6. Tokens and Fields
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## 6. Tokens and Fields

### 6.1. Defining Tokens and Fields

A token is one of the byte-sized pieces that make up the machine code instructions being modeled. Instruction fields must be defined on top of them. A field is a logical range of bits within an instruction that can specify an opcode, or an operand etc. Together tokens and fields determine the basic interpretation of bits and how many bytes the instruction takes up. To define a token and the fields associated with it, we use the define token statement.

    define token tokenname ( integer )
      fieldname=(integer,integer) attributelist
      ...
    ;

The first part of the definition defines the name of a token and the number of bits it uses (this must be a multiple of 8). Following this there are one or more field declarations specifying the name of the field and the range of bits within the token making up the field. The size of a field does not need to be a multiple of 8. The range is inclusive where the least significant bit in the token is labeled 0. The endianess of the processor will effect this labeling when defining tokens that are bigger than 1 byte. After each field declaration, there can be zero or more of the following attribute keywords:

    signed
    hex
    dec

These attributes are defined in the next section. There can be any manner of repeats and overlaps in the fields so long as they all have different names.

### 6.2. Fields as Family Symbols

Fields are the most basic form of family symbol; they define a natural map from instruction bits to a specific symbol as follows. We take the set of bits within the instruction as given by the field’s defining range and treat them as an integer encoding. The resulting integer is both the display portion and the semantic meaning of the specific symbol. The display string is obtained by converting the integer into either a decimal or hexadecimal representation (see below), and the integer is treated as a constant varnode in any semantic action.

The attributes of the field affect the resulting specific symbol in obvious ways. The signed attribute determines whether the integer encoding should be treated as just an unsigned encoding or if a twos-complement encoding should be used to obtain a signed integer. The hex or dec attributes describe whether the integer should be displayed with a hexadecimal or decimal representation. The default is hexadecimal. [Currently the dec attribute is not supported]

### 6.3. Attaching Alternate Meanings to Fields

The default interpretation of a field is probably the most natural but of course processors interpret fields within an instruction in a wide variety of ways. The attach keyword is used to alter either the display or semantic meaning of fields into the most common (and basic) interpretations. More complex interpretations must be built up out of tables.

#### 6.3.1. Attaching Registers

Probably the most common processor interpretation of a field is as an encoding of a particular register. In SLEIGH this can be done with the attach variables statement:

    attach variables fieldlist registerlist;

A fieldlist can be a single field identifier or a space separated list of field identifiers surrounded by square brackets. A registerlist must be a square bracket surrounded and space separated list of register identifiers as created with define statements (see Section Section 4.4, “Naming Registers”). For each field in the fieldlist, instead of having the display and semantic meaning of an integer, the field becomes a look-up table for the given list of registers. The original integer interpretation is used as the index into the list starting at zero, so a specific instruction that has all the bits in the field equal to zero yields the first register (a specific varnode) from the list as the meaning of the field in the context of that instruction. Note that both the display and semantic meaning of the field are now taken from the new register.

A particular integer can remain unspecified by putting a ‘_’ character in the appropriate position of the register list or also if the length of the register list is less than the integer. A specific integer encoding of the field that is unspecified like this does not revert to the original semantic and display meaning. Instead this encoding is flagged as an invalid form of the instruction.

#### 6.3.2. Attaching Other Integers

Sometimes a processor interprets a field as an integer but not the integer given by the default interpretation. A different integer interpretation of the field can be specified with an attach values statement.

    attach values fieldlist integerlist;

The integerlist is surrounded by square brackets and is a space separated list of integers. In the same way that a new register interpretation is assigned to fields with an attach variables statement, the integers in the list are assigned to each field specified in the fieldlist. [Currently SLEIGH does not support unspecified positions in the list using a ‘_’]

#### 6.3.3. Attaching Names

It is possible to just modify the display characteristics of a field without changing the semantic meaning. The need for this is rare, but it is possible to treat a field as having influence on the display of the disassembly but having no influence on the semantics. Even if the bits of the field do have some semantic meaning, sometimes it is appropriate to define overlapping fields, one of which is defined to have no semantic meaning. The most convenient way to break down the required disassembly may not be the most convenient way to break down the semantics. It is also possible to have symbols with semantic meaning but no display meaning (see Section 7.4.5, “Invisible Operands”).

At any rate we can list the display interpretation of a field directly with an attach names statement.

    attach names fieldlist stringlist;

The stringlist is assigned to each of the fields in the same manner as the attach variables and attach values statements. A specific encoding of the field now displays as the string in the list at that integer position. Field values greater than the size of the list are interpreted as invalid encodings.

### 6.4. Context Variables

SLEIGH supports the concept of context variables. For the most part processor instructions can be unambiguously decoded by examining only the bits of the instruction encoding. But in some cases, decoding may depend on the state of processor. Typically, the processor will have some set of status flags that indicate what mode is being used to process instructions. In terms of SLEIGH, a context variable is a field which is defined on top of a register rather than the instruction encoding (token).

    define context contextreg
      fieldname=(integer,integer) attributelist
      ...
    ;

Context variables are defined with a define context statement. The keywords must be followed by the name of a defined register. The remaining part of the definition is nearly identical to the normal definition of fields. Each context variable defined on this register is listed in turn, specifying the name, the bit range, and any attributes. All the normal field attributes, signed, dec, and hex, can also be used for context variables.

Context variables introduce a new, dedicated, attribute: noflow. By default, globally setting a context variable affects instruction decoding from the point of the change, forward, following the flow of the instructions, but if the variable is labeled as noflow, any change is limited to a single instruction. (See Section 8.3.1, “Context Flow”)

Once the context variable is defined, in terms of the specification syntax, it can be treated as if it were just another field. See Section 8, “Using Context”, for a complete discussion of how to use context variables.

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