Subject: [stella] SC demo unit bootstrap code From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx> Date: Thu, 22 Jul 1999 17:57:59 +0200 |
Lee was kind enough to send me a sample of the bootstrap code for the Supercharger demo unit. I converted it into a binary and had a look at the code. From what I can see, loading though the joystick port works like this: The 'right' line of the right joystick port is used for output to the demo unit. The 'left', 'up', 'down' an 'button' lines of this port are used for input. A state change on the output line triggers the demo unit to send the next halfbyte. The high nibble gets send before the low nibble. The demo unit seems to need at least 12 6507 processor cycles to prepare the next nibble to be send. I think that pulling the output line low and then setting it high again imediately triggers the demo unit to switch to the next 8kb ROM load. The loading code seems to load 65 bytes at a time. 64 of those bytes get stored back to front into SC-RAM. One bit in the 65th byte seems to make the loading code branch into a test routine that gets executed after each 64 byte load anyway. I don't know why it does this. The test code branches into the VSYNC routine if nessessary, since the loading code seems to try to make sure that no frame gets dropped during loading time. The bootstrap code only prepares to load one 64 bytes block which then has to hold data to tell the loading code where to load what block next. Therefore I'm not sure what happens after this block has been loaded, but I think that the loading code stays intact and gets called to load the other 8kb ROM banks too. This would mean that it would be possible to replace the loading code with some code that does normal SC tape loads. If the 8kb ROM banks can be converted into a SC tape format, running the demo from a tape should be possible. Note to Glenn: Would it be alright if I'd post the commented bootstrap code here and would anyone want to see it? Note to Kevin: With you knowledge of the hardware of the SC demo unit, does anything I said above make sense to you? Do you think it would be possible to convert the 8kb ROM banks inside the demo unit into the SC tape format and have you already put the binaries up on your webpage? Thanks, Eckhard Stolberg -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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