PCjs Machines

Space Invaders (1978)

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.

Space Invaders Emulation Notes

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.

Space Invaders Hardware Notes

Memory Map

0000-1FFF  r   ROM code
2000-23FF  rw  RAM
2400-3FFF  rw  bitmapped screen (224x256)

I/O Map

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.

Other Online References

See Computer Archeology for an excellent collection of materials on the original Space Invaders, including commented ROM disassemblies.