An apparatus and method are provided for storing source operands for operations. The apparatus comprises execution circuitry for performing operations on data values, and a register file comprising a plurality of registers to store the data values operated on by the execution circuitry. Issue circuitry is also provided that has a pending operations storage identifying pending operations awaiting performance by the execution circuitry and selection circuitry to select pending operations from the pending operation storage to issue to the execution circuitry. The pending operations storage comprises an entry for each pending operation, each entry storing attribute information identifying the operation to be performed, where that attribute information includes a source identifier field for each source operand of the pending operation. The source identifier field has a field size sufficient to enable a register identifier to be stored within the source identifier field to identify the register used to store the data value forming the source operand. However, the field size is insufficient to store the data value as stored in the register. Value analysis circuitry is responsive to the execution circuitry generating a data value that will be used as a source operand for a pending operation, to determine whether a reduced size representation of that generated data value can be accommodated within the associated source identifier field of the entry for that pending operation. If so, the reduced size representation is generated and a control signal is issued to the issue circuitry to cause the register identifier for that source operand to be replaced by the reduced size representation of the data value. By such an approach, it is possible to increase the performance of the apparatus and/or to simplify the construction of the register file.