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