Bitfields tutorial

In this tutorial, you will learn how to enhance disassembly output by using bitfields.

Suppose the source code looked like this:

// '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 


int m65_opflags(const op_t &x)
{
  switch ( x.type )
  {
    case o_displ:
      return OOF_ADDR|OOFS_NOSIGN|OOFW_16;
    case o_near:
    case o_mem:
      return OOF_ADDR|OOF_NUMBER|OOFS_NOSIGN|OOFW_16|OOF_ZSTROFF;
    default:
      return 0;
  }
}

We have a disassembly that looks like this:

Let’s improve it by using bitfields.

1. We first define a bitfield type by going to the Local types window (menu Open subviews -> Local types). We press Ins to add a new enum and make it a bitfield. The name given to the bitfield does not matter much.

Note that Bitmask has been checked. Click OK.

1. Then we edit the enum and update it using the Declaration tab as shown in the screenshot below.

Click OK.

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. The enum definition becomes:

1. We finally switch to the disassembly window. Through the Edit -> Operand types -> Enum member menu (or by pressing M on the second operand at addresses 0x130003E39 and 0x130003E40) we select the enum type we just defined and get this result…

That’s all folks!