1. A method for customizing a test instrument, comprising: providing a plurality of pairs of code modules, wherein each pair of code modules comprises a first code module having program instructions for execution by a processor of the test instrument and a second code module for implementation on a programmable hardware element of the test instrument; wherein, for each pair of code modules, the first code module and the second code module collectively implement a function in the test instrument, wherein the first code module of a pair of code modules is executable on the processor to perform a first portion of a function, and wherein the second code module of the pair of code modules is configured to be implemented on the programmable hardware element to perform a corresponding second portion of the function; wherein the plurality of pairs of code modules are executable to perform a test operation on a system under test (SUT); receiving user input specifying modification of a second code module of a first one of the plurality of pairs of code modules; wherein other ones of the plurality of pairs of code modules are not affected by the modification of the second code module of the first one of the plurality of pairs of code modules; generating a hardware description for the programmable hardware element of the test instrument based on the modified second code module.