Re: Apple //e Upgrade to 640x192 Resolution?

["David Wilson" <mcs6502@gmail.com>]

Bryan Parkoff wrote:
>     Do you think that Apple IIgs have similiar timing?  It has clever design
> than Apple //e's design which 28MHz crystal osc chip is used.  I can't
> imagine if Apple IIgs' timing use 28MHz crystal osc chip to work with 14MHz,
> 7MHz, 3.5MHz, 2MHz, and 1MHz.  Maybe, it has extra signal like 2.5MHz for
> 65C816 MPU and 3.5" AppleDrive.  It looks like that it switches to use 1 MHz
> and 2.5 MHz from timing.

The IIgs has a single 28.63636MHz oscillator (I will ignore the 32768Hz
crystal used by the RTC). The normal speed of a IIgs is 2.8MHz or
XTAL/10. The slow speed is ~1MHz or XTAL/28 (plus the extended cycle).
Every timing frequency on the IIgs will be derived from this one clock
source. The reason for 2.5MHz is that not every cycle can be used - the
RAM needs to be refreshed periodically and in fast mode it is not done
by the video scanner. Code running from ROM will run faster than the
same code in RAM.