In a data processing apparatus 2 having a first mode of operation in which JAVA.RTM. bytecodes 46, 48 specify the processing operations and a second mode of operation in which other instructions specify the processing operations. In order to speed operation, the JAVA.RTM. Local Variable 0, or another such variable, is stored within a register of a register bank 14 to be available for rapid access. This storage is in a write-through manner such that reads of the value will be directly serviced from the register R4 and writes to the data value will be made in both the register R4 and back in the original memory location for that data value as determined by the JAVA.RTM. Virtual Machine.