The document discusses the Dalvik virtual machine (VM) used in Android. It begins by explaining what a VM is and the basic parts that make up a VM. It then discusses the differences between stack-based and register-based VMs, noting that Dalvik uses a register-based architecture. The document explains that Dalvik was chosen for Android because it executes instructions more efficiently than Java VM and requires less memory. It also discusses just-in-time (JIT) compilation techniques used to improve performance of interpreted code. Specifically, Dalvik uses a trace JIT that compiles short sequences of instructions to optimize mobile performance.

1. Dalvik VM & JITAnkit Somani July 25, 2010

2. What is VM ? A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine.Basic Parts: A set of registers

3. A stack (optional)

4. An execution environment

5. A garbage-collected heap

6. A constant pool

7. A method storage area

8. An instruction set Types of VM ? Based on its Working &Functionality : 1. System Virtual Machine (supports execution of a complete operating system)2. Process Virtual Machine (supports execution of a single process) Based on its architecture :1. Stack based VM (uses instructions to load in a stack for execution)2. Register based VM (uses instructions to be encoded in source and destinationregisters)

9. Dalvik Virtual MachineDalvik architecture is register based

10. Can run on slow CPU, with little ram & in OS with lesser or even without swap space

11. It is optimized to use less space

12. The interpreter is simplified for faster execution

13. It executes its own Dalvik byte code rather than Java byte code Why android choose Dalvik?Dalvik (Register based) take average 47 % less executed VM instruction then JVM (Stack based).

14. Register code is 25% larger than the corresponding stack code.

15. This increased cost of fetching more VM instructions due to larger code size involves only 1.07% extra real machine loads per VM instruction. Which is negligible.

16. Some Marketing Reasons too. .Dex file anatomy

17. Conversion of Java Byte Code to Dalvik Byte Code

18. Shared Constant Poolpublic interface Zapper { public String zap(String s, Object o);}public class Blort implements Zapper { public String zap(String s, Object o) { ...; }}public class ZapUser { public void useZap(Zapper z) { z.zap(...); }}

19. Shared Constant Pool (.Class File)

20. Shared Constant Pool (.Dex File)

21. Shared Constant Pool (Memory Saved Via) minimal repetition

22. per-type pools (implicit typing)

23. implicit labeling*After the Obfuscation & compression it will be much lesser.

24. Example #1: Sourcepublic static long sumArray(int[] arr) { long sum = 0; for (inti : arr) { sum += i; } return sum;}