Re: ROM 01 vs ROM 3

[David.Empson@bbs.actrix.gen.nz]

In article <346cXB4w165w@lablues.UUCP> larry@lablues.UUCP (Lawrance A. Schneider) writes:
> George Zamroz scribed the following:
> 
> |Programs in ROM can run at the full speed of the processor
> |(normally 2.8 Mhz). Programs in RAM lose some cpu cycles due
> |to the memory refresh required and therefore run slower
> |(about 2.5 Mhz).
> 
> While I admit to NOT BEING a gurue, I believe you are wrong.  The
> CPU is connected to the bus; the CPU defines the address on one
> half of a cycle and read/writes on the other half of the cycle.
> Ram is refreshed every other cycle and has nothing to do with
> CPU speed other than being connected to the same bus (cycle).

No, that only applies to the II+/IIe/IIc and the 'slow' RAM of the
IIgs.  The memory accesses in the other half of the cycle are used to
fetch data for the video display, and they also act to refresh the RAM.

The IIgs 'fast' RAM has no video buffers, so it has to be refreshed
independently.  This is done by the Fast Processor Interface 'stealing'
cycles from the processor.  This only needs to be done if the
processor is accessing the fast RAM during the refresh.  If the
processor is accessing ROM, I/O or slow RAM, there is no additional
delay caused by the refresh of the fast RAM.

This means the effective speed of fast RAM is 2.6 MHz (or thereabouts)
while ROM can be accessed at the full 2.8 MHz.
-- 
David Empson

Internet: David.Empson@bbs.actrix.gen.nz    EMPSON_D@kosmos.wcc.govt.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand