Hard and soft real time systems require, for each process, the
worst-case execution time (WCET), which is needed by the scheduler's
admission tests and subsequently limits a task's execution time during
operation.  A worst-case execution time analysis is usually performed
in two distinct steps: first the program is analyzed to extract
semantic information and determine maximal bounds on the number of
iterations for each basic block. In a second step the duration of the
different program's instructions is computed with respect to the used
hardware platform.  Modern systems with preemption and modern
architectures with non-constant instruction duration (due to
pipelining, branch prediction and different level of caches) hinder a
fast and precise computation of a program's WCET.  We present a
technique to approximate the instruction duration on modern processors
using precise block bounds. Instead of simulating the CPU behavior on
all the possible paths we apply the principle of locality limiting the
effects of a given instruction to a restricted time allowing us to
analyze large applications in linear time.