IDA Bitfields

Bitfields

There is a special kind of enums:
bitfields. A bitfield is an enum where the 32bits are divided into 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.
IDA will display the operand like this:

mov ax, 70h

will be replaced by

mov ax, OOFS_IFSIGN or OOFW_8 or OOF_ADDR