Re: Questions about IWM

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

rg.jones@rogers.com wrote:
> Not counting MacIntosh models, the IWM chip was used in the
>
> IIc - 5.25 (rom 03 added unidisk support)

Actually it was the 3.5" ROM aka ROM0 that added support. Only the
original (ROM255) version does not have Smartport support.

> Liron Card - Unidisk 3.5
> UniDisk Drive - (internal)
> IIe PDS card (for the Mac). 5.25 and Unidisk 3.5
> IIc+ - 5.25, unidisk or Apple 3.5
> IIgs - 5.25, unidisk or Apple 3.5
>
> a couple of quesions
>
> a) packaging aside where these all the same chip?

AFAIK they are the same chip - in either 28 pin DIP or 28 pin PLCC.

> b) what was the deciding factor on wether a system could use the Apple
> 3.5 vs the Unidisk - was it faster CPU or ?

A 2MHz 65c02 is fast enough to drive an Apple 3.5 via the IWM without
additional hardware (the IIc+ has an extra 2KiB static RAM chip plus
the MIG IC to allow a 1MHz processor to drive the Apple 3.5 drive as
the circuitry was designed before the 4MHz accelerator was added late
in the design).

> c) The Superdrive Contoller (aka 3.5 Apple disk controller) used which
> chip, the IWM or SWIM?

The SWIM hence the support for 1440KiB disks using MFM in addition to
800KiB GCR disks.