Subject: RE: [stella] 7800 programs -- potential problems? From: Kevin Horton <khorton@xxxxxxxxxx> Date: Thu, 20 Jan 2000 18:42:11 -0500 (EST) |
At 16:37 1/20/00 +0100, you wrote: >>Would it then be possible to write a game that flips back and forth between >>7800 and 2600 mode, maintaining state? > >That would be very difficult, since the register that controls the >state of the RAM, ROM and graphics chip can be accessed by writing to the >low nibble of all addresses between $00 and $1F. Since that overlaps >with many TIA registers (of which only the sound registers are used >in MARIA mode), you would have to be very carefull that you don't >accidentally change the state of something that your code relies on. >Therefore it's better to choose one setting and lock that in. Not to mention you only get two writes to the control register! After 2 writes, it halts the processor!! There are two chained flipflops that shift a "1" through each bit, and on the third write to the control register, the last output goes high, which runs thru a transistor to the processor's HALT line, stopping the processor. -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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