From CHAPTER 3 of the JVM Spec

The Structure of the Java Virtual Machine

3.11 Instruction Set Summary

3.11.2 Load and Store Instructions

The load and store instructions transfer values between the local variables (§3.6.1) and the operand stack (§3.6.2) of a Java virtual machine frame (§3.6):

Load a local variable onto the operand stack: iload, iload_<n>, lload, lload_<n>, fload, fload_<n>, dload, dload_<n>, aload, aload_<n>.
Store a value from the operand stack into a local variable: istore, istore_<n>, lstore, lstore_<n>, fstore, fstore_<n>, dstore, dstore_<n>, astore, astore_<n>.
Load a constant onto the operand stack: bipush, sipush, ldc, ldc_w, ldc2_w, aconst_null, iconst_m1, iconst_<i>, lconst_<l>, fconst_<f>, dconst_<d>.
Gain access to more local variables using a wider index, or to a larger immediate operand: wide.

Instructions that access fields of objects and elements of arrays (§3.11.5) also transfer data to and from the operand stack.

Instruction mnemonics shown above with trailing letters between angle brackets (for instance, iload_<n>) denote families of instructions (with members iload_0, iload_1, iload_2, and iload_3 in the case of iload_<n>). Such families of instructions are specializations of an additional generic instruction (iload) that takes one operand. For the specialized instructions, the operand is implicit and does not need to be stored or fetched. The semantics are otherwise the same (iload_0 means the same thing as iload with the operand 0). The letter between the angle brackets specifies the type of the implicit operand for that family of instructions: for <n>, a nonnegative integer; for <i>, an int; for <l>, a long; for <f>, a float; and for <d>, a double. Forms for type int are used in many cases to perform operations on values of type byte, char, and short (§3.11.1).

This notation for instruction families is used throughout The Java^TM Virtual Machine Specification.

3.11.3 Arithmetic Instructions

The arithmetic instructions compute a result that is typically a function of two values on the operand stack, pushing the result back on the operand stack. There are two main kinds of arithmetic instructions: those operating on integer values and those operating on floating-point values. Within each of these kinds, the arithmetic instructions are specialized to Java virtual machine numeric types. There is no direct support for integer arithmetic on values of the byte, short, and char types (§3.11.1), or for values of the boolean type; those operations are handled by instructions operating on type int. Integer and floating-point instructions also differ in their behavior on overflow and divide-by-zero. The arithmetic instructions are as follows:

Add: iadd, ladd, fadd, dadd.
Subtract: isub, lsub, fsub, dsub.
Multiply: imul, lmul, fmul, dmul.
Divide: idiv, ldiv, fdiv, ddiv.
Remainder: irem, lrem, frem, drem.
Negate: ineg, lneg, fneg, dneg.
Shift: ishl, ishr, iushr, lshl, lshr, lushr.
Bitwise OR: ior, lor.
Bitwise AND: iand, land.
Bitwise exclusive OR: ixor, lxor.
Local variable increment: iinc.
Comparison: dcmpg, dcmpl, fcmpg, fcmpl, lcmp.

The semantics of the Java programming language operators on integer and floating-point values (§2.4.2, §2.4.4) are directly supported by the semantics of the Java virtual machine instruction set.

The Java virtual machine does not indicate overflow during operations on integer data types. The only integer operations that can throw an exception are the integer divide instructions (idiv and ldiv) and the integer remainder instructions (irem and lrem), which throw an ArithmeticException if the divisor is zero.

Java virtual machine operations on floating-point numbers behave as specified in IEEE 754. In particular, the Java virtual machine requires full support of IEEE 754 denormalized floating-point numbers and gradual underflow, which make it easier to prove desirable properties of particular numerical algorithms.

The Java virtual machine requires that floating-point arithmetic behave as if every floating-point operator rounded its floating-point result to the result precision. Inexact results must be rounded to the representable value nearest to the infinitely precise result; if the two nearest representable values are equally near, the one having a least significant bit of zero is chosen. This is the IEEE 754 standard's default rounding mode, known as round to nearest mode.

The Java virtual machine uses the IEEE 754 round towards zero mode when converting a floating-point value to an integer. This results in the number being truncated; any bits of the significand that represent the fractional part of the operand value are discarded. Round towards zero mode chooses as its result the type's value closest to, but no greater in magnitude than, the infinitely precise result.

The Java virtual machine's floating-point operators do not throw runtime exceptions (not to be confused with IEEE 754 floating-point exceptions). An operation that overflows produces a signed infinity, an operation that underflows produces a denormalized value or a signed zero, and an operation that has no mathematically definite result produces NaN. All numeric operations with NaN as an operand produce NaN as a result.

Comparisons on values of type long (lcmp) perform a signed comparison. Comparisons on values of floating-point types (dcmpg, dcmpl, fcmpg, fcmpl) are performed using IEEE 754 nonsignaling comparisons.

3.11.4 Type Conversion Instructions

The type conversion instructions allow conversion between Java virtual machine numeric types. These may be used to implement explicit conversions in user code or to mitigate the lack of orthogonality in the instruction set of the Java virtual machine.

The Java virtual machine directly supports the following widening numeric conversions:

int to long, float, or double
long to float or double
float to double

The widening numeric conversion instructions are i2l, i2f, i2d, l2f, l2d, and f2d. The mnemonics for these opcodes are straightforward given the naming conventions for typed instructions and the punning use of 2 to mean "to." For instance, the i2d instruction converts an int value to a double. Widening numeric conversions do not lose information about the overall magnitude of a numeric value. Indeed, conversions widening from int to long and int to double do not lose any information at all; the numeric value is preserved exactly. Conversions widening from float to double that are FP-strict (§3.8.2) also preserve the numeric value exactly; however, such conversions that are not FP-strict may lose information about the overall magnitude of the converted value.

Conversion of an int or a long value to float, or of a long value to double, may lose precision, that is, may lose some of the least significant bits of the value; the resulting floating-point value is a correctly rounded version of the integer value, using IEEE 754 round to nearest mode.

A widening numeric conversion of an int to a long simply sign-extends the two's-complement representation of the int value to fill the wider format. A widening numeric conversion of a char to an integral type zero-extends the representation of the char value to fill the wider format.

Despite the fact that loss of precision may occur, widening numeric conversions never cause the Java virtual machine to throw a runtime exception (not to be confused with an IEEE 754 floating-point exception).

Note that widening numeric conversions do not exist from integral types byte, char, and short to type int. As noted in §3.11.1, values of type byte, char, and short are internally widened to type int, making these conversions implicit.

The Java virtual machine also directly supports the following narrowing numeric conversions:

int to byte, short, or char
long to int
float to int or long
double to int, long, or float

The narrowing numeric conversion instructions are i2b, i2c, i2s, l2i, f2i, f2l, d2i, d2l, and d2f. A narrowing numeric conversion can result in a value of different sign, a different order of magnitude, or both; it may thereby lose precision.

A narrowing numeric conversion of an int or long to an integral type T simply discards all but the N lowest-order bits, where N is the number of bits used to represent type T. This may cause the resulting value not to have the same sign as the input value.

In a narrowing numeric conversion of a floating-point value to an integral type T, where T is either int or long, the floating-point value is converted as follows:

If the floating-point value is NaN, the result of the conversion is an int or long 0.
Otherwise, if the floating-point value is not an infinity, the floating-point value is rounded to an integer value V using IEEE 754 round towards zero mode. There are two cases:
- If T is long and this integer value can be represented as a long, then the result is the long value V.
- If T is of type int and this integer value can be represented as an int, then the result is the int value V.
Otherwise:
- Either the value must be too small (a negative value of large magnitude or negative infinity), and the result is the smallest representable value of type int or long.
- Or the value must be too large (a positive value of large magnitude or positive infinity), and the result is the largest representable value of type int or long.

A narrowing numeric conversion from double to float behaves in accordance with IEEE 754. The result is correctly rounded using IEEE 754 round to nearest mode. A value too small to be represented as a float is converted to a positive or negative zero of type float; a value too large to be represented as a float is converted to a positive or negative infinity. A double NaN is always converted to a float NaN.

Despite the fact that overflow, underflow, or loss of precision may occur, narrowing conversions among numeric types never cause the Java virtual machine to throw a runtime exception (not to be confused with an IEEE 754 floating-point exception).

3.11.5 Object Creation and Manipulation

Although both class instances and arrays are objects, the Java virtual machine creates and manipulates class instances and arrays using distinct sets of instructions:

Create a new class instance: new.
Create a new array: newarray, anewarray, multianewarray.
Access fields of classes (static fields, known as class variables) and fields of class instances (non-static fields, known as instance variables): getfield, putfield, getstatic, putstatic.
Load an array component onto the operand stack: baload, caload, saload, iaload, laload, faload, daload, aaload.
Store a value from the operand stack as an array component: bastore, castore, sastore, iastore, lastore, fastore, dastore, aastore.
Get the length of array: arraylength.
Check properties of class instances or arrays: instanceof, checkcast.

3.11.6 Operand Stack Management Instructions

A number of instructions are provided for the direct manipulation of the operand stack: pop, pop2, dup, dup2, dup_x1, dup2_x1, dup_x2, dup2_x2, swap.

3.11.7 Control Transfer Instructions

The control transfer instructions conditionally or unconditionally cause the Java virtual machine to continue execution with an instruction other than the one following the control transfer instruction. They are:

Conditional branch: ifeq, iflt, ifle, ifne, ifgt, ifge, ifnull, ifnonnull, if_icmpeq, if_icmpne, if_icmplt, if_icmpgt, if_icmple, if_icmpge, if_acmpeq, if_acmpne.
Compound conditional branch: tableswitch, lookupswitch.
Unconditional branch: goto, goto_w, jsr, jsr_w, ret.

The Java virtual machine has distinct sets of instructions that conditionally branch on comparison with data of int and reference types. It also has distinct conditional branch instructions that test for the null reference and thus is not required to specify a concrete value for null (§3.4).

Conditional branches on comparisons between data of types boolean, byte, char, and short are performed using int comparison instructions (§3.11.1). A conditional branch on a comparison between data of types long, float, or double is initiated using an instruction that compares the data and produces an int result of the comparison (§3.11.3). A subsequent int comparison instruction tests this result and effects the conditional branch. Because of its emphasis on int comparisons, the Java virtual machine provides a rich complement of conditional branch instructions for type int.

All int conditional control transfer instructions perform signed comparisons.

3.11.8 Method Invocation and Return Instructions

The following four instructions invoke methods:

invokevirtual invokes an instance method of an object, dispatching on the (virtual) type of the object. This is the normal method dispatch in the Java programming language.
invokeinterface invokes a method that is implemented by an interface, searching the methods implemented by the particular runtime object to find the appropriate method.
invokespecial invokes an instance method requiring special handling, whether an instance initialization method (§3.9), a private method, or a superclass method.
invokestatic invokes a class (static) method in a named class.

The method return instructions, which are distinguished by return type, are ireturn (used to return values of type boolean, byte, char, short, or int), lreturn, freturn , dreturn, and areturn. In addition, the return instruction is used to return from methods declared to be void, instance initialization methods, and class or interface initialization methods.

3.11.9 Throwing Exceptions

An exception is thrown programmatically using the athrow instruction. Exceptions can also be thrown by various Java virtual machine instructions if they detect an abnormal condition.

3.11.10 Implementing `finally`

The implementation of the finally keyword uses the jsr, jsr_w, and ret instructions. See Section 4.9.6, "Exceptions and finally," and Section 7.13, "Compiling finally."

3.11.11 Synchronization

The Java virtual machine supports synchronization of both methods and sequences of instructions within a method using a single synchronization construct: the monitor .

Method-level synchronization is handled as part of method invocation and return (see Section 3.11.8, "Method Invocation and Return Instructions").

Synchronization of sequences of instructions is typically used to encode the synchronized blocks of the Java programming language. The Java virtual machine supplies the monitorenter and monitorexit instructions to support such constructs.

Proper implementation of synchronized blocks requires cooperation from a compiler targeting the Java virtual machine. The compiler must ensure that at any method invocation completion a monitorexit instruction will have been executed for each monitorenter instruction executed since the method invocation. This must be the case whether the method invocation completes normally (§3.6.4) or abruptly (§3.6.5).

The compiler enforces proper pairing of monitorenter and monitorexit instructions on abrupt method invocation completion by generating exception handlers (§3.10) that will match any exception and whose associated code executes the necessary monitorexit instructions (§7.14).