Five short randomised exercises on the idea every later page assumes: one bit pattern is two numbers, and which one you get depends only on how you read the top bit. They start at 4 bits, where there are only sixteen patterns, and offer 8 bits once that feels mechanical. Each asks for your answer before it shows the working, drawn as bits.
Read a pattern both ways. Give the unsigned and signed readings; the reveal shows the place values, with the top bit worth minus its power of two under the signed reading.
Write a value as a pattern. The reveal shows both routes: count back from zero round the wheel, or invert and add 1.
Step round the wheel. Predict where a subtraction lands and what it reads as; at 4 bits the reveal draws the wheel with the arc.
Negate by hand. Invert, add 1, check the reading. The one pattern that cannot be negated turns up.
Does it fit? Judge whether the unsigned and signed readings of a sum are the true sum: the C and V flags before they have names.
Next¶
Bit Lab: the ALU that does this arithmetic, with the flags drawn from gates.