4. Basic Definitions
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## 4. Basic Definitions

SLEIGH files must start with all the definitions needed by the rest of the specification. All definition statements start with the keyword define and end with a semicolon ‘;’.

### 4.1. Endianness Definition

The first definition in any SLEIGH specification must be for endianness. Either

    define endian=big;            OR
    define endian=little;

This defines how the processor interprets contiguous sequences of bytes as integers or other values and globally affects values across all address spaces. It also affects how integer fields within an instruction are interpreted, (see Section 6.1, “Defining Tokens and Fields”), although it is possible to override this setting in the rare case that endianness is different for data versus instruction encoding. The specification designer generally only needs to worry about endianness when labeling instruction fields and when defining overlapping registers, otherwise the specification language hides endianness issues.

### 4.2. Alignment Definition

An alignment definition looks like

    define alignment=integer;

This specifies the byte alignment of instructions within their address space. It defaults to 1 or no alignment. When disassembling an instruction at a particular address, the disassembler checks the alignment of the address against this value and can opt to flag an unaligned instruction as an error.

### 4.3. Space Definitions

The definition of an address space looks like

    define space spacename attributes ;

The spacename is the name of the new space, and attributes looks like zero or more of the following lines:

    type=(ram_space|register_space)
    size=integer
    default
    wordsize=integer

The only required attribute is size which specifies the number of bytes needed to address any byte within the space, for example a 32-bit address space has size 4.

A space of type ram_space is defined as follows:

- It is read/write.
- It is part of the standard memory map of the processor.
- It is addressable in the sense that the processor may load and store from dynamic pointers into the space.

A space of type register_space is intended to model the processor’s general-purpose registers. In terms of accessing and manipulating data within the space, SLEIGH and p-code make no distinction between the type ram_space or the type register_space. But there are still some distinguishing properties of a space labeled with register_space.

- It is read/write.
- It is not part of the standard memory map of the processor.
- In terms of GHIDRA, there will not be separate windows for the space and references into the space will not be stored.
- Named symbols within the space will have Register objects associated with them in GHIDRA.
- It is not addressable. Data-flow analysis will assume that data within the space cannot be manipulated indirectly via pointer, so there is no pointer aliasing. Make sure this is true!

At least one space needs to be labeled with the default attribute. This should be the space that the processor accesses with its main address bus. In terms of the rest of the specification file, this sets the default space referred to by the ‘*’ operator (see Section 7.7.1.2, “The '*' Operator”). It also has meaning to GHIDRA.

The average 32-bit processor requires only the following two space definitions.

    define space ram type=ram_space size=4 default;
    define space register type=register_space size=4;

The wordsize attribute can be used to specify the size of the memory location referred to with a single address. If a space has wordsize two, then each address of the space refers to 16 bits of data, rather than 8 bits. If the space has size two, then there are still 216 different addresses, but since each address accesses two bytes, there are twice as many bytes, 217, in the space. If the wordsize attribute is not specified, the size of a memory location defaults to one byte (8 bits).

### 4.4. Naming Registers

The general purpose registers of the processors can be named with the following define syntax:

    define spacename offset=integer size=integer stringlist ;

A stringlist is either a single string or a white space separated list of strings in square brackets ‘[’ and ‘]’. A string of just “_” indicates a skip in the sequence for that definition. The offset corresponding to that position in the list of names will not have a varnode defined at it.

This defines specific varnodes within the indicated address space. Each name in the list is assigned to a varnode in turn starting at the indicated offset within the space. Each varnode occupies the indicated number of bytes in size. There is no restriction on size, and by reusing the same offset in different define statements, overlapping varnodes are allowed. This is most often used to give registers their standard names but could be used to label any semantic variable that might need to be accessed globally by the processor. Overlapping register sequences like the x86 EAX/AX/AL can be easily modeled with overlapping varnode definitions.

Here is a typical example of register definition:

    define register offset=0 size=4
        [EAX ECX EDX EBX ESP EBP ESI EDI ];
    define register offset=0 size=2
        [AX _ CX _ DX _ BX _ SP _ BP _ SI _ DI];
    define register offset=0 size=1
        [AL AH _ _ CL CH _ _ DL DH _ _ BL BH ];

### 4.5. Bit Range Registers

Many processors define registers that either consist of a single bit or otherwise don't use an integral number of bytes. A recurring example in many processors is the status register which is further subdivided into the overflow and result flags for the arithmetic instructions. These flags typically have labels like ZF for the zero flag or CF for the carry flag and can be considered logical registers contained within the status register. SLEIGH allows registers to be defined like this using the define bitrange statement, but there are some important caveats with its use. A bit register like this is problematic for the underlying p-code instructions that SLEIGH models because the smallest object they can manipulate directly is a byte. In order to manipulate single bits, p-code must use a combination of bitwise logical, extension, and truncation operations. So a register defined as a bit range is not really a varnode as described in Section 1.2, “Varnodes”, but is really just a signal to the SLEIGH compiler to fill in the proper operators to simulate the bit manipulation. Using this feature may greatly increase the complexity of the compiled specification with little indication within the specification file itself.

    define register offset=0x180 size=4 [ statusreg ];
    define bitrange zf=statusreg[10,1]
                    cf=statusreg[11,1]
                    sf=statusreg[12,1];

A bit range register must be defined on top of another normal register. In this example, statusreg is defined first as a 4 byte register, and the bit registers themselves are built by the following define bitrange statement. A single bit register definition consists of an identifier for the register, followed by ‘=’, then the name of the register containing the bits, and finally a pair of numbers in square brackets. The first number indicates the lowest significant bit in the containing register of the bit range, where bit 0 is the least significant bit. The second number indicates the number of bits in the new register. Multiple definitions can be included in a single define bitrange statement, and the command is finally terminated with a semicolon. In the example, three new registers are defined on top of statusreg, each made up of 1 bit. The new registers zf, cf, and sf represent the tenth, eleventh, and twelfth bit of statusreg respectively.

The syntax for defining a new bit register is consistent with the pseudo bit range operator, described in Section 7.7.1.5, “Bit Range Operator”, and the resulting symbol is really just a placeholder for this operator. Whenever SLEIGH sees this symbol it generates p-code precisely as if the designer had used the bit range operator instead. Section 7.7.1.5, “Bit Range Operator”, provides some additional details about how p-code is generated, which apply to the use of bit range registers.

If a defined bit range happens to fall on byte boundaries, the new symbol will in fact be a normal varnode, so the define bitrange statement can be used as an alternate syntax for defining overlapping registers.

### 4.6. User-Defined Operations

The specification designer can define new p-code operations using a define pcodeop statement. This statement automatically reserves an internal form for the new p-code operation and associates an identifier with it. This identifier can then be used in semantic expressions (see Section 7.7.1.8, “User-Defined Operations”). The following example defines a new p-code operation arctan.

    define pcodeop arctan;

This construction should be used sparingly. The definition does not specify how the new operation is supposed to actually manipulate data, and any analysis routines cannot know what the specification designer intended. The operation will be treated as a black box. It will hold its place in syntax trees, and the routines will understand how data flows into and out of it. But, no other analysis will be possible.

New operations should be defined only after considering the above points and the general philosophy of p-code. The designer should have a detailed description of the new operation in mind, even though this cannot be put in the specification. If it all possible, the operation should be atomic, with specific inputs and outputs, and with no side-effects. The most common use of a new operation is to encapsulate actions that are too esoteric or too complicated to implement.

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