Registers

Ben Gruver edited this page Jun 19, 2016 · 4 revisions

Introduction

In dalvik's bytecode, registers are always 32 bits, and can hold any type of value. 2 registers are used to hold 64 bit types (Long and Double).

Specifying the number of registers in a method

There are two ways to specify how many registers are available in a method. the .registers directive specifies the total number of registers in the method, while the alternate .locals directive specifies the number of non-parameter registers in the method. The total number of registers would therefore include the registers needed to hold the method parameters.

How method parameters are passed into a method

When a method is invoked, the parameters to the method are placed into the last n registers. If a method has 2 arguments, and 5 registers (v0-v4), the arguments would be placed into the last 2 registers - v3 and v4.

The first parameter to a non-static methods is always the object that the method is being invoked on.

For example, let's say you are writing a non-static method LMyObject;->callMe(II)V. This method has 2 integer parameters, but it also has an implicit LMyObject; parameter before both integer parameters, so there are a total of 3 arguments to the method.

Let's say you specify that there are 5 registers in the method (v0-v4), with either the .registers 5 directive or the .locals 2 directive (i.e. 2 local registers + 3 parameter registers). When the method is invoked, the object that the method is being invoked on (i.e. the this reference) will be in v2, the first integer parameter will be in v3, and the second integer parameter will be in v4.

For static methods it's the same thing, except there isn't an implicit this argument.

Register names

There are two naming schemes for registers - the normal v naming scheme and the p naming scheme for parameter registers. The first register in the p naming scheme is the first parameter register in the method. So let's go back to the previous example of a method with 3 arguments and 5 total registers. The following table shows the normal v name for each register, followed by the p name for the parameter registers

Local Param
v0 the first local register
v1 the second local register
v2 p0 the first parameter register
v3 p1 the second parameter register
v4 p2 the third parameter register

You can reference parameter registers by either name - it makes no difference.

Motivation for introducing parameter registers

The p naming scheme was introduced as a practical matter, to solve a common annoyance when editing smali code.

Say you have an existing method with a number of parameters and you are adding some code to the method, and you discover that you need an extra register. You think "No big deal, I'll just increase the number of registers specified in the .registers directive!".

Unfortunately, it isn't quite that easy. Keep in mind that the method parameters are stored in the last registers in the method. If you increase the number of registers - you change which registers the method arguments get put into. So you would have to change the .registers directive and renumber every parameter register.

But if the p naming scheme was used to reference parameter registers throughout the method, you can easily change the number of registers in the method, without having to worry about renumbering any existing registers.

Note: by default baksmali will use the p naming scheme for parameter registers. If you want to disable this for some reason and force baksmali to always use the v naming scheme, you can use the -p/--no-parameter-registers option.

Long/Double values

As mentioned previously, long and double primitives (J and D respectively) are 64 bit values, and require 2 registers. This is important to keep in mind when you are referencing method arguments. For example, let's say you have a (non-static) method LMyObject;->MyMethod(IJZ)V. The parameters to the method are LMyObject;, int, long, bool. So this method would require 5 registers for all of its parameters.

Register Type
p0 this
p1 I
p2, p3 J
p4 Z

Also, when you are invoking the method later on, you do have to specify both registers for any double-wide arguments in the register list for the invoke-instruction.