Subject: Re: [stella] Superchip: What if.... From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx> Date: Fri, 10 Jun 2005 16:37:57 -0400 |
> It depends on the hardware doing the reading. On a DEVOS 7800 they read > as $ff. On my home made cart dumper I didn't have anything pulling the > lines high, so they read random values. If DevOS detects a Superchip game, it skips the Superchip addresses in each bank. It's the PC software that fills this area in the binary with $ff, because that's what we use in z26 to autodetect Superchip ROMs. ;-) It seems that the 6507/6502 processor is pretty good at maintaining the state of the data bus. So, if nothing is driving the data bus during a read cycle, the CPU will read the value that was last on the data bus. Depending on the addressing mode used for the read, this is usually the low or high byte of the address to read. For example if I use DevOS to read the entire ROM area with no cartridge inserted, the ROM will contain a series of 256 times the same byte counting up from $40 to $ff. This is because I read the area from $4000 to $FFFF with indirect-y-indexed addressing. Also, if you read from the TIA collision registers, the TIA will only drive the two topmost bits of these bytes. The other six bits on the data bus will be left floating. Still the values read by the CPU are reliable enough for some games, like Video Pinball, to rely on the state of all eight bits. Ciao, Eckhard Stolberg Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://stella.biglist.com
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