Subject: Re: Hopefully last dumb newbie question. From: dempson@actrix.gen.nz (David Empson) Date: Wed, Jun 24, 1998 4p42 EDT Message-id: <1db5aaj.vui26t11kvoiN@dempson.actrix.gen.nz> Mitchell Spector wrote: > In article dempson@actrix.gen.nz (David Empson) writes... > > >The only slowdown on a ROM 3 would be due to the synchronization delays > >introduced by shadowing writes in locations $0800-$0BFF of bank 0 into > >bank $E0, i.e. 1 MHz writes instead of 2.8 MHz. Nothing outside that > >address range is affected. > > > >This would have a variable effect depending on what was using that area > >of memory. Applesoft BASIC programs normally run in that area, but they > >aren't written to (except while editing or being loaded). Under GS/OS, > >it would tend to be application or toolbox direct page/stack space > >(reasonably major impact). > > Hmm, perhaps it is not as trivial a glitch as I first thought. Is > it possible my ROM 3 (even with 15 MHz Zip GS accelerator) is running > slower than it should be, and has for all these years? The ZIP GS does not cache write operations, so the only speed penalty is 1 MHz vs 2.8 MHz, even on a highly accelerated machine. I doubt you'd notice much. > Is there a way I can confirm, for certain, whether Alternate > Display Mode is constantly active on my system? (perhaps by looking > up an address in memory). You could look at the contents of the shadow register ($C035). Bit 5 is zero if text page 2 shadowing is enabled (on the 1MB/ROM 3 motherboard only). The catch: this register is fiddled with by the monitor firmware, so you can't reliably read it from the command line. The best way to read it is to use a small machine code routine to read the register and save it elsewhere. For example (don't type the prompts, but you do need the leading spaces in the mini-assembler): ]CALL-151 *! !300:LDA C035 ! STA 00 ! RTS ! *300G 0 00/0000-xy * The 'xy' indicates the value in the shadow register. Bit 5 has a value of 20 (hex), so if the 'x' digit is 0, 1, 4, 5, 8, 9, C or D then shadowing is enabled for text page 2. I haven't disassembled the Alternate Display Mode CDA, so I don't know where it stores its setting. This would be used to derive the value for the shadow register. It would be necessary to fix this value as well as updating the shadow register itself. -- David Empson dempson@actrix.gen.nz Snail mail: P.O. Box 27-103, Wellington, New Zealand