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Thread: Space Invaders

  1. #1
    EmuTalk Member hap's Avatar
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    Space Invaders

    If you think CHIP8 is too easy, or GameBoy/NES/MasterSystem is too hard, writing a Space Invaders emulator is not. Most work will be put into creating an Intel 8080 CPU emulator (shouldn't be too hard for the GameBoy crowd ), the rest is peanuts.

    Code:
    Space Invaders, (C) Taito 1978, Midway 1979
    
    CPU: Intel 8080 @ 2MHz (CPU similar to the (newer) Zilog Z80)
    
    Interrupts: $cf (RST 8) at the start of vblank, $d7 (RST $10) at the end of vblank.
    
    Video: 256(x)*224(y) @ 60Hz, vertical monitor. Colours are simulated with a
    plastic transparent overlay and a background picture.
    Video hardware is very simple: 7168 bytes 1bpp bitmap (32 bytes per scanline).
    
    Sound: SN76477 and samples.
    
    Memory map:
    	ROM
    	$0000-$07ff:	invaders.h
    	$0800-$0fff:	invaders.g
    	$1000-$17ff:	invaders.f
    	$1800-$1fff:	invaders.e
    	
    	RAM
    	$2000-$23ff:	work RAM
    	$2400-$3fff:	video RAM
    	
    	$4000-:		RAM mirror
    
    Ports:
    	Read 1
    	BIT	0	coin (0 when active)
    		1	P2 start button
    		2	P1 start button
    		3	?
    		4	P1 shoot button
    		5	P1 joystick left
    		6	P1 joystick right
    		7	?
    	
    	Read 2
    	BIT	0,1	dipswitch number of lives (0:3,1:4,2:5,3:6)
    		2	tilt 'button'
    		3	dipswitch bonus life at 1:1000,0:1500
    		4	P2 shoot button
    		5	P2 joystick left
    		6	P2 joystick right
    		7	dipswitch coin info 1:off,0:on
    	
    	Read 3		shift register result
    	
    	Write 2		shift register result offset (bits 0,1,2)
    	Write 3		sound related
    	Write 4		fill shift register
    	Write 5		sound related
    	Write 6		strange 'debug' port? eg. it writes to this port when
    			it writes text to the screen (0=a,1=b,2=c, etc)
    	
    	(write ports 3,5,6 can be left unemulated, read port 1=$01 and 2=$00
    	will make the game run, but but only in attract mode)
    
    I haven't looked into sound details.
    
    16 bit shift register:
    	f              0	bit
    	xxxxxxxxyyyyyyyy
    	
    	Writing to port 4 shifts x into y, and the new value into x, eg.
    	$0000,
    	write $aa -> $aa00,
    	write $ff -> $ffaa,
    	write $12 -> $12ff, ..
    	
    	Writing to port 2 (bits 0,1,2) sets the offset for the 8 bit result, eg.
    	offset 0:
    	rrrrrrrr		result=xxxxxxxx
    	xxxxxxxxyyyyyyyy
    	
    	offset 2:
    	  rrrrrrrr	result=xxxxxxyy
    	xxxxxxxxyyyyyyyy
    	
    	offset 7:
    	       rrrrrrrr	result=xyyyyyyy
    	xxxxxxxxyyyyyyyy
    	
    	Reading from port 3 returns said result.
    
    Overlay dimensions (screen rotated 90 degrees anti-clockwise):
    	,_______________________________.
    	|WHITE            ^             |
    	|                32             |
    	|                 v             |
    	|-------------------------------|
    	|RED              ^             |
    	|                32             |
    	|                 v             |
    	|-------------------------------|
    	|WHITE                          |
    	|         < 224 >               |
    	|                               |
    	|                 ^             |
    	|                120            |
    	|                 v             |
    	|                               |
    	|                               |
    	|                               |
    	|-------------------------------|
    	|GREEN                          |
    	| ^                  ^          |
    	|56        ^        56          |
    	| v       72         v          |
    	|____      v      ______________|
    	|  ^  |          | ^            |
    	|<16> |  < 118 > |16   < 122 >  |
    	|  v  |          | v            |
    	|WHITE|          |         WHITE|
    	`-------------------------------'
    	
    	Way of out of proportion :P
    attachments:
    8080asm.7z: i8080 assembly programming manual, needed for 8080 emulation,
    8080ds.pdf: i8080 datasheet, the table on page 8 and 9 is handy,
    76477.pdf: SN76477 (I've hardly looked at that)

    Attached Files Attached Files
    Last edited by hap; October 12th, 2006 at 23:47.

  2. #2
    EmuTalk Member hap's Avatar
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    Here's mine .. source code only.

    *edit* made a mistake in the documentation: Read 0 should be 1, 1 should be 2, 2 should be 3. The above post has been corrected, but I won't release an updated version of "It's a trap!". (it's fine in the code)
    Last edited by hap; October 12th, 2006 at 23:47.

  3. #3
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    Looks good. This should only be a little bit harder than chip8, not counting sound. Is "attract mode" auto-play demonstration?

    Other small question - this shouldn't make a difference for Space Invaders (and probably almost any software) but the way you're storing flags, as represented by the result of the previous calculation - it should be possible for the program to compute flags (using pop psw) and thus have both zero, negative, etc set at the same time. IE, if the user did (assume b is already something like 0xFF, loaded from memory or something)

    push b
    pop psw
    push psw
    pop b

    You'd end up with a different b. Like I said, I doubt this matters because no sane programmers would construct the flags, it's just something I've thought about before.
    Last edited by Exophase; October 12th, 2006 at 18:42.

  4. #4
    EmuTalk Member hap's Avatar
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    Yes, it's like having no pocket change.

    *edit* you're right, I didn't think of that. I'm using the same method on my 6502 emulator (NES)... and should probably change that considering the large library of NES games.
    Last edited by hap; October 12th, 2006 at 21:59.

  5. #5
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    Looks like something I can try next after I finish the chip8 ... And looking at you source briefly it seems like space invaders would be easier, would it not?

  6. #6
    EmuTalk Member hap's Avatar
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    If you've successfully created a CHIP8 interpreter, you've gained experience. In that sense, it's easier to create a Space Invaders emulator. I mean, going from zero to CHIP8 is harder than going from CHIP8 to Space Invaders, get it?

  7. #7
    Emulator Developer ShizZy's Avatar
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    Nice plastic transparent overlay This looks like a good project for a rainy day though sometime when I'm bored. Thanks for all of the docs. Oh, and by the way, is the rom PD? If not, you have any more specific name or something so it can be located?
    ~****Zy

    6Bit.net Emulation & Gaming | Forums
    Gekko GameCube Emulator

  8. #8
    EmuTalk Member hap's Avatar
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    invaders.h, invaders.g, invaders.f, invaders.e, each 2KB in size. MAME supports the game, called "invaders.zip", search for it on Google.

    If this link is against the rules, please remove it: mame roms "space invaders" - Google Search

  9. #9
    EmuTalk Member Garstyciuks's Avatar
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    I think that I'm going to make space invaders emulator It looks quite interesting. But can someone explain me why does nes, gameboy, space invaders, and a lot of other systems need "RAM mirror"?

  10. #10
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    I can't say the specific reasons, I can only guess the following two things..

    - It's easier/faster to implement in hardware. Seems counterintuitive, but throwing out bits on an address bus is a lot easier to do than detecting bits and having it do something else. That's basically what happens with mirroring, it's like a big binary AND off the top bits, so you can just ignore them.

    - Sometimes different mirrored region have different specifications, like on GBA the different cartridge spaces have different wait state configurations, and on PSP the different mirrors of VRAM affect swizzling somehow (I don't totally recall at the moment)

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