1. A computer system for executing a sequence of instructions and effecting changes in data held in one or more registers during execution of the instructions, which computer system includes instruction fetch circuitry, decode circuitry to decode instructions and identify any registers to be used in execution of the instruction, and dispatch circuitry to dispatch instructions to one or more execution units after decoding, said computer system including emulator circuitry for debug operations which emulator circuitry is arranged to watch data values in one or more selected registers modified during execution of the instructions, which computer circuitry further comprises a register watch store for identifying one or more registers to be watched, comparator circuitry for comparing registers identified by said decode circuitry with registers identified in said register watch store and providing a hit signal for hits in the comparison, and instruction insertion circuitry responsive to hit signals to insert in the instruction sequence to an execution unit a store instruction to store in a location accessible by the emulation circuitry the data value in a data register identified by a hit signal after execution of the instruction using the data register.