Re: Disk images: from //e to Mac via PC?

[dempson@actrix.gen.nz (David Empson)]

<m.kelsey@spam.eecs.wsu.edu> wrote:

> In article <1998052801482900.VAA00646@ladder01.news.aol.com>,
>       supertimer@aol.com (Supertimer) writes:
> >
> > For the same reason, an AppleTalked GS can link at 230kbit.
> > When AppleTaking, the GS uses the hardware characteristics
> > of the serial chip and motherboard to transfer at these high
> > speeds.

"Hardware characteristics of the motherboard?"

There is no special hardware on the IIgs motherboard to support high
speed serial comms - the SCC chip may be able to transfer data at up to
230400 bps, but the only special handling required by the IIgs is to
dedicate most of the CPU time to transferring data in and out of the
chip (i.e. no interrupts).

Even the IIe is almost capable of sustaining this data rate.  For
example, a standard 5.25" disk controller is doing raw data transfer at
250 kilobits per second (32 microseconds per byte).  It can only just
cope with this.

In the IIgs, the IWM doubles this data rate for the Apple 3.5 Drive.

> > Remember...the GS can pump MUCH faster than 230kbit
> > with an SCSI card...or even with a floppy controller (which is
> > serial).  The serial hardware is the limiting factor, not the CPU
> > speed (except when doing error checking stuff like Zmodem).
> 
> That is a good point, I hadn't considered DMA as possibility for 
> onboard dedicated hardware.  From what I've benchmarked, my IIgs 
> doesn't pull more than 150K/sec from hard drives capable of 600K/sec
> on a faster PC.

This depends greatly on circumstances.  When doing bulk transfers, the
Apple HS SCSI card is capable of peak rates up to 1 megabyte per second.
The GS/OS SCSI drivers work best when transferring large contiguous
blocks of data.

Under ProDOS-8, the main speed limitation is that the OS only asks for
512 bytes at a time, and has all the overhead of SCSI commands, seek
delays, etc.  The RamFast gets around most of this by minimising SCSI
comms delays due to its cache.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand