wtm@neoucom.UUCP (Bill Mayhew) (07/10/86)
/////// Thank you very much to everyone who sent mail on my question. To sumarize briefly, if the need is absoulute, the Apple //e (enhanced) can be downgraded by substituting the older CD, EF and character generator ROMs. If one is desperate, 2764 ROMs and be burned with the desired contents. Access time needs to be less than 250 uS. Some people suggested that the ROMs were brand-sensitive. I think the name Hitachi was mentioned as being o.k. I one doesn't need the extra memory, the bankswitched RAM could be loaded and used to look like an old plain //e, as long as ProDOS is not used. The restructuring of interrupt processing is to accomode interrupts that occur at totally random times (such as with a mouse device of some kind). In the older //e ROMs interrupts bombed when the lower 48K bankswitched memory was in operation, and there was also greater damger of the interrupt return vector or $45 scratchpad register being trashed. With the new ROM, it takes about 150 uS to enter an interrupt handler, and about another 50 uS to get out. These times vary, depending upon which memory banks are switched in. I assume the above overhead is in addition to the ProDOS overhead (if you use ProDOS, of course). Thanks again, all. Bill Mayhew NEOUCOM Rootstown, OH 44272