Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!wuarchive!udel!princeton!phoenix!tbrakitz
From: tbrakitz@phoenix.Princeton.EDU (Byron Rakitzis)
Newsgroups: comp.sys.apple2
Subject: Re: Xapple2
Message-ID: <14340@phoenix.Princeton.EDU>
Date: 7 Mar 90 18:06:15 GMT
References: <53040@bbn.COM> <53134@bbn.COM>
Reply-To: tbrakitz@phoenix.Princeton.EDU (Byron Rakitzis)
Organization: Princeton University, NJ
Lines: 37

In article <53134@bbn.COM> kanarick@vax.bbn.com (Craig M. Kanarick) writes:

>Basically, I have written, in C, a 6502 emulator.  It has an array of
>16-bit values, and is the memory of the Apple.  I also have 5 16-bit
>registers, and a program counter.  Basically, the emulator is just a
>big case statement, each branch being one 6502 instruction (yes, there
>are a lot, because each instruction/addressing mode pair is a seperate
>branch).  I am about halfway done with this part.  Essentially, I just

I wrote a rudimentary 6502 emulator in C last summer which I have not
gotten around to debugging yet. The code is really messy, and I'm not
very proud of it, but there's a MUCH better way to go about finding
the next instruction! 

1) I keep a 256-valued array of struct containing the info:
	instruction, addressing mode, cycles.
This way, by checking the array for any particular opcode, I can
determine in constant time what the instruction, opcode is supposed to
be. 

2) (the sneaky part) the "instruction" datum is simply an index to
another array of pointer to function! So, each function is an
instruction which does the obvious thing (load, rol whatever) and the
trick is to set up the addressing mode first (just pass the right
pointers to accumulator, memory, whatever) and then in one statement 
execute_instruction[x](), go there!

If anybody wants the code they are free to have it, but it doesn't
work and I really can't be bothered to look at it. It was written in
great haste at the end of the summer, but most of the grungy work is
done. 

-- 
Just try taking your VAX down to Jiffy-Lube these days!
			
Byron "Bo knows parallel computational geometry" Rakitzis. 
				(tbrakitz@phoenix.Princeton.EDU)