# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1684335179 -7200
# Node ID d444825c4fa1af85a94db4844e1ed79bc077c233
# Parent  9a19c0cb0c45936397ce58ca4152a8371eb815e6
Added remarks about text mode screen sizes and memory layouts.

diff -r 9a19c0cb0c45 -r d444825c4fa1 ULA.txt
--- a/ULA.txt	Wed May 17 15:26:15 2023 +0200
+++ b/ULA.txt	Wed May 17 16:52:59 2023 +0200
@@ -1514,7 +1514,45 @@
 
 Character bitmap data would be stored in RAM, since this is the only possible
 source of data for the ULA as delivered. The use of ROM would require changes
-to the broader system architecture.
+to the broader system architecture. Thus, the total memory requirements of
+such a mode would be the locations for character positions plus the storage
+requirements of the bitmaps to be supported.
+
+  Columns  Rows  Screen size  Bitmaps  Bitmaps size  Total size
+  -------  ----  -----------  -------  ------------  ----------
+  40       25    1000         256      2048          3048
+  40       25    1000         128      1024          2024
+  40       25    1000         96       768           1768
+  40       32    1280         256      2048          3328
+  40       32    1280         128      1024          2304
+  40       32    1280         96       768           2048
+
+The simplest arrangement would involve bitmap definitions for all 256 possible
+character codes, demanding a total of around 3K of RAM. Reducing the number of
+supported bitmaps to 96 (codes 32 to 127 inclusive) would bring this total to
+a maximum of 2K, but this would incur additional complexity in the ULA itself
+if the codes not corresponding to bitmaps were to be specially mapped to, say,
+the bitmap for the space character or to a null character.
+
+With the screen start address controllable, it is conceivable that with a
+256-entry bitmap table, the screen memory could be made to overlap the bitmap
+table for bitmaps not likely to be used. For example, the bitmap table might
+be situated at &7700, with this leaving enough space for 128 entries (&400 or
+1024 bytes) and a 40x32 text screen (&500 or 1280 bytes):
+
+  &8000 +---------------+---------------+
+  &7F00 +---------------+               |
+        |               |    Display    |
+        | Bitmaps (128) |    (40x32)    |
+        |               |               |
+  &7B00 +---------------+---------------+
+        |               |
+        | Bitmaps (128) |
+        |               |
+  &7700 +---------------+
+
+Care would then need to be taken to avoid the use of codes from 128 to 255 in
+the screen memory as these would replicate character data as bitmap data.
 
 Enhancement: MODE 7 Emulation using Character Attributes
 --------------------------------------------------------