Added separate debugger/non-debugger machine configs for the PC8080 machines (like most PCx86 machines do)

devices/pc8080/machine/invaders/debugger/README.md

@@ -12,8 +12,9 @@ Space Invaders (1978) with Debugger
 -----------------------------------
 
 The [PC8080](/modules/pc8080/) machine below is configured to run [Space Invaders](/devices/pc8080/machine/invaders/)
-with a Control Panel and Debugger.  It is currently playable only in desktop browsers and uses the following hard-coded
-key mappings:
+with a Control Panel and Debugger.
+
+It is currently playable only in desktop browsers and uses the following hard-coded key mappings:
 
 - 1: One Player
 - 2: Two Player
@@ -22,6 +23,72 @@ key mappings:
 - D: Move Right
 - L: Fire Missile
 
-Click the "Run" button when you're ready to start the simulation. 
+Click the "Run" button to start the simulation.  You'll also find assorted
+[Space Invaders Hardware Notes](#space-invaders-hardware-notes) 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 PCx86) has a *[Bus](/modules/pc8080/lib/bus.js)* 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 PCx86), 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](http://www.computerarcheology.com/Arcade/SpaceInvaders/) for an excellent collection
+of materials on the original Space Invaders, including commented ROM disassemblies.