Instruction manual

Fig. 5: Memory Map for Shape Table
Shape File Initialization
The principle shape-creating pro-
gram requires a previously allocated
disk file
for shape table storage. The in-
itialization program (Figure
6) creates
the
disk file and also establishes the
name and length
of the file. The program
allocates space for the shape table
directory based on the number
of
shapes to be stored in the file, a number
that is declared by you during initializa-
tion. The memory
map_ for a shape table
is stored in the first byte
of the table; its
maximum value is therefore
255, and
this is the maximum number of shapes
that can be'stored in one shape table.
The directory contains addressing infor-
mation
that allows random access to
r
)
\
J
N, No. shapes
Empty
Directory
First Shape
Byte 1
Byte 2
2* N Bytes
any shape in the table.
The directory falls between the first
byte
of the table and the beginning of
the first shape. The amount of space
allocated to the directory is determined
by the number of shapes ultimately to be
stored in the table; each shape requires
two byte in the directory for addressing.
The shape tables themselves may
be
any length, up to a total length consis-
tent
with the 15 x 15 matrix in which the
shapes are created. The shape tables
are stored end-to-end as they are added
to the file, each shape determining in a
zero byte as end-of-record mark. The
layout
of the shape file requires that any
tables added to the file be accurately
done, because once a table is buried in
the file, it cannot
be simply replaced
unless the replacement has precisely
the same length.
The file initialization program is
also used for creating the cursor
re-
quired for mapping shapes on the
15 x 15 working grid produced by the
principal program. This relieves the user
of the need to generate the cursor
himself everytime
he opens a new shape
file. The cursor is stored as the first
shape in the shape file, and the shape-
creating program assumes
that the cur-
sor has already been stored
for its use.
As a consequence
of this arrangement,
you
must remember that the user-
generated shapes start with the second
shape table in the file.
Although the file initialization pro-
gram zeroes out all
of the bytes in the
directory, there is no substantial reason
for doing this, except that the string of
zero bytes make it easy to determine
where the directory ends and the shape
tables begin in a memory dump. This ad-
vantage will last only until the directory
is filled.
The Shape Creating Program
The BASIC program (Figure
7) that
enables shape generation requires the
use
of dual floppy disks, but can be easi-
ly changed
for single floppy use by
replacing "02" in step 110 by "01."
(Similar adjustments will have to be
made in the initialization and display
programs, which store and access the
shape file from
disk 02). Tape users will
have
to replace disk 1/0 by suitable tape
I/O in steps 100, 110 and 1360.
The program loads a pre-existing
shape file (created
by the initialization
program, if necessary) from disk, using
the shape file name supplied by you on
request from the program. The file is
loaded
into a memory location which
you are also asked for by the program. A
check is made (step
220) that there is
room in the shape file
-directory for
another entry.
If not, you will be so advis-
ed and the program will abort. A pointer
to the shape file required by the APPLE
system is set up in step
260. The 15 x 15
plotting grid is turned on (steps 300-330)
and you will be asked to input the star-
ting grid coordinates for the shape.
Note, these are
grid coordinates and not
screen coordinates that are asked for.
The cursor will
be displayed on the
center
of the grid square that you have
just selected as the starting point. Some
user helps are displayed in the text area
under the grid (steps
410-440), and you
are
off and running. Manipulation of the
R,L,D, and U keys will move the cursor in
the appropriate directions. The REPEAT
key will work with these commands.
Pressing the P key will plot a small circle
inside the square in which the cursor
currently resides, and this plotted point
will become part
of the shape table be-
ing built in memory. An image of the cur-
sor will persist in the initial
square-as a
"negative" image if you happened to
plot at that square. The persistent cur-
sor image serves as a reminder
to you of
the location of the start of the shape.
The cursor is made to disappear and
reappear in adjacent squares as you
press the move keys by XDRAW com-
mands at steps
500 and 530; the IF state-
ment at step
1040 in the subroutine that
draws the plotting circle is responsible
for keeping the persistent image
of the
cursor
at the starting square. The flag,
FLAG,
that appears in step 480 and
elsewhere is used to allow the cursor
to
be turned off in a plotted square and to
be turned on again when the cursor
moves to the next square.
Keystrokes are recorded in step
570. A previous step (550) saves the
previous two keystrokes in KI$ and
KSVE$. The former record, KI$, is
re-
quired to allow the erase feature, con-
trolled by the E key and discussed
below.
KSVE is needed for proper
generation of plot-then-move 3-bit
codes, also discussed below. Interpreta-
tion
of a keystroke takes place in steps
590-710, a sequence
of IF's called a
sieve. This particular form
of key screen
was chosen because it gives almost
complete protection against inadvertent
entry
of incorrect keys. Once you are in
the program, you will find
that the
keyboard is effectively locked out for all
keys except those required by the pro-
gram. If a non-applicable key is pressed,
the sieve eventually routes the program
through step
710 back to another key ac-
cess
at step 570. Inside the sieve, when
a keystroke has bee identified as a move
command (L,R,U,D), the appropriate
X- or
Y- coordinate adjustment is made and
the decimal value
of the 3-bit code ap-
plicable
to the move is stored where the
variable
KSVE$ is checked to see if the
previous keystroke was a Plot com-
mand.
If it were, SYMBOL is in-
cremented by a 4 (remember Figure
4?),
and SYMBOL is then transmitted to the
byte assembly area, more of this later.
December,
1979'
MICRO -- The 6502 Journal
19:15