Abstract: A method for modifying a user program and running it under the control of another program is provided. This provides ability to save states during the operation of a program in a loop and ability to jump instantaneously between them, either forward or backward in execution time. The invention also provides a way to optionally attach a debugger to an active state and to maintain debug context for all the saved states. The invention provides a means to run a computer program in virtual time machine, in galloping mode and in a safe mode.