# Bit Fields tutorial

Suppose the source text looked like this:

```
void out_operand(int opnum, int flags);

// 'flags' parameter is combination of the following bits:
// (don't use OOF_SIGNMASK and OOF_WIDTHMASK, they are for the kernel)

#define OOF_SIGNMASK    0x0003      // sign output:
#define   OOFS_IFSIGN   0x0000      //   output sign if needed
#define   OOFS_NOSIGN   0x0001      //   should not out sign     ()
#define   OOFS_NEEDSIGN 0x0002      //   always out sign         (+-)
#define OOF_SIGNED      0x0004      // output as signed if 


// This function output the first 2 operands of instruction
void out_operands(void)
{
  // the first operand is a signed value
  out_operand(0, OOFS_IFSIGN|OOF_SIGNED|OOFW_IMM);
  // the first operand is a unsigned 32bit address
  out_operand(1, OOFS_NOSIGN|OOF_ADDR|OOFW_32);
}
```

We have a disassembly like this:

![](https://1800237466-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FmbcivpLb9jyc0Sv4VOMC%2Fuploads%2Fgit-blob-1ebb0d68a992e8d62ccbf19de6e9ebd6f16d8752%2FBefore.gif?alt=media)

Let’s improve it by using bitfields. We first define a bitfield type by opening an enumeration window (menu View|Enumerations) where we press Ins to create a new object and make it a bitfield. The name given to the bitfield does not matter much. We press Ctrl-N to define the bitfield values.

![](https://1800237466-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FmbcivpLb9jyc0Sv4VOMC%2Fuploads%2Fgit-blob-4071251516c97743b44256008bd5529ef7708299%2FDefine1.gif?alt=media)

The first bitfield mask is 3 (or 2 bits). The name of the mask is not used by IDA, it is intended as a memory helper. Out of the 4 values this field can take, we only define the first value, zero, and assign a name to it : OOFS\_IFSIGN. If we want to define other values, within the fields limits, we just repeat the process. With some comments, the definition becomes

![](https://1800237466-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FmbcivpLb9jyc0Sv4VOMC%2Fuploads%2Fgit-blob-84ba6a39029fe718c64455031e2c7241f12751fb%2FDefined.gif?alt=media)

We switch to the disassembly window (or close the enumeration window with Alt-F3). Through the Edit|Operand types|Enum member menu we select the enum type we just defined and get this result…

![](https://1800237466-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FmbcivpLb9jyc0Sv4VOMC%2Fuploads%2Fgit-blob-88504af08b8b46bb193ea4ce5f8704539c822c8e%2Fafter-1.gif?alt=media)

That’s all folks !