During program code conversion, such as in a dynamic binary translator, automatic code generation provides target code 21 executable by a target processor 13. Multiple instruction ports 610 disperse a group of instructions to functional units 620 of the processor 13. Disclosed is a mechanism of preparing an instruction group 606 using a plurality of pools 700 having a hierarchical structure 711-715. Each pool represents a different overlapping subset of the issue ports 610. Placing an instruction 600 into a particular pool 700 also reduces vacancies in any one or more subsidiary pools in the hierarchy. In a preferred embodiment, a counter value 702 is associated with each pool 700 to track vacancies. A valid instruction group 606 is formed by picking the placed instructions 600 from the pools 700. The instruction groups are generated accurately and automatically. Decoding errors and stalls are minimised or completely avoided.