94 lines
3.4 KiB
Markdown
94 lines
3.4 KiB
Markdown
---
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layout: page
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title: Space Invaders (1978) with Debugger
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permalink: /devices/pc8080/machine/invaders/debugger/
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machines:
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- id: invaders
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type: pc8080
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debugger: true
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---
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Space Invaders (1978) with Debugger
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-----------------------------------
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The [PC8080](/modules/pc8080/) machine below is configured to run [Space Invaders](/devices/pc8080/machine/invaders/)
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with a Control Panel and Debugger.
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It is currently playable only in desktop browsers and uses the following hard-coded key mappings:
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- 1: One Player
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- 2: Two Player
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- 3: Insert Coin
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- A: Move Left
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- D: Move Right
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- L: Fire Missile
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Click the **Run** button to start the simulation. You'll also find assorted
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[Space Invaders Hardware Notes](#space-invaders-hardware-notes) below.
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{% include machine.html id="invaders" %}
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Space Invaders Hardware Notes
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-----------------------------
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### Memory Map
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0000-1FFF r ROM code
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2000-23FF rw RAM
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2400-3FFF rw bitmapped screen (224x256)
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### I/O Map
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00 r
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01 r control inputs
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bit 7
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bit 6 right
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bit 5 left
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bit 4 fire
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bit 3
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bit 2 1 player start
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bit 1 2 player start
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bit 0 coin slot
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02 r control inputs
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bit 7 0=display coin info
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bit 6 right 2
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bit 5 left 2
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bit 4 fire 2
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bit 3 bonus (1500,1000) / preset mode
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bit 2 1=tilt
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bit 0-1 initial lives (3,4,5,6) / (3,4)
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02 w shift count (0-7)
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03 r shifted value (low then high)
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03 w sound
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bit 4 bonus base
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bit 3 invader hit
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bit 2 base hit
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bit 1 base fire
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bit 0 saucer
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04 w value to shift
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05 w sound
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bit 5 flip video
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bit 4 saucer hit
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bit 3 invader movement 4
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bit 2 invader movement 3
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bit 1 invader movement 2
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bit 0 invader movement 1
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06 w watchdog timer clear
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Note that, unlike most emulators, PC8080 (like PCx86) has a *[Bus](/modules/pc8080/lib/bus.js)* architecture,
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allowing components to "plug in" different kinds of memory or memory-mapped devices at different addresses,
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and to register specific functions for specific I/O ports.
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For example, Space Invaders has 8Kb of ROM at addresses 0x0000 through 0x1FFF, which means that if any 8080 code
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attempts to write to those addresses, nothing should happen. Unfortunately, most emulators treat the entire address
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space as one contiguous array of bytes. Which means either that the ROM is susceptible to corruption *or* that every
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write operation must check the address to determine its validity, which hurts the performance of *all* writes.
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In PC8080 (and PCx86), all writes are equally fast, and all ROMs are fully protected. An exception is made for the
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Debugger, which allows you to use the "e" command to modify ("patch") ROM code on the fly, but that is completely
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outside and independent of the 8080 code being emulated.
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### Other Online References
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See [Computer Archeology](http://www.computerarcheology.com/Arcade/SpaceInvaders/) for an excellent collection
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of materials on the original Space Invaders, including commented ROM disassemblies.
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