pcjs/devices/pc8080/machine/invaders

layout	title	permalink	machines
page	Space Invaders (Under Construction)	/devices/pc8080/machine/invaders/	type id debugger pc8080 invaders true

Space Invaders (Under Construction)

This is a test of PC8080, a new 8080-based machine emulator recently added to the PCjs Project. It's a work-in-progress, so there's not much to look at yet. And the goal is not to build yet-another Space Invaders/Arcade Machine emulator -- that's been done many times over -- but to start with a well-understood 8080-based machine architecture that doesn't need a lot of peripherals, and use it as an early test case for another classic CPU that we can add to the PCjs toolbox.

Assorted Space Invaders Hardware Notes are collected below.

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

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 most emulators, PC8080 (like PCjs) has 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 PC8080 (and PCjs), 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.