Re: 40x24 Resolution Question

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

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

>     I forgot to ask one more question.  Are Apple I's text resolution and
> Apple II/II+'s text resolution the EXACT same with all uppercase, inverse,
> and flash?

I cannot answer that question, as I haven't seen enough detail about the
Apple I to know how its text screen is implemented (let alone the
character mapping).

>     I understand that //e and later have only one difference with hex 0x7F
> that shows dotted box instead of white solid box and also lowercase support
> is added for II+, but I don't know if II does support lowercase unless ROM
> is switched.

The ][ and ][+ have identical video ROMs - neither contains any lower
case characters (unless you replace the ROM).

I don't have a real machine here, but the Apple ][ reference manual
(1981 edition, which covers the ][ and ][+, but only the "revision 0"
and "revision 1" motherboards) claims that the $E0-$FF characters are
duplicates of $A0-$BF (digits and symbols).

The manual's description of the video circuit says that only the lower
order six bits of each byte read from the text screen are used to look
up the video ROM.  This means that it is not possible to display more
than 64 unique characters on early revisions of the ][ and ][+.

Some additional research in the "differences between the ][ and IIe"
appendix of "Understanding the Apple IIe" reveals that my Apple II
Technical Reference manual is somewhat out of date.  Does anyone know if
there was a later edition of this manual before the IIe was released?

On revision 7 of the Apple ][ motherboard, Apple replaced the 2513
character generator (probably a mask programmed ROM) with a standard
ROM, and allowed 7-bit characters to be supplied to the ROM in NORMAL
mode (i.e. $A0-$FF can be distinct characters).  As supplied by Apple,
the characters in the $E1-$FA range (lower case letters) were the same
as $C1-$DA (upper case letters).  I'm not sure about $E0 (reverse quote)
and $FB-$FF (left brace, vertical bar, right brace, tilde and DEL).

The end user can replace the video ROM with a custom EPROM which
contains arbitrary characters.  Installing a "lower case" EPROM was a
popular modification.

This tallies with my recollection of the late model Apple ][+ machines
("Europlus" models) I used at school in 1981 and 1982.  In 1983, the
school upgraded to the Apple IIe, which I used extensively and became
intimately familiar with its details, courtesy of much experimentation
and poring through manuals.

In earlier revisions of the Apple ][, character $7F would be a flashing
question mark, and $FF would be a normal question mark.  I don't know
off-hand what character appears for $FF in an unmodified revision 7
Apple ][ or ][+, but it isn't a checkerboard.  (The flashing space used
by the ][ and ][+ as a cursor is actually character $60, or possibly a
software-timed alternation between $20 and $A0.)

As for the IIe:

The major change in the character set is the addition of lower case as a
standard feature.  In addition, character $FF is a checkerboard symbol,
which the IIe uses as a cursor.

The IIe adds a second character set, which can be selected under
software control.  The "primary" character set is designed to be
compatible with the ][+, and is used by default when in 40 column mode.
The "alternate" character set eliminates FLASH support, but allows the
display of inverse lower case characters, and is used by default when
the 80-column firmware is active (even if it is being used to display 40
columns).

The unenhanced IIe's alternate character set has two sets of inverse
upper case characters, in the $00-$1F and $40-$5F ranges.  The $00-$1F
range is the standard one, compatible with inverse upper case in the ][
and ][+.  In the IIgs, IIc and enhanced IIe, Apple replaced the $40-$5F
characters with "mousetext" symbols, which can be used to draw elements
of a windowed user interface on the text screen, and for special symbols
such as Open-Apple and Solid-Apple.

Applications are supposed to use $00-$1F to display inverse uppercase,
but some used $40-$5F, causing display problems due to unexpected
mousetext characters when the software is used on an enhanced IIe or
later machine.

-- 
David Empson
dempson@actrix.gen.nz