitmapped screen (224x256)
00 r
01 r control inputs
bit 7
bit 6 right
bit 5 left
bit 4 fire
bit 3
bit 2 1 player start
bit 1 2 player start
bit 0 coin slot
02 r control inputs
bit 7 0=display coin info
bit 6 right 2
bit 5 left 2
bit 4 fire 2
bit 3 bonus (1500,1000) / preset mode
bit 2 1=tilt
bit 0-1 initial lives (3,4,5,6) / (3,4)
02 w shift count (0-7)
03 r shifted value (low then high)
03 w sound
bit 4 bonus base
bit 3 invader hit
bit 2 base hit
bit 1 base fire
bit 0 saucer
04 w value to shift
05 w sound
bit 5 flip video
bit 4 saucer hit
bit 3 invader movement 4
bit 2 invader movement 3
bit 1 invader movement 2
bit 0 invader movement 1
06 w watchdog timer clear
Note that, unlike other emulators, PCjs machines have a Bus architecture, allowing components to "plug in" different kinds of memory or memory-mapped devices at different addresses, and to register specific functions for specific I/O ports.
For example, Space Invaders has 8Kb of ROM at addresses 0x0000 through 0x1FFF, which means that if any 8080 code attempts to write to those addresses, nothing should happen. Unfortunately, most emulators treat the entire address space as one contiguous array of bytes. Which means either that the ROM is susceptible to corruption or that every write operation must check the address to determine its validity, which hurts the performance of all writes.
In PCjs machines, all writes are equally fast, and all ROMs are fully protected. An exception is made for the Debugger, which allows you to use the "e" command to modify ("patch") ROM code on the fly, but that is completely outside and independent of the 8080 code being emulated.
See Computer Archeology for an excellent collection of materials on the original Space Invaders, including commented ROM disassemblies.