# Jasmin Instruction Syntax

Jonathan Meyer, July '96

## Introduction

This document shows the syntax and the types of parameters required by each Java VM instruction in Jasmin. It also shows brief illustrative examples.

See The Jasmin User Guide for a description of other aspects of the Jasmin syntax.

## 1) Local variable instructions

The following instructions use local variables:

        ret <var-num>
        aload <var-num>
        astore <var-num>
        dload <var-num>
        dstore <var-num>
        fload <var-num>
        fstore <var-num>
        iload <var-num>
        istore <var-num>
        lload <var-num>
        lstore <var-num>

for example:

        aload 1    ; push local variable 1 onto the stack
        ret 2      ; return to the address held in local variable 2

## 2) The bipush, sipush and iinc instructions

The bipush and sipush instructions take an integer as a parameter:

        bipush <int>
        sipush <int>

for example:

        bipush 100    ; push 100 onto the stack

The iinc instruction takes two integer parameters:

        iinc  <var-num> <amount>

for example:

        iinc 3 -10    ; subtract 10 from local variable 3

## 3) Branch instructions

The following instructions take a label as a parameter:

        goto  <label>
        goto_w  <label>
        if_acmpeq  <label>
        if_acmpne  <label>
        if_icmpeq  <label>
        if_icmpge  <label>
        if_icmpgt  <label>
        if_icmple  <label>
        if_icmplt  <label>
        if_icmpne  <label>
        ifeq  <label>
        ifge  <label>
        ifgt  <label>
        ifle  <label>
        iflt  <label>
        ifne  <label>
        ifnonnull  <label>
        ifnull  <label>
        jsr  <label>
        jsr_w  <label>

For example:

        Label1:
           goto Label1    ; jump to the code at Label1
                          ; (an infinite loop!)

## 4) Class and object operations

The following instructions take a class name as a parameter:

        anewarray  <class>
        checkcast  <class>
        instanceof <class>
        new        <class>

For example:

        new java/lang/String   ; create a new String object

## 5) Method invokation

The following instructions are used to invoke methods:

        invokenonvirtual  <method-spec>
        invokestatic      <method-spec>
        invokevirtual     <method-spec>

for example:

        ; invokes java.io.PrintStream.println(String);

        invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V

A method specification is formed of three parts: the characters before the last '/' form the class name. The characters between the last '/' and '(' are the method name. The rest of the string is the descriptor.

         foo/baz/Myclass/myMethod(Ljava/lang/String;)V
         ---------------         ---------------------
               |         --------         |
               |            |             |
             class        method       descriptor

A special case is invokeinterface, which takes a <method-spec> and an integer indicating how many arguments the method takes:

         invokeinterface <method-spec> <num-args>

for example:

         invokeinterface foo/Baz/myMethod(I)V 1

## 6) Field manipulation instructions

The four instructions getfield, getstatic, putfield and putstatic have the form:

        getfield  <field-spec> <descriptor>
        getstatic  <field-spec> <descriptor>
        putfield  <field-spec> <descriptor>
        putstatic  <field-spec> <descriptor>

for example:

        ; get java.lang.System.out, which is a PrintStream
        getstatic java/lang/System/out Ljava/io/PrintStream;

<field-spec> is composed of two parts, a classname and a fieldname. The classname is all of the characters in the <field-spec> up to the last '/' character, and the fieldname is the rest of the characters after the last '/'. For example:

          foo/baz/AnotherClass/anotherFunField
          -- class name ------ --field name --

<descriptor> is the Java type descriptor of the field. For example:

        Ljava/io/PrintStream;

## 7) The newarray instruction

The newarray instruction is followed by the type of the array,

        newarray  <array-type>

for example:

        newarray int
        newarray short
        newarray float
        etc.

## 8) The multianewarray instruction

The multianewarray instruction takes two parameters, the type descriptor for the array and the number of dimensions to allocate:

         multianewarray  <array-descriptor> <num-dimensions>

for example:

         multianewarray [[[I 2

## 9) The ldc and ldc_w instructions

The ldc and ldc_w instructions are followed by a constant:

         ldc  <constant>
         ldc_w  <constant>

<constant> is either an integer, a floating point number, or a quoted string. For example:

         ldc 1.2              ; push a float
         ldc 10               ; push an int
         ldc "Hello World"    ; push a String
         ldc_w 3.141592654    ; push PI as a double

## 10) The lookupswitch instruction

The lookupswitch instruction has the syntax:

        <lookupswitch> ::=
            lookupswitch
                <int1> : <label1>
                <int2> : <label2>
                ...
                default : <default-label>

For example:

        ; If the int on the stack is 3, jump to Label1.
        ; If it is 5, jump to Label2.
        ; Otherwise jump to DefaultLabel.

        lookupswitch
                3 : Label1
                5 : Label2
          default : DefaultLabel

        Label1:
           ... got 3

        Label2:
           ... got 5

        DefaultLabel:
           ... got something else

## 11) The tableswitch instruction

The tableswitch instruction has the syntax:

        <tableswitch> ::=
            tableswitch <low>
                <label1>
                <label2>
                ...
                default : <default-label>

For example:

      ; If the int on the stack is 0, jump to Label1.
      ; If it is 1, jump to Label2.
      ; Otherwise jump to DefaultLabel.

      tableswitch 0
            Label1
            Label2
         default : DefaultLabel

      Label1:
         ... got 0

      Label2:
         ... got 1

      DefaultLabel:
         ... got something else

## 12) No parameter

The following instructions (the majority) take no parameters:

  aaload aastore aconst_null aload_0 aload_1 aload_2 aload_3 areturn arraylength astore_0 astore_1 astore_2 astore_3 athrow baload bastore breakpoint caload castore d2f d2i d2l dadd daload dastore dcmpg dcmpl dconst_0 dconst_1 ddiv dload_0 dload_1 dload_2 dload_3 dmul dneg drem dreturn dstore_0 dstore_1 dstore_2 dstore_3 dsub dup dup2 dup2_x1 dup2_x2 dup_x1 dup_x2 f2d f2i f2l fadd faload fastore fcmpg fcmpl fconst_0 fconst_1 fconst_2 fdiv fload_0 fload_1 fload_2 fload_3 fmul fneg frem freturn fstore_0 fstore_1 fstore_2 fstore_3 fsub i2d i2f i2l iadd iaload iand iastore iconst_0 iconst_1 iconst_2 iconst_3 iconst_4 iconst_5 iconst_m1 idiv iload_0 iload_1 iload_2 iload_3 imul ineg int2byte int2char int2short ior irem ireturn ishl ishr istore_0 istore_1 istore_2 istore_3 isub iushr ixor l2d l2f l2i ladd laload land lastore lcmp lconst_0 lconst_1 ldiv lload_0 lload_1 lload_2 lload_3 lmul lneg lor lrem lreturn lshl lshr lstore_0 lstore_1 lstore_2 lstore_3 lsub lushr lxor monitorenter monitorexit nop pop pop2 return saload sastore swap

for example:

        pop         ; remove the top item from the stack
        iconst_1    ; push 1 onto the stack
        swap        ; swap the top two items on the stack

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