1 description of third column of em_table:
3 -: pop item indicated by next character
4 +: push item indicated by next character
5 0: no effect on the stack
7 characters describing items:
9 w: target machine word (1, 2 or 4)
10 d: double target machine word (2, 4 or 8)
11 p: target machine address
12 a: item with size specified in argument
13 x: item with size specified by top item of stack
14 y: item with size specified by second item on stack
15 ?: one or more items of unknown size