1. A method of setting a breakpoint at an address in a guest software program in a memory of a virtualized data-processing system, the method comprising automatically performing the following steps using a processor: a) locating a division of memory corresponding to the address; b) creating a copy of at least part of the contents of the division; c) modifying the contents of an instruction set of the division to set a breakpoint at the address; d) configuring an environment of the guest software program to permit executable but not readable access to the division; and e) installing an access violation handler so that: i) when the guest software program attempts to read at least some of the contents of the modified division, the processor will provide a data view using the copy of the division and configure the environment to permit readable but not executable access to the data view in place of the division; and ii) when the guest software program attempts to execute at least some of the contents of the division, the processor will configure the environment to provide a code view using the division having the modified contents which grants executable but not readable access.