As presented by Hao Wang (1954, 1957), his basic machine B is an extremely simple computational model equivalent to the Turing machine. It is "the first formulation of a Turing-machine theory in terms of computer-like models" (Minsky, 1967: 200). With only 4 sequential instructions it is very similar to, but even simpler than, the 7 sequential instructions of the Post–Turing machine. In the same paper, Wang introduced a variety of equivalent machines, including what he called the W-machine, which is the B-machine with an "erase" instruction added to the instruction set.
Description
As defined by Wang (1954) the B-machine has at its command only 4 instructions:
A sample of a simple B-machine instruction is his example (p. 65):
1. *, 2. →, 3. C2, 4. →, 5. ←He rewrites this as a collection of ordered pairs:
{ ( 1, * ), ( 2, → ), ( 3, C2 ), ( 4, → ), ( 5, ← ) }Wang's W-machine is simply the B-machine with the one additional instruction