Assembly Language for Beginners

4.1. JAVA

public class calc
{
public static int mult_add(int a, int b, int c)
{
return a*b+c;
}
}

public static int mult_add(int, int, int);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=3, args_size=3

0: iload_0

1: iload_1

2: imul

3: iload_2

4: iadd

5: ireturn

The first step is multiplication. The product is left at theTOS:

+---------+
TOS ->| product |
+---------+

iload_2loads the third argument (c) in the stack:

+---------+
TOS ->| c |
+---------+
| product |
+---------+

Now theiaddinstruction can add the two values.

4.1.4 JVMmemory model

x86 and other low-level environments use the stack for argument passing and as a local variables storage.

JVMis slightly different.

It has:

• Local variable array (LVA^7 ). Used as storage for incoming function arguments and local variables.

Instructions likeiload_0load values from it.

istorestores values in it. At the beginning the function arguments are stored: starting at 0 or at 1

(if the zeroth argument is occupied bythispointer).

Then the local variables are allocated.

Each slot has size of 32-bit.

Hence, values oflonganddoubledata types occupy two slots.

• Operandstack(orjust“stack”). It’susedforcomputationsandpassingargumentswhilecallingother
  functions.

Unlikelow-levelenvironmentslikex86, it’snotpossibletoaccessthestackwithoutusinginstructions

which explicitly pushes or pops values to/from it.

• Heap. It is used as storage for objects and arrays.

These 3 areas are isolated from each other.

(^7) (Java) Local Variable Array