Subject: [stella] Larger games... brainstorming|
From: emooney@xxxxxxxxxxxxxxxx (Erik Mooney)
Date: Sat, 12 Apr 1997 20:14:55 GMT
As we know, the 6507 address space between 297h (T1024T) and 1000h is unused on the 2600. The cartridge address space is supposed to be 4k, but all 13 address lines do connect to the cart slot, so the entire 8k could be addressed on the cart. Suppose one used a 2764 8K EPROM, with the chip enable tied to a logical OR of A11 and A12 (and inverted if necessary - I think it would be), and the EPROM's 13 address lines simply connected to the cart slot's 13 address lines. Would this allow access to ROM data between addresses 0800h and 0FFFh, reclaming 2k of address space and allowing 6k carts with no bankswitching? The only problem I can see is if the TIA only checks address lines 0-5 (address), 7 (select between Stella and RIOT) and 12 and ignores the others - meaning it would respond to writes to addresses $00 - $2C (the highest register - CXCLR), and map to the registers any writes to $40 - $6C, $100 - $12C, $140 - $16C, ... , $800 - $82C, $840 - $86C, etc. Even in this case, it might be feasible to map a 4k RAM chip on the cart between $000 and $FFF (chip enable A12), and only use addresses $30 - $3F, $70 - $7F, $130 - $13F, etc. Stella shouldn't respond to those addresses, so no conflicts would happen. This would provide an extra 512 bytes of RAM, or 256 bytes if separate read-write addresses must be used like on Superchip and other programs with cart-based RAM. One definite advantage of this is it can be done to an existing board, recycled with a new EPROM... it doesn't affect the function of the EPROM, and it involves adding nothing but the RAM chip (no bankswitching hardware.) Even if I'm completely wrong about all of this, is there any way to reclaim any of the lower 4k address space and direct it into the cart slot? -- Archives available at http://www.biglist.com/lists/stella/archives/ E-mail UNSUBSCRIBE in the body to stella-request@xxxxxxxxxxx to be removed.