1. A computer system for maintaining liveness information for executing programs, the system comprising: a processor configured to communicate with a main storage, the processor comprising an instruction fetcher and one or more execution units for executing instructions, the processor configured to perform a method comprising: maintaining, by a processor, current operand state information indicating whether corresponding current operands will be used again, wherein current operands that will be used again are enabled for use by a first program module and current operands that will not be used again are disabled for use by the first program module, the first program module comprising machine instructions of an instruction set architecture (ISA), the first program module currently being executed by the processor; accessing a current operand by a machine instruction of said first program module, the accessing comprising using the current operand state information to determine whether a previously stored current operand value is accessible by the first program module; and based on the current operand being disabled, the accessing comprising at least one of a) and b) comprising: a) returning an architecture-specified value as the value of the current operand, where the architecture-specified value is any one of an undefined value, a zero value, a value consisting of all ‘1’s, or a program specified default value; and b) performing a notification, wherein the notification is any one of raising an exception, suppressing an exception, raising and suppressing an exception under control of a machine state, providing debug information, and setting at least one register to indicate occurrence of an access to a disabled operand, wherein the current operand state information further comprises an operand enablement control for controlling whether current operand state information can be changed between enabled and disabled by a current program.