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Re: Apple II vs Atari 800

Russg wrote:

<heuser.marcus@freenet.de> wrote in message
408ecbae-5123-4571-81a8-0daf48109f2a@l64g2000hse.googlegroups.com">news:408ecbae-5123-4571-81a8-0daf48109f2a@l64g2000hse.googlegroups.com...


On Apr 15, 3:22 pm, "micrologix1...@yahoo.com"
<micrologix1...@yahoo.com> wrote:


you know what I want to know?  How does the apple interface
to a floppy drive with so little hardware?


In short:

Atari drives as you surely know are intelligent - with a 6502-
variant, ROM, RAM and interfaces.
Apple drives are "dumb" - most of the "digital stuff" happens
inside the Apple on a small but very, very clever interface card.

Read the respective chapter in the seminal Apple hardware
explanation book "Understanding the Apple II" which you can
do here: http://victa.jamtronix.com/display/page/4313
If you don't recognize the wizardry of Woz here, then you'll
never will ;-)

Some other bits:

The Apple drives peak at around 11KB/sec and - because of
their design - very flexible to program for. A multitude of DOSses
exist but there are really only two standards: DOS 3.3 and ProDOS.

I don't know if the Apple drives & controller were cheaper but
they surely were cheaper to produce and made good profits.

The Atari drives for all their complexity were flawed from the
beginning: Shitty interface with 2KB/sec which was planned for
tape recorders and modems only, low capacity compared to
nearly all competitors, the need for a DOS in the Atari's RAM
in spite of the ROM in the drive (the Commodore 1541 saved
much memory - other flaws aside).

The only real technical advantage I see for the Atari drives is
that they can be used to stream data into RAM and the
application runs mostly undisturbed (display list interrupts
have problems, IIRC) while the Apple has to "stop" the game,
loads and then continues.

bye
Marcus


I remember someplace that Wozniak's floppy drive
interface/controller was a major reason Apple ][
could be made/sold for the price.  ie. Floppy controllers
were very expensive and Woz made one from
cheap parts.


Right--and virtually every system sold had a floppy
controller and at least one floppy drive.

Not only was the controller very simple, inexpensive
hardware, but the drive was a "gutted" Shugart SA390,
with the complex Shugart logic board replaced by the
very simple Apple analog board.

Making the Apple II completely responsible for control
of the drive reduced the cost of the disk subsystem
substantially--*while significantly improving performance*!

This was a principle that Woz seemed to understand
almost intuitively--and his experience with "algorithmic
state machines" in HP calculators certainly helped.


I read the article, sorta, up to page 43 or so.
I don't know the speed of a Apple ][ floppy,
I do know the basic speed was 10 times faster
than a Commodore 1541.  Seems I remember
that the Atari can go about 52K baud, which is
about 5K bytes/sec.  The bus speed of an Atari
is 115K cps I think, so that is a limit on its
I/O.  The Atari PBI with hard drive interface
goes at about that 115K bit/sec rate, I think.


Any "smart" drive will wind up reading the disk data
into an internal buffer and then transferring the data
to the host computer when requested.  This double move
performed by a single processor guarantees that a smart
drive cannot move data at the rate the disk can support.

Most other computer manufacturers seemed to put disk I/O
in the "slow" category, and so did not devote much effort
to speeding up transfers.  In the case of the C=64 and
the Ataris, the transfer speed on the link to the computer
was a small fraction of the speed supported by the drives.

Apple drives communicate directly with the disk controller
which communicates directly with the Apple's processor bus.
Data transfers are controlled by software, but it runs at the
speed of the disk, and the bus connection is not a limiter.

The raw speed of a single-density 5.25" drive is 250kBits/sec.
The "standard" single-density encoding required at least 16
bit times per byte of data, so they could support about 15KB/sec.

Commodore and Apple both used Group Code Recording, which uses
fewer "overhead" bits per byte.

Apple 16-sector formats encode 3 bytes of data in 32 bits,
increasing density and speed by a factor of 1.5 compared to
single-density encoding.  This is all done in software running
on the Apple II--another piece of Woz' cleverness.

Practical disk speeds will, of course, be affected by many other
factors, including seek and rotational latency and the computer's
ability to sustain data transfers over multiple sectors.

In Apple's DOS 3.3, data was doubly buffered on its way to
memory, cutting transfer speed and requiring sector interleaving
to achieve respectable speeds (still the fastest in the PC world
at the time).  With the introduction of ProDOS, the double-buffering
was eliminated and the interleave factor was cut substantially,
providing significantly faster floppy I/O.


I'm a hardware wannabe, but not.
Rick Cortese is a hardware man, and maybe
Rick Detlefsen (A8Maestro).
For instance, does Apple ][ have a serial,
parallel or USB interface disk drive emulator
for the PC? or Mac?  Atari has a very useful
interface that allows using a PC for a hard or
floppy drive.


Such devices are a "natural" for Commodore and Atari systems,
since they function normally as a drive connected to the computer
by a data link.

They are much more challenging for the Apple II, since the drive
is interactively controlled by the Apple II in real time--and
many copy-protection schemes used this control in novel ways to
achieve their ends.  So the only really "compatible" interface
to a Disk ][ is a real-time faithful emulation of an Apple II
running the actual code that accesses the disk.

Of course, if you are willing to settle for read-only access only
to normally formatted disks, then there are several hardware (and
even one software) solutions for doing so.  Read/write access is
more difficult, and access to copy-protected disks is not available,
in general.

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it's seriously underused."