Sp00 Final Exam Review: Virtual Machines

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Sp00 Final Exam Review: Virtual Machines

See Final Exam Review - Sp2000

a)
Why VMs are better than native code:

more portable. instead of entire system, just port the VM code
better memory security so you don't shoot yourself in the foot. or, have the toaster never turn off...
internal garbage collection ( generational to make faster ) allows the luxary of not having to worry about memory leaks
once the VM is ported, you don't have to write any new code. You're done. But if it's in native code, you have to re-write the code for EACH system it is required to run on

b)
HotSpot and Squeak VM Similarities:

both have generation scaveging and mark-sweep GC algorithms
use stacks, not registers
you can compile JAVA's VM bytecode to Squeak VM bytecode

HotSpot and Squeak VM Differences:

JAVA is typed, squeak isn't
JAVA handles exceptions in VM, Squeak handles them in Squeak, not VM
Squeak's VM is smaller than JAVA's
you canNOT compile Squeaks's VM bytecode to JAVA VM bytecode ( types and operands )

c)
guessing here, but floating point math?

anyone remember what mark said about frames/contexts and stacks in class? something like, contexts are in stacks, but for java, the stacks are in the contexts? or vice versa???

Stephen Belknap

Java: contexts are in stack
Squeak: stack in each context

 Java is faster than Squeak. popping off stack requires much less time than context switching

Coredump

c) if i remember correctly, mark was talking about boxing and unboxing being the hardest part of primitives. meaning, moving between the native types and objects is a challenge. but i could be completely off :)

sheetal

no.. that sounds right. i didn't think it was floating point math. your answer does look correct, and i remember mark discussing this in class

Sheetal's not off. The issue that Stephen's grappling with is where the stack is that the bytecodes handle: Inside the contexts (the method execution objects) or global. Stephen's (a) and (b) lists are good, but there are more possible items in each. Mark Guzdial

a) Portability, garbage collection, security, better for OO. Also the programs run on the VM and not on top of something else (ie, this is good for small devices without an OS).
b) The big difference that we came up with is the difference in garbage collection. Java has a 3 generational, and Squeak has a 2 generational AND a mark/sweep. Also, Squeak has JIT compilation.

I think JAVA's VM also has a M-S GC algorithm as well as the generational scaveging. Not 100% sure, though -Stephen Belknap

Susi Rathmann, jeremy, Matt Flagg

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Final Exam Review - Sp2000 last edited on 20 April 2000 at 5:18 am by r66h24.res.gatech.edu.
VMs and Bytecodes last edited on 2 May 2006 at 7:48 am by lawn-199-77-211-10.lawn.gatech.edu