Instruction manual

Time of Day Clock
and Calendar for the SYM·1
Now you can have a Clock and Calander running in your
SYM at the same time you are running programs in
BASIC. The concepts presented can be easily generaliz·
ed into other 'multi-task' operations.
Casmir J. Suchyta, III
and Paul
W. Zitzewitz
Univ.
of Michigan, Dearborn
4901 Evergreen Road
Dearborn, MI 48128
~ .. )
Here is a machine language
subroutine for the SYM-1 BASIC which
keeps track of time and date while allow-
ing BASIC programs
to be run.
A useful
adjunct to a microcom-
puter, especially one used in a system,
is a
continuously running clock which
can be used to record the time at which
events occur or to generate signals at
specified times. The SYM·1 includes
timers on the 6522 VIA chips which make
implementation of such a clock easy.
The
clock can be started, set, and read
from BASIC.
The clock is based on the use of the
6522
to generate a train of accurately
spaced interrupts. The April, 1979, issue
of MICRO contained an article by John
Gieryic (page 31) which presented the
techniques
of setting up and servicing
the interrupts. The clock is an adapta-
tion of those techniques. The program
consists of sections which set the clock,
initialize the interrupt, service the inter·
rupt, and
update the clock. The clock-
calendar needs to be reset only on
February 29!
The program is loaded
into the
highest bytes
of available memory. On a
4K machine this is $OF54-$OFFF. After
the program is loaded, BASIC is initializ-
ed with Memory Size set at 3920 to avoid
overwriting the program. The clock is set
and started
by the command PRINT
USR(3924,M,d,h,m), where the four
parameters represent the month, date,
December, 1979
hour, and minute, respectively. The pro-
gram stores the times, then initializes
the interrupt and starts the timer as
described in MICRO 11:31. The
timer
located at $ACxx was used to avoid in-
terference with the cassette tape
routines. Once every 1/20 second an in·
terrupt
occurs which is serviced in the
routines starting at $OF90. Accumulator
and registers are pushed on to the stack,
then the 1/20 of seconds, seconds,
minutes, and hours are incremented as
needed. These four updates are done in
an indexed loop, using a table
of com-
parison values (20 fractions, 60 seconds,
60 minutes, 24 hours) stored at $OFE9 to
see if the next timing unit should be in·
cremented. The days and
months cannot
be incremented in the same loop, and so
are done in the routines starting at
$OFBD. There is a comparison table giv-
ing the number
of days (plus one) in each
month starting at $OFF4 used to deter-
mine if the month should be in·
cremented. When all needed increments
are made the flag is cleared and the sav-
ed registers pulled back from the stack.
The
clock may be read from BASIC
by PEEKing at the appropriate storage
locations.
To print the date and time in
the form 7/20/1979 17:45:02 execute the
commandPRINTPEEK(4083)"1"
PEEK(4082)"/1979
"PEEK(4081)":"PEEK(4080)":"PEEK·
(4079). The number of the month in the
date can be replaced by a three letter abo
breviation by using the following short
program
to print the date.
MICRO
-- The 6502 Journal
1 A$
= "JANFEBMARAPRMAYJUN-
JULAUGSEPOCTNOVDEC"
2 MO =1 + 3"(PEEK(4083) -1)
3 PRINT
MID$(A$,MO,3);PEEK)4082);",1979"
Starting each program With
this
routine will let you know exactly when
you did each job. Another use
of the
clock is to serve as an alarm clock. You
may want the
SYM to turn on a light, or
start an experiment at a certain time. To
do this include a tight loop which in-
cludes an IF statement comparing one
or more of the storage locations with the
desired time. When the comparison is
good, the loop will
be exited and the
computer can execute the command.
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19:67