Re: Downgrade Apple //e?

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

Bryan Parkoff <BParkoff@satx.rr.com> wrote:

>     I have unenhanced Apple //e that displays "Apple ][" at cold start.  How
> do I read "revision A or B" on the motherboard?

Probably by looking at the letter on the end of the part number, which
is printed on the motherboard: 820-0064-A or 820-0064-B.

> How do I downgrade unenhanced Apple //e that it does not support double
> resolution?  I mean to switch from Revision B to Revision A.

I can't think of any reason to want to do this on purpose, but as far as
I know, the only way to do so is to swap the motherboard, installing a
revision A board instead of revision B.

> Can you explain how the revision B was fixed to enable double resolution?

According to appendix G of "Understanding the Apple IIe":

Revision B rewired 1/3 of the C5 74LS10 NAND gate so that pulling pin 55
of the auxiliary slot low forces TEXT processing at the timing HAL,
instead of disabling the motherboard ROM as it did in revision A.  There
were also changes in the timing HAL and the MMU chip.  Revision B also
had some unrelated motherboard changes relating to jumper pads, changing
the function of X1 and X2, and adding X3 and X7.

> Is it because of fixing 80 column?

The only fundamental change in revision B is that it supports double
high resolution graphics mode (560x192 in monochrome).  80-column text
mode is supported on both revision A and B (as is double low resolution
graphics mode, I expect).

>     Will MouseText be supported on unenhanced Apple //e if Character ROM is
> swapped?

No.  Software which attempts to display MouseText characters will get
inverse uppercase letters.

>     It looks like that Apple II+ does not have double resolution support
> because it does not have softswitch for double resolution, but unenhanced
> Apple //e does have additional softswitch.

The IIe has considerably more softswitches than the II+, notably all of
the ones in the $C000-$C01F area (except for the keyboard latch at $C000
and keyboard latch reset at $C010, which are common to both models).

These softswitches relate to the major new features of the Apple IIe:
auxiliary memory and 80-column video, and incidental ones like the
alternate character set and vertical blanking detection.  The II+ has
none of these features, though some similar ones can be added with
peripheral cards.

> Is it possible to program double resolution on Apple II+

No.  None of the required hardware support is available on the
motherboard.

> If it is not possible, 6502 has no way to switch between high and double
> resolution without using softswitch.  Is it true that 6502 requires
> softswitch so 6502 would be able to manipulate the hardware through
> softswitch.

Whether the processor is a 6502 or 65C02 has nothing to do with the
issue.  There is no hardware on the II+ motherboard which supports
80-column text mode and/or double hi-res graphics mode, so there is no
way to enable it.

>     Why is softswitch built?  Why can't 6502 can do without it?

Softswitches are typically control or status registers provided by
hardware components on the motherboard, which are mapped into the
address space of the 6502 in order to allow software to control aspects
of the hardware.  If you didn't have any softswitches, then software
would have no control over the behaviour of the computer (except for
fixed areas of memory which had special functions, like video output
buffers).

An alternative to softswitches is to use "hard switches", i.e. a
physical switch that the user must set to control an aspect of the
computer.  The international models of the IIe have at least one example
of this: a switch on the underside of the keyboard which toggles between
the US and localised character sets for the keyboard and text mode
display.

The 6502 family (including the 65C02 and 65816) only has one address
space, so I/O locations (softswitches) must be mapped into the main
address space.  In the Apple II, the $C000-$CFFF area was chosen for
this purpose.

Some other processor families (such as the 8080/Z80 and 8086/8088) have
separate memory and I/O address spaces, allowing the soft switches to be
independently accessed using special instructions, which in more
advanced cases are restricted to privileged software.  Even in these
cases, the I/O locations could still be described as "softswitches".

-- 
David Empson
dempson@actrix.gen.nz