Separating instructions, logic and memory
The mechanism represents ten without requiring a person to remember the carry.
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The ideas to retain
1. From automating calculations to programming procedures
The first calculators: automation is not programming
The Pascaline, built by Blaise Pascal between 1642 and 1644, automated addition and subtraction through geared wheels. A few decades later, Leibniz's Step Reckoner extended the idea and…
Jacquard: when instructions separated from the mechanism
The next decisive advance did not come from mathematics, but from the textile industry. The Jacquard loom, developed in 1804-1805, used interchangeable punched cards to control complex…
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A carry connects two positions
A units wheel represents nine and a tens wheel represents zero.
Adding one unit returns the first wheel to zero and advances the second to one.
The mechanism represents ten without requiring a person to remember the carry.
Change the pattern, keep the mechanism
A pattern of holes encodes instructions for selecting positions.
A second card activates another sequence without rebuilding the mechanism.
Separating instructions from execution lets different procedures be repeated.
A logical operation can control a circuit
Two switches in series must both close to complete the path.
If A or B remains open, the output is zero.
When both close, the output is one: the circuit implements A AND B.
A local rule can traverse a tape
The tape contains three ones followed by an empty cell.
The rule moves right while it reads a one.
At the empty cell it writes another one and stops, incrementing a unary count.
Instructions and data can reside in memory
Memory contains load, add and store instructions alongside two data values.
The execution unit reads one instruction at a time and updates its register.
In this example it loads three, adds four and stores seven.
Four equally likely alternatives require two bits
A message can be A, B, C or D, each equally likely.
Two binary decisions distinguish the four possibilities.
Bits quantify uncertainty in this choice, not the message’s meaning.
