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and easy to provide a circuit modification such that
when the data was put out as a 1 the light -would be
lit rather that turned off, such as addition to the
circuit would have cost you more that the cost for
byte of memory.
can be accomplished by' adding one instruction to the loop
which complements the data, that is, changes all ones to
0 's and all 0's to l's.
same as Test Program 1 with the addition of one instruction
between the input instruction and the output instruction,
which will complement the data read in from the switches
before it is output.
Test Program 2 may be entered exactly the same way as Test
Program 1 was and checked and then run through one or
more cycles with the operation of the machine and to
double-check that the program truly has been entered
correctly.
permit the loop to run at high speed.
With this change in the program, thejPROGRAMMED. OUTPUT
port will show a light lit when the switch is positioned
up to enter a 1 bit.
way to achieve the function of causing the lights to
turn on when the bit is entered as a 1, but it is a
much more versatile solution since the operator can
change his mind at a later date and either remove the
complement instruction or change it to yet another
instruction for a different result.
When single stepping through Test Program 2, the compli
ment data instruction is seen to use up only one cycle of
the processor.
executed, and when the SINGLE STEP switch is operated
again, we are immediately fetching the next instruction.
This will be true of any instructions which operate
only on data which is already stored within the processor.
Additional cycles are only necessary if additional
information must be read in or out of the program
processor itself.
After either loop is running,
be depressed to STOP at any time and the operation
processor will stop during the fetch of the next
instruction.
operates, it is impossible to tell beforehand at what
point in the loop the processor will be at the exact
instant that the RUN/STOP switch is moved to STOP, so
that the processor will stop at different places in the
loop for different times when the switch is actuated.
40
The same function as the added circuit
Test Program 2 is exactly the
If the machine is stopped and reset.
Then the RUN switch may be actuated to
Not only is this a less expensive
We are able to see it being fetched to be
Due to the speed at which the processor
IMSAI 8080
General Assembly and
Test Instructions
the RUN/STOP switch may
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