Re: Nostalgia - Apple ][ europlus

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

In article <437qk4$37h@news.mel.aone.net.au>,
Philip Stephens <philip@labtam.OZ.AU> wrote:
> Philip Mumford writes:
> 
> >P.S. I don't know very much about the GS - I vaguely remember it having 
> >some strange CPU which was based on the 6502 and being quite excited about
> >the graphics and sound. Exactly how powerful is it? I wrote a few games on
> >the ][, does the GS play a mean game of defender etc?!!!
> 
>   The GS is powered by the 65816 CPU, which is a 16-bit version of the
> 6502.  The 65816 can either run in emulation mode (behaving like a 65C02),
> or it can run in native mode whereby all instructions work with 16-bit
> values rather than 8-bit.  The accumulator and index registers can
> individually be toggled between 8-bit and 16-bit operation.  The 65816
> also adds a couple of new instructions and addressing modes. 

A "couple"?  :-)

The 65816 uses all 256 opcodes (except one which is reserved for
future expansion of the family).  This is 77 more than 65C02, and 104
more than the 6502.  (If I counted them correctly.)

Comparing the 65816 to the 6502, it has 35 new instructions
(mnemonics), and many of the existing instructions have new addressing
modes.

The 65816 has a 24-bit address bus, allowing it to directly access 16MB
of memory.  The IIgs can support up the 8.125MB of RAM, all of which
can be directly accessed by software.  There is no physical bank
switching (except for the emulated Apple II areas - auxiliary memory
and language card, which only appear in four banks).

The processor has a data bank register which specifies which 64K bank
is used for the original 6502 absolute and indirect addressing modes.
There are also "absolute long" and "indirect long" addressing modes
that support 24-bit addresses.

The original IIgs has 256K of RAM and 128KB of ROM.  The later
revision has 1.125MB of RAM and 256KB of ROM.  Both models have a
memory expansion slot that can be used to expand total RAM to 8 MB.

The use of the extra RAM is somewhat limited for old 8-bit software.
The IIgs does support setting memory aside as a RAM disk, which can be
accessed from 8-bit ProDOS software.  Some 8-bit programs (e.g.
AppleWorks) can make direct use of the additional memory of the IIgs.

The extra ROM space is used for parts of the IIgs toolbox (more is in
ROM in the later model) - the rest is loaded from the system disk, and
for other firmware components (serial firmware, AppleTalk, 3.5" drive
firmware, significantly extended system monitor, Control Panel, etc.)

>   How powerful is it?  Well, in 16-bit mode programs can run as much as
> twice as fast at the same clock rate (this assumes they are written to
> process information in 16-bits, I don't mean 8-bit programs run twice as
> fast).

This is somewhat of a simplification - the same amount of code can
process twice as much data (with a few modifications), but it won't
necessarily run twice as fast.  In some cases, it could be faster
(e.g. not having to deal separately with each half of 16-bit pointers
or values, resulting in much simpler code).

>  A stock GS only runs at 2.8 Mhz, although you can add an
> accelerator card and get it up to 7-14 Mhz.

Key point here: a stock IIgs running 8-bit software will run it up to
2.8 times faster than an unaccelerated 8-bit Apple II.  (The actual
speed will vary depending on I/O accesses, writes to screen memory,
etc., which are forced to run at 1 MHz.)  The IIgs can be forced to
run at 1 MHz, e.g. for running games (which would be unplayable if the
computer ran faster).

>   The GS can emulate an Apple //e almost perfectly, or can run in native
> mode whereby you have access to a Macintosh-like OS and toolbox.

Native mode has many other benefits and applications.  Most of the
native mode programs I write run in text mode under a shell, not
on the graphical desktop.

The 65816 in native mode is much more able to deal with high-level
compiled languages than the 6502.  Most 6502-based compilers produce
an intermediate code which is then interpreted.  The compilers which
produce actual machine code tend to generate very big programs.


The IIgs is based on the IIe, but has also inherited features from the
IIc, particularly the built-in I/O devices.  The IIgs has the
following peripherals built in:

- two serial ports (using the same serial chip and connector as the
  Macintosh), with firmware that provides interrupt-driven buffering.
  AppleTalk (LocalTalk) networking hardware and firmware is built in.
  The firmware only allows for booting over an AppleTalk network from
  an AppleShare server.  Other AppleTalk functions are implemented by
  software loaded from the system disk.

- 80-column card is built in (firmware and hardware).

- Apple Desktop Bus (as used on the Macintosh) for keyboard and mouse.
  Mouse firwmare is built in.

- 3.5" drive controller (SmartPort), supporting the Apple 3.5 Drive,
  UniDisk 3.5, other intelligent SmartPort drives, RAM disk, optional
  ROM disk.  5.25" drives may also be daisy-chained from the same disk
  port (they are controlled separately from SmartPort devices).

Each of the above devices appears in the normal locations used by the
slots.  The IIgs has a built-in Control Panel that allows the slots to
be switched between the internal function and the physical slot (a
reboot is required for changes to take effect).

The following peripherals are also provided, but are not accessed
through the slot space:

- Clock

- Ensoniq sound chip (providing up to 15 voices playing waveforms from
  a dedicated 64K RAM area, separate from the IIgs's memory).

The IIgs has all of the IIe's built-in I/O devices, with the exception
of the cassette input/output and the "strobe" signal on the internal
game connector.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand