Document the new FG handling.

1 files changed, 22 insertions, 0 deletions

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+Changes in release DE1-18:
+--------------------------
+* Use a 6x10 character matrix as the original ABC80/800 did, instead
+  of a 8x16.  To make this compatible with a VGA monitor, we use
+  double scanning (each pixel row drawn as two scanlines.)
+* Add an ABC800C/M-style "fine graphics" (FG) unit.  Since there is
+  not enough on-chip memory, we store the FG data in the SRAM, which
+  is multiplexed with the CPU.  This works without stalling either
+  unit, since the CPU never needs the SRAM for two consecutive CPU
+  cycles, so we can always get one access every two CPU cycles for the
+  graphics unit, which is more than enough.  In fact, we probably
+  could supply quite a bit more bandwidth than this (especially since
+  the SRAM is really 16 bits wide), but this is good enough for now.
+
+  In ABC800, the FGCTL control is OUT 6; since this is the audio
+  generator on ABC80 we use OUT 158 for now.  OUT 159 can be used to
+  select which 16K page in the SRAM contains the graphics; this
+  defaults to 4 (= 64K) on reset.
+
+  This is intended to be a first step toward implementing ABC800.
+
+
 Changes in release DE1-17:
 --------------------------
 * Fix handling of "abcprintd -d directory" with a relative pathname