IM System Architecture Notes
We use binary logic in order to have an abstract representation of
logic gates. A digital logic circuit has a number of input lines A, B,
C, ... and a number of output lines. We will consider one output
line called Z at the moment.
We write Z = f(A,B,C,...) to mean that the value of Z is determined
by the values of the inputs A, B, C.... Z is said to be a
function of its inputs.
In the case of real digital circuits currents are passed through the
lines, but voltages between about 5 and 15 are called
on or TRUE and near zero called
off or FALSE. We identify TRUE
with the digit 1 and FALSE with the digit 0.
- Binary Every expression can only evaluate to 0
or 1, there are no other possibilities.
- NOT For any A, not(A) has the value 1 if A=0,
and 0 if A=1. not(A) is called the complement of
- OR Given two inputs A, B the expression A+B
represents "A OR B" and evaluates to 1 if at least one of A or B is
1, otherwise it is 0.
- AND Given two inputs A, B the expression A.B
represents "A AND B" and evaluates to 1 if both A and B are 1,
otherwise it is 0.
- NOR Given two inputs A, B the expression
not(A+B) (not-or) is the complement of A+B.
- NAND Given two inputs A, B the expression
not(A.B) (not-and) is the complement of A.B.
- EOR Given two inputs A, B the expression A^B is
the exclusive-or of A, B and has the value 1 if one and only one of
A or B are 1 (but not both).
These operations may be expressed as Truth Tables, and using truth tables a number of fundamental laws of logic, often called Boolean Algebra (after the mathematician George Boole) may be constructed.
Back to System Architecture Home Page