Re: MAP of memory video

[nathan@visi.com (Nathan Mates)]

In article <339C9146.586E@interaccess.cl>,
Alex G. <hgonzale@interaccess.cl> wrote:
>Somebody knows where finding information on the map of memory of video
>in manner text of 80 columns, for a MULTITECH MPF-III that is compatible
>with the Apple-II.

   Ready? Good. Take an aspirin now; you'll probably be getting a
headache from all this :)

   The Apple standard for 80 column starts with 40 column text, so
I'll start there. The primary (page 1) text screen is located from
$400-$7FF in the first 64K of memory; the secondary from $800-$BFF.
The first 40 bytes are the top row of the screen (left to right), then
the next 40 bytes are the 9th screen row, and the next 40 the 17th
row. There are then 8 bytes that do not contribute to the screen, but
are used by cards; do NOT touch these screen holes unless you have a
very good idea what you're doing.

    If you've been keeping track, this is 128 bytes, bringing us to
$480 on page 1. Starting there, we have the second screen row, the
10th, and the 18th, followed by 8 more screen hole bytes. Repeat this
pattern until the 40x24 screen is filled.

   The 80-column modes are worse. There is still a 40-column screen at
400.7FF and 800.BFF. However, that contains the odd numbered columns
in the 80 column screen. The even columns come from "auxillary
memory", which is at the location of the first screen plus 64K. Of
course, pre-GS Apple IIs can't directly access more than 64K at once,
so there's a whole 'nother nightmare of softswitches, etc to deal with,
and that's the subject of another posting.

   [Of course, all of the above assumes that this card follows the
standard for 80-column text as initially showed up on the //e's aux
slot. If this is for a ][+ or the like, the above may not be useful or
correct]

   Here endeth today's lesson. 

Nathan Mates

--
<*> Nathan Mates http://www.visi.com/~nathan/      <*>
# What are the facts? Again and again and again-- what are the _facts_?
# Shun wishful thinking, avoid opinion, care not what the neighbors
# think-- what are the facts, and to how many decimal places?  -R.A. Heinlein