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In article <1929@uw-june.UUCP> roper@uw-june.UUCP (Michael Roper) writes:
>1)  Since only 128K of main memory is reserved for display buffers (from
>    A0000 to BFFFF), where do EGA's with "a full 256K of RAM", put the 
>    other 128K?  Or is it just bank-switched?

The EGA can be mapped into memory space in several ways, such as B0000-B7FFF
or B8000-BFFFF (for mono & CGA emulation, resp.), or A0000-AFFFF (for
most non-emulation graphic modes), or theoretically A0000-BFFFF (I have
not seen this used; it would conflict with another color/mono adapter).
When in graphic mode 10h, 640x350 color, it's memory is mapped to A0000-AFFFF,
which is only 64K.  It can, however, support up to 4 bit planes in this
address slot (256K Bytes total).  There are various registers which allow
one to read/write single or groups of planes.

>2)  By my calculations, 112K is required for a full page of graphics in
>    video mode 10h, using a 16-color palette.  Is the rest of the (128K)
>    display memory wasted?

You are right, with 16 colors any memory beyond 112K is not displayed;
with 256K this leaves room for a second full page of graphics for ping
ponging if you wish.  Or one could scroll around within a picture larger
than 640x350.  (With only 64K of memory installed in the IBM EGA, only
56K is used, with 4 colors).

>3)  Advanced MS-DOS says that mode 10h uses either a 4- or 16-color palette,
>    depending upon the amount of display memory provided by the adapter.
>    Is this selection made automatically?  What is the cut-off?

Yes, it is automatic; 64K gets 4 colors, 128K or more gets 16.  Handled by
new BIOS (onboard EGA, C0000) during initialization.

>4)  Norton makes a vague reference to the difficulty of writing directly
>    to the EGA's display buffer.  What's the story?  Are the pros and cons
>    of accessing the EGA's buffer directly any different from say, the MA
>    or CGA?

The major pros and cons are the same, speed versus portablility.  All you
get with the BIOS is dot plot/read.  A line drawing primitive can run much
faster if it directly accesses the screen.  The major "con" with the EGA
is that its mapping is more complex regarding multiple bit maps, although
simpler regarding x,y to address translation (no "interleaving" like the
CGA and Herc do in mapping memory to screen).  

In brief: The mode 10h screen is mapped as 4 bit maps, all at location
A0000-A6FFF (28 KBytes).  A0000 is the upper left byte (most significant
bit leftmost, as with CGA/Herc; IBM displays are mixed-endian #%$@).
A0001 is just to the right of that. A0050 (80 decimal after start of buffer)
is just below the first byte, A00A0 just below that.
You can write to any set of planes (all 4, just one, or anything between)
without modifying the others - Provided you do a read of the location 
before writing (if writing all 4 it doesn't matter).  You can read from any
one plane, or from a comparison of any set of planes with a color compare
register.  You can mask which bits of the byte are changed and which are
not (again, must read first unless writing to all bits).  You can do an
AND, OR, XOR or replace when writing (... must read first unless doing
a replace).  There is also a rotation function built in, which I have
not found any way to use (the cpu rotate is more useful).  Perhaps you
can see why this aspect is more difficult to set up (especially with poor
documentation) than CGA stuff.  The good part is that sometime you can
get a real speed gain (you can read from one location, (8 bits x 4 planes)
and write the contents to another, thus transferring 32 bits in one byte
copy).  Good luck.  I have not seen good documentation; IBM now sell the
EGA tech ref material without the Options and Adapters Tech Ref Manuals
set, I hear.  PC tech journal has had some articles which cover everything
in the IBM manuals; have to look in a library to find which issues.
Be prepared to use a debugger and try things out to verify your guesses
at to what the documentation means.

A challenge: find out how to get a 1024x1024 virtual screen (4 colors).
It can be done. (you only get a 640x350 window to display, of course, but
you can move it around).       ~z~

>---Michael Roper---                    *
-- 
Zhahai Stewart
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