of Microprocessors

CONTROL SEQUENCE DISPLAY AND EDITING 343
2. Store Xl into port 1 (the coordinate port).
3. Set "store X" and "move X" bits to ones and then zeros to cause an
immediate move to Xl.
4. Store Y I into port 1.
5. Set "move Y" bit to a one then a zero to cause an immediate move to
Yl.
6. Store X2 into port 1.
7. Set "store X" bit on then off to store X2 In the display generator
without affecting the X DAC.
8. Store Y 2 into port 1.
9. Set "move X," "move Y," and "draw" bits on.
10. Wait 50 j-tsec for the line to be drawn.
11. Clear the control port to zeros.
Usually the majority of an image is formed by line segments joined end-toend.
In this case, each additional segment after the first only requires Steps 6
to 11 to be executed.
Display List Interpreter
In actual use, a display subroutine would be written that would display
all of the line segments.needed for the desired image once and then return. A
display list in memory can specify the line segments in a form that allows
rapid retrieval and display yet easy manipulation of the list for editing. The
display subroutine then becomes a display list interpreter executing commands
from the list not unlike a BASIC interpreter. Although considerably more
sophisticated list formats are possible, let us describe one that is simple yet
well suited for this display.
The display list consists of individual "list ,elements" strung end to
end. Each element contains an "operation code" byte followed by "operand"
bytes. Code Olt6, for example, will specify a move with the beam off;
therefore, it will be followed by two bytes specifying X,Y coordinates of the
destination. Code 02 specifies an isolated line segment; thus, XI,YI and
X2,Y2 will follow as four bytes. For drawing connected line segments, it is
most efficient if they are drawn consecutively. Accordingly, code 03 indicates
that a count byte and a series of coordinates follows. A move will be
done to the first coordinate of the series and thereafter lines will be drawn
from point to point through the series. The count byte specifies up to 255
lines in the series.
In music synthesis applications, it is common to display graphs in
which one axis, usually X, simply increments for each line in the graph.
Considerable space saving is therefore possible by defining a "graph" segment
type. Code 04 is used to specify a graph. It is followed by a count byte,