# Bitfields

There is a special kind of enums: bitfields. A bitfield is an enum divided into bit groups. When you define a new symbolic constant in a bitfield, you need to specify the group to which the constant will belong to. By default, IDA proposes groups containing one bit each. If a group is not defined yet, it is automatically created when the first constant in the group is defined. For example:

```
        name    CONST1
        value   0x1
        mask    0x1
```

will define a constant named CONST1 with value 1 and will create a group containing only one bit. Another example. Let's consider the following definitions:

```
 #define OOF_SIGNMASK    0x0003
 #define   OOFS_IFSIGN   0x0000
 #define   OOFS_NOSIGN   0x0001
 #define   OOFS_NEEDSIGN 0x0002
 #define OOF_SIGNED      0x0004
 #define OOF_NUMBER      0x0008
 #define OOF_WIDTHMASK   0x0030
 #define   OOFW_IMM      0x0000
 #define   OOFW_16       0x0010
 #define   OOFW_32       0x0020
 #define   OOFW_8        0x0030
 #define OOF_ADDR        0x0040
 #define OOF_OUTER       0x0080
 #define OOF_ZSTROFF     0x0100
```

How do we describe this?

```
   name           value    mask   maskname

   OOFS_IFSIGN   0x0000   0x0003 OOF_SIGNMASK
   OOFS_NOSIGN   0x0001   0x0003 OOF_SIGNMASK
   OOFS_NEEDSIGN 0x0002   0x0003 OOF_SIGNMASK
 OOF_SIGNED      0x0004   0x0004
 OOF_NUMBER      0x0008   0x0008
   OOFW_IMM      0x0000   0x0030 OOF_WIDTHMASK
   OOFW_16       0x0010   0x0030 OOF_WIDTHMASK
   OOFW_32       0x0020   0x0030 OOF_WIDTHMASK
   OOFW_8        0x0030   0x0030 OOF_WIDTHMASK
 OOF_ADDR        0x0040   0x0040
 OOF_OUTER       0x0080   0x0080
 OOF_ZSTROFF     0x0100   0x0100
```

If a mask consists of more than one bit, it can have a name and a comment. A mask name can be set when a constant with the mask is being defined. IDA will display the mask names in a different color.

In order to use a bitfield in the program, just convert an instruction operand to [enum](https://github.com/HexRaysSA/docs/blob/IDA-8.4/user-guide/operand-types/convert-operand-to-symbolic-constant-enum.md). IDA will display the operand like this:

```
        mov     ax, 70h
```

will be replaced by

```
        mov     ax, OOFS_IFSIGN or OOFW_8 or OOF_ADDR
```


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