Subject: Re: How to ** WRITE ** an emulator?
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From: czempel@ns.net (Chip Zempel)
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Charles-

Thanks for the tip. I had pretty much figured out most of what you said
already. The "big switch" statement seems like the simplest way to do it,
but also the slowest. Something like a jump table seems more like the
"right" way to do it - an array of pointers to the routines, using the
instruction as the index into the array. I haven't tried this in C, but it
looks like fun. 

FWIW, I'm toying with the idea of emulating a Z80 running CP/M, on a
PowerPC as the host system. This has already been done (and I have the
source code) but that seems like a good way to start. I'll have someone
else's code to learn from. I'm not at all familiar with 680x0 or PowerPC
assembly language, but I used to know my way around a Z80 very well.

Paul Robson sent me a reply that mentioned a couple of web sites. It took
me a few minutes to track them down, so I'm going to put them here in hopes
that someone else with a question like mine will be more likely to stumble
across them in a DejaNews search:

EmuSchool            http://members.home.net/teaguecl/
Dan's Emulator Page  http://www.atarihq.com/danb/emulate.htm

Thanks again for responding!

Chip Zempel


In article <7voiuf$5d$1@nntp2.atl.mindspring.net>, "Charles E. Bortle, Jr."
<cbrtjr@ix.netcom.com> wrote:

>Hello Chip,
>
>Basically, an emulator is a program that makes one processor (cpu)
>"look" like a different processor.
>
>Because of your experience with assembly programming you should
>have some idea of what a processor (machine) instruction looks like
>and what it does.  Well...an emulator just makes another processor
>appear to be that processor that you already know.
>
>There are a number of ways to build one, depending upon the implementation
>language, the processor you are running it on, and the processor you intend
>to
>emulate.  If you want to emulate a 6800 on a Pentium, and you were using C
>you could us the brute-force method and just write a huge Switch statement
>that took all possible values of the 8bit instruction op code and vectored
>control
>to a routine designed to handle that instruction.  Now that routine just
>does
>processing to, in effect, execute that instruction.
>
>This is simplistic, thougth, since you might want to handle addressing
>modes,
>but on the other hand, the single Switch is faster.
>
>Just build a "fetch" routine that fetches each instruction from a memory
>buffer that
>emulates the processor's memory address space.  Pass that to the Switch,
>which
>will dispatch that instruction to the proper handler, and, after that one is
>done, fetch
>another instruction and do the same.
>
>Hope this gets you started :-)
>
>
>--
>Charles          cbrtjr@ix.netcom.com
>"For God So Loved The World, That He Gave His
>Only Begotten Son, That Whosoever Believeth
>In Him Should Not Perish, But Have Everlasting
>Life"John3:16  * http://pw2.netcom.com/~cbrtjr/wrdthing.html *

>Chip Zempel wrote in message ...
>>(I apologize for cross-posting this to several NGs, but since I can't seem
>>to find one that deals specifically with the design of emulators, I'm
>>hoping a programmer who can help me will stumble across this question!)
>>
>>I'm looking for information (in print or online) about writing emulators -
>>theory, design, optimization, that sort of thing. Source code would be nice
>>too!
>>
>>I'm just an amateur programmer and this is mostly idle curiosity - I'll
>>probably never get around to actually _writing_ an emulator! On the other
>>hand, I'm pretty familiar with C and I did some assembly language
>>programming way back in CP/M's heyday, so I'm fairly comfortable with
>>high-level and low-level programming and concepts.
>>
>>Thanks in advance for any info.
>>
>>-------------------------------------------------------------------
>>Chip Zempel
>>czempel@ns.net
>>-------------------------------------------------------------------
>>If a cluttered desk is a sign of a cluttered mind,
>>then what does an empty desk signify?

-------------------------------------------------------------------
Chip Zempel
czempel@ns.net
-------------------------------------------------------------------
If a cluttered desk is a sign of a cluttered mind, 
then what does an empty desk signify?