The Conditional Branch
The ENIAC was one of the first electronic devices to have a conditional
branch. Modern computer languages use IF...THEN...ELSE for branching.
The wires that ran around the ENIAC carried two kinds of pulses: number-pulses
and control-pulses. The number-pulses represented numbers from zero (no
pulses) to nine (nine pulses). The control-pulse was used to trigger the
start of a panel.
The ENIAC operators found that if they connected a number-pulse wire to
the control-pulse input, it could be used to control the execution of the
program. Here is how it worked: If the output on the number-pulse wire was
a non-zero number (one or more pulses), then it could be used to start another
panel. But, if the output was zero (no pulses), then the operation would
halt.
Some scholars contend that it is the ability to branch that separates a
computer from a calculator. Here is another way to say it:
IF a machine has the ability to branch,
THEN
it's a computer,
ELSE
it's just a calculator.
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