Graphics Programming for the WP-2
 
   Graphics programming on the WP-2 is relatively simple and
easy.  The video RAM for the LCD screen is directly accessed as
memory locations 9900 to A7FF (hex) and each byte of memory
corresponds with eight pixels on the screen.  Two I/O ports, 60H
and 61H, are also available for LCD control, but are not
necessary to access the video RAM.
   The screen is 480 pixels wide and 64 pixels high.  Each byte
of video RAM is displayed as 8 horizontal dots, black or blank
depending on the corresponding bit (1 for black, 0 for blank). 
Bytes are arranged sequentially from left to right, and top to
bottom on the screen.  Thus, the video RAM consists of 64 rows
of 60 bytes.
   The most significant bit (bit 7) of each byte corresponds
with the leftmost pixel in that byte's group of 8 pixels, and
the least significant bit (bit 0) corresponds with the rightmost
pixel.  For example, suppose memory location 9B4F contained the
value 4.  9B4F is 9900 + 24F in hex.  Converting 24F to decimal,
we find that this location is byte 591 of video RAM, making it
byte 51 of row 9 (591 = 9*60 +51).  Remember, however, that this
is counting 0 as the first byte and first row, so row 9 refers
to the 10th line from the top of the screen, and byte 51 refers
to the 52nd byte from the left.  Because each byte represents 8
pixels, byte 51 translates into pixels 408 to 415, or the 409th
to the 416th pixels from the left.  Since the value in our
memory location is 4, or 00000100 in binary, only one pixel will
be set (black).  The rest will be reset (blank).  Counting in
from the left of the byte, the pixel to be set will be the 6th
pixel.  That makes it pixel 413, or the 414th pixel from the
left.  The XY coordinates of this pixel would be (413,9).
   Don't let the wordiness of this example mislead.  The set-up
of the WP-2's video memory is quite straightforward, and since
it is direct memory accessed, quite fast as well.  Recommended
Z-80 instructions to be used include LDIR; the rotate group of
instructions; BIT, SET, and RESET; and the 8-bit logic
instructions such as OR, AND, and XOR.  The graphics demo
program Exempli Graphia (which can be found in library 15)
relies heavily on LDIR for displaying blocks of memory, and
RR (HL) for moving an image a single pixel to the right.
 
 
   Michael F. Klar - 4 Fleetwood Dr. - Somerville, NJ 08876