Java virtual machine: Difference between revisions

Revision as of 05:21, 10 August 2004

Revision as of 12:18, 14 August 2004

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Each [[Thread (computer programming)|thread]] has its own program counter.

Code verification is applied to bytecode class files by the runtime at program load time. This means that only a limited amount of bytecode sequences form valid programs, e.g. a JUMP (branch) instruction can only target an instruction within the same function. Because of this, the fact that JVM is a stack architecture does not imply a speed penalty for emulation on register based architectures when using a JIT compiler: In face of the codeverified JVM architecture, it makes no difference to a JIT compiler whether it gets named imaginary registers or imaginary stack positions that need to be allocated to the target architectures registers. In fact, code verification makes JVM different to a classic stack architecture whose efficient emulation with a JIT compiler is more complicated and typically carried out by a more slowslower interpreter.

Code verification also ensures that arbitrary bitpatterns cannot get used as an address. [[Memory protection]] is achieved without the need for an [[Memory management unit|MMU]]. Thus, JVM is an efficient way of getting memory protection on simple silicon that has no MMU.