Re: Hardware emulation of 6509 using 6502?

From: smf <>
Date: Fri, 27 Jul 2018 11:15:10 +0100
Message-ID: <>
You're right, I was simplifying.

> The VIC-I chip can address 16K, but only internal memory, not on the 
> cartridge bus. Normally the operating system nearly all of the lower 
> 1K, but in certain demos like e.g. VIMM and VIMMII, Albert of Pu-239 
> stored the video matrix in that area in order to use all of the 4K for 
> bitmap. (The unexpanded VIC-20 has 5.5K RAM in total of which 3.5K is 
> available to the user)

It's largely impractical to put the screen matrix in the first 1k from 
BASIC. Although you could blank out areas you can't touch with a 
foreground colour that matches the background colour. Theoretically that 
could be supported too though.

> Some individuals have installed internal 3K expansion to their VIC-20 
> by piggybacking chips. It is highly non-standard, but allows the VIC-I 
> chip to access that memory as well. I suppose in theory one could 
> install internal 8K expansion and access that memory for the VIC-I 
> chip as well but I don't know anyone who has done it.

vice only allows access in the standard locations

     } else if ((addr < 0x0400) || ((addr >= 0x1000) && (addr < 0x2000))) {
         /* RAM */
         b = mem_ram[addr];

Theoretically you could hook 64k (or more) and then add extra banking 
bits (like c64 has). I think that is out of scope.
Received on 2018-07-27 13:00:05

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