Dalvik VM Instruction Formats

Introduction and Overview

This document lists the instruction formats used by Dalvik bytecode and is meant to be used in conjunction with the bytecode reference document.

Bitwise descriptions

The first column in the format table lists the bitwise layout of the format. It consists of one or more space-separated "words" each of which describes a 16-bit code unit. Each character in a word represents four bits, read from high bits to low, with vertical bars ("|") interspersed to aid in reading. Uppercase letters in sequence from "A" are used to indicate fields within the format (which then get defined further by the syntax column). The term "op" is used to indicate the position of an eight-bit opcode within the format. A slashed zero ("Ø") is used to indicate that all bits must be zero in the indicated position.

For the most part, lettering proceeds from earlier code units to later code units, and low-order to high-order within a code unit. However, there are a few exceptions to this general rule, which are done in order to make the naming of similar-meaning parts be the same across different instruction formats. These cases are noted explicitly in the format descriptions.

For example, the format "B|A|op CCCC" indicates that the format consists of two 16-bit code units. The first word consists of the opcode in the low eight bits and a pair of four-bit values in the high eight bits; and the second word consists of a single 16-bit value.

Format IDs

The second column in the format table indicates the short identifier for the format, which is used in other documents and in code to identify the format.

Most format IDs consist of three characters, two digits followed by a letter. The first digit indicates the number of 16-bit code units in the format. The second digit indicates the maximum number of registers that the format contains (maximum, since some formats can accomodate a variable number of registers), with the special designation "r" indicating that a range of registers is encoded. The final letter semi-mnemonically indicates the type of any extra data encoded by the format. For example, format "21t" is of length two, contains one register reference, and additionally contains a branch target.

Suggested static linking formats have an additional "s" suffix, making them four characters total. Similarly, suggested "inline" linking formats have an additional "i" suffix. (In this context, inline linking is like static linking, except with more direct ties into a virtual machine's implementation.) Finally, a couple oddball suggested formats (e.g., "20bc") include two pieces of data which are both represented in its format ID.

The full list of typecode letters are as follows. Note that some forms have different sizes, depending on the format:

Mnemonic Bit Sizes Meaning
b 8 immediate signed byte
c 16, 32 constant pool index
f 16 interface constants (only used in statically linked formats)
h 16 immediate signed hat (high-order bits of a 32- or 64-bit value; low-order bits are all 0)
i 32 immediate signed int, or 32-bit float
l 64 immediate signed long, or 64-bit double
m 16 method constants (only used in statically linked formats)
n 4 immediate signed nibble
s 16 immediate signed short
t 8, 16, 32 branch target
x 0 no additional data

Syntax

The third column of the format table indicates the human-oriented syntax for instructions which use the indicated format. Each instruction starts with the named opcode and is optionally followed by one or more arguments, themselves separated with commas.

Wherever an argument refers to a field from the first column, the letter for that field is indicated in the syntax, repeated once for each four bits of the field. For example, an eight-bit field labeled "BB" in the first column would also be labeled "BB" in the syntax column.

Arguments which name a register have the form "vX". The prefix "v" was chosen instead of the more common "r" exactly to avoid conflicting with (non-virtual) architectures on which a Dalvik virtual machine might be implemented which themselves use the prefix "r" for their registers. (That is, this decision makes it possible to talk about both virtual and real registers together without the need for circumlocution.)

Arguments which indicate a literal value have the form "#+X". Some formats indicate literals that only have non-zero bits in their high-order bits; for these, the zeroes are represented explicitly in the syntax, even though they do not appear in the bitwise representation.

Arguments which indicate a relative instruction address offset have the form "+X".

Arguments which indicate a literal constant pool index have the form "kind@X", where "kind" indicates which constant pool is being referred to. Each opcode that uses such a format explicitly allows only one kind of constant; see the opcode reference to figure out the correspondence. The four kinds of constant pool are "string" (string pool index), "type" (type pool index), "field" (field pool index), and "meth" (method pool index).

Similar to the representation of constant pool indices, there are also suggested (optional) forms that indicate prelinked offsets or indices. There are two types of suggested prelinked value: vtable offsets (indicated as "vtaboff") and field offsets (indicated as "fieldoff").

In the cases where a format value isn't explictly part of the syntax but instead picks a variant, each variant is listed with the prefix "[X=N]" (e.g., "[A=2]") to indicate the correspondence.

The Formats

Format ID Syntax Notable Opcodes Covered
N/A 00x N/A pseudo-format used for unused opcodes; suggested for use as the nominal format for a breakpoint opcode
ØØ|op 10x op  
B|A|op 12x op vA, vB  
11n op vA, #+B  
AA|op 11x op vAA  
10t op +AA goto
ØØ|op AAAA 20t op +AAAA goto/16
AA|op BBBB 20bc op AA, kind@BBBB suggested format for statically determined verification errors; A is the type of error and B is an index into a type-appropriate table (e.g. method references for a no-such-method error)
AA|op BBBB 22x op vAA, vBBBB  
21t op vAA, +BBBB  
21s op vAA, #+BBBB  
21h op vAA, #+BBBB0000
op vAA, #+BBBB000000000000
 
21c op vAA, type@BBBB
op vAA, field@BBBB
op vAA, string@BBBB
check-cast
const-class
const-string
AA|op CC|BB 23x op vAA, vBB, vCC  
22b op vAA, vBB, #+CC  
B|A|op CCCC 22t op vA, vB, +CCCC  
22s op vA, vB, #+CCCC  
22c op vA, vB, type@CCCC
op vA, vB, field@CCCC
instance-of
22cs op vA, vB, fieldoff@CCCC suggested format for statically linked field access instructions of format 22c
ØØ|op AAAAlo AAAAhi 30t op +AAAAAAAA goto/32
ØØ|op AAAA BBBB 32x op vAAAA, vBBBB  
AA|op BBBBlo BBBBhi 31i op vAA, #+BBBBBBBB  
31t op vAA, +BBBBBBBB  
31c op vAA, string@BBBBBBBB const-string/jumbo
A|G|op BBBB F|E|D|C 35c [A=5] op {vC, vD, vE, vF, vG}, meth@BBBB
[A=5] op {vC, vD, vE, vF, vG}, type@BBBB
[A=4] op {vC, vD, vE, vF}, kind@BBBB
[A=3] op {vC, vD, vE}, kind@BBBB
[A=2] op {vC, vD}, kind@BBBB
[A=1] op {vC}, kind@BBBB
[A=0] op {}, kind@BBBB

The unusual choice in lettering here reflects a desire to make the count and the reference index have the same label as in format 3rc.

 
35ms [A=5] op {vC, vD, vE, vF, vG}, vtaboff@BBBB
[A=4] op {vC, vD, vE, vF}, vtaboff@BBBB
[A=3] op {vC, vD, vE}, vtaboff@BBBB
[A=2] op {vC, vD}, vtaboff@BBBB
[A=1] op {vC}, vtaboff@BBBB

The unusual choice in lettering here reflects a desire to make the count and the reference index have the same label as in format 3rms.

suggested format for statically linked invoke-virtual and invoke-super instructions of format 35c
35mi [A=5] op {vC, vD, vE, vF, vG}, inline@BBBB
[A=4] op {vC, vD, vE, vF}, inline@BBBB
[A=3] op {vC, vD, vE}, inline@BBBB
[A=2] op {vC, vD}, inline@BBBB
[A=1] op {vC}, inline@BBBB

The unusual choice in lettering here reflects a desire to make the count and the reference index have the same label as in format 3rmi.

suggested format for inline linked invoke-static and invoke-virtual instructions of format 35c
AA|op BBBB CCCC 3rc op {vCCCC .. vNNNN}, meth@BBBB
op {vCCCC .. vNNNN}, type@BBBB

where NNNN = CCCC+AA-1, that is A determines the count 0..255, and C determines the first register

 
3rms op {vCCCC .. vNNNN}, vtaboff@BBBB

where NNNN = CCCC+AA-1, that is A determines the count 0..255, and C determines the first register

suggested format for statically linked invoke-virtual and invoke-super instructions of format 3rc
3rmi op {vCCCC .. vNNNN}, inline@BBBB

where NNNN = CCCC+AA-1, that is A determines the count 0..255, and C determines the first register

suggested format for inline linked invoke-static and invoke-virtual instructions of format 3rc
AA|op BBBBlo BBBB BBBB BBBBhi 51l op vAA, #+BBBBBBBBBBBBBBBB const-wide