Subject: Re: [stella] What's the purpose of address $29E?|
From: "Eric Ball" <ericball@xxxxxxxxxxxx>
Date: Thu, 2 Dec 2004 19:58:57 -0500
> What is $29E used for? > Could these be accessing the NUSIZx registers? > > BTW, thanks Eckhard for answering my question about the $288 address. I > looked at Jawbreaker again and yes this is the same as SWCHA. How does > that work? Am I missing something? These addresses don't show in any > documentation I have. The TIA and RIOT don't completely decode the 6507 addressing lines so there are "shadows" throughout the $0000-$0FFF address range. The TIA !CS0 and !CS3 are connected to A12 and A7, and it fully decodes A0-5. So the TIA is mapped to address %0xxxx0x?????? The RIOT CS1, !CS2 and !RS (RAM select) are connected to A7, A12 and A9 respectively and it fully decodes A0-6. So the RIOT is mapped to addresses %0xxRx1??????? $29E = %0001010011110 since A7 ($080) is high, it's a RIOT address. Since A9 ($200) is high, it's a RIOT register address. RIOT register $1E is (according to http://www.6502.org/documents/datasheets/mos/mos_6532_riot.pdf) the same as TIM8T except it enables the interrupt from timer to !IRQ.