Date   : Mon, 18 Jul 1994 16:28:00 +0100 (BST)
From   : amh15@... (Alan Hart)
Subject: Re: Steve's emulators.

> I have a bit of a problem which I think I could really speed up - how can 
> I do relative branches? What I'm doing at the moment (basically) is:
> 
>  load branch distance byte
>  if <branch distance> is less than 128
>  move P back <branch distance>
>  goto end
>  move P forward <branch distance>
>  .end
> 
> Is there a better way of doing this? In the 256^3 speed test this is *by 
> far* where I lose my speed.

Surely the byte is a signed 8-bit number. All you have to do is add this to 
the current PC+1 (or whatever it is that branch uses on the 6502). You might 
want to sign-extend the byte when you load it into a 16-bit register and then 
add it to the current PC+1. In fact I think the system you have proposed 
above will not work for forward jumps, will it [I assume it does work for 
backwards and that it can't work for both].

Alan

 Alan Hart  -  amh15@...  -  University of Cambridge, UK