Segments

This submenu allows you to manipulate segments of the program:

Create a new segment

Action    name: CreateSegment

This command allows you to create a new segment.

If you select a range using the anchor, IDA will propose the start address and the end address of the selection as defaults for the segment bounds.

You need to specify at least:

    - the segment start address
    - the segment end address (excluded from the range)
    - the segment base

Click here to learn about addressing model used in IDA.

If "sparse storage" is set, IDA will use special sparse storage method for the segment. This method is recommended for huge segments. Later, it is possible to change the storage method of any region using set_storage_type IDC function.

If another segment already exists at the specified address, the existing segment is truncated and the new segment lasts from the specified start address to the next segment (or specified end address, whichever is lower). If the old and the new segments have the same base address, instructions/data will not be discarded by IDA. Otherwise, IDA will discard all instructions/data of the new segment.

An additional segment may be created by IDA to cover the range after the end of the new segment.

IDA address space concepts

nternally, IDA has 32-bit linear address space (IDA64 uses 64-bit address space). The internal addresses are called "linear addresses". The input program is loaded into this linear address space.

Please note that usually the linear addresses are not used in the program directly. During disassembling, we use so-called "virtual addresses", which are calculated by the following formula:

        VirtualAddress = LinearAddress - (SegmentBase << 4);

We see that the SegmentBase determines what addresses will be displayed on the screen. More than that, IDA allows to create several segments with the same virtual address in them. For this, you just need to create segments with correct segment base values.

Normally a SegmentBase is a 16bit quantity. To create a segment with base >= 0x10000, you need to use selectors. However, if you try to create a segment with a segment base >= 0x10000, IDA will automatically choose appropriately a free selector and setup for the new segment.

All SegmentBases are looked up in the selector table.

There are some address restrictions in IDA.

There is a range of addresses that are used for internal housekeeping. This range can be specified by the configuration variable PRIVRANGE (start address and size). It is not recommended to use these addresses for other purposes.

There is also one address which must never be used in the disassembly. It is the 'all ones' address, or -1. Internally, it is used as a BADADDR (bad address). No address or address range can include BADADDR.

Create segment - simple case (PC)

IBM PC case -----------

Suppose we need to create a segment occupying addresses F000:1000..F000:2000 Let's calculate linear addresses:

        start = (0xF000 << 4) + 0x1000 = 0xF1000
        end   = (0xF000 << 4) + 0x2000 = 0xF2000

The segment base must be selected so that the first offset in our segment will be 0x1000. Let's find it using the following equation:

        VirtualAddress = LinearAddress - (SegmentBase << 4);
        0x1000         = 0xF1000 - (base << 4);

After solving this equation, we see that the segment base is equal to 0xF000. (you see, this is really a very simple case :) )

Now, we can create a segment entering:

        segment start address:  0xF1000
        segment end address:    0xF2000
        segment base:           0xF000

Please note that the end address never belongs to the segment in IDA.

Create segment - simple case (Z80)

Z80 case
 --------

Suppose we need to create a segment occupying virtual addresses 8000-C000. Since we are free to place our segment anywhere in the linear address space, we choose the linear addresses at our convenience. Let's say we choose a linear address 0x20000:

        start = 0x20000
        end   = start + 0x4000 = 0x24000

The segment base must be selected so that the virtual address in our segment will be 0x8000. Let's find it using the following equation:

        VirtualAddress = LinearAddress - (SegmentBase << 4);
        0x8000         = 0x20000 - (base << 4);
        base << 4      = 0x20000 - 0x8000
        base << 4      = 0x18000
        base           = 0x1800

After solving this equation, we see that the segment base is equal to 0x1800.

Now we can create a segment entering:

        segment start address:  0x20000
        segment end address:    0x24000
        segment base:           0x1800

Please note that the end address never belongs to the segment in IDA.

Create segment - automatically chosen selector case

Suppose we need to create a segment occupying linear addresses 200000-200C00 and the virtual addresses must have be 0000..0C00. If we simply enter

        segment start address:  0x200000
        segment end address:    0x200C00
        segment base:           0x20000

Then IDA will notice that the segment base is too big and does not fit into 16bits. Because of this IDA will find a free selector (let's say it has found selector number 5), define it to point at paragraph 0x20000 and create a segment. After all this we will have:

        - a new selector is defined (5 -> 0x20000)
        - a new segment is created. Its attributes:
                start = 0x200000
                end   = 0x200C00
                base  = 5

The first virtual address in the segment will be 0:

        VirtualAddress = LinearAddress - (SelectorValue(SegmentBase) << 4)
                       = 0x200000      - (SelectorValue(5) << 4)
                       = 0x200000      - (0x20000 << 4)
                       = 0x200000      - 0x200000
                       = 0

Please note that the end address never belongs to the segment in IDA.

Create segment - user-defined selector case

If the previous example we saw how IDA allocates a selector automatically. You could make it yourself:

1. Create a selector. For this, open the selectors window and press Ins. Enter a selector number and its value.

2. Create a segment. Specify the selector number as the segment base.

Delete a segment

Action    name: KillSegment

This command allows you to delete a segment.

IDA will ask your the permission to disable the addresses occupied by the segment. If you allow this operation, all information about the segment will be deleted. In other words, IDA will discard the information about instructions or data, comments etc.

If you check the "disable addresses" checkbox, IDA will mark the addresses occupied by the segment as "nonexistent" in the program. You will lose *ALL* information, including byte values.

It is impossible to disassemble the content of addresses not located in any segment, therefore you must create a new segment if you want to resume the disassembly of that part of the code.

You can also edit (see below) an adjacent segment to expand it to those addresses.

IDA will ask your the permission to disable addresses occupied by the segment. If you give your permission, information about the segment will be deleted, otherwise IDA will discard information about instruction/data, comments etc, but retain byte values so that you will be able to create another segment afterwards.

To disassemble the addresses occupied by the segment, you need to create a new segment again (i.e. you cannot disassemble bytes without a segment). You can also expand another adjacent segment to these addresses.

Change segment attributes

Changing the segment class may change the segment type.

MOVE ADJACENT SEGMENTS: means that the previous and next segments will be shrunk or expanded to fill gaps between segments. Click here for more information.

DISABLE ADDRESSES: if set, when a segment is shrunk, all information about bytes going out of the segment will be completely removed.. Otherwise, IDA will discard information about instructions/data, comments etc, but will retain byte values so that another segment can be created later and it will use the existing byte values.

If IDA creates 2 segments where only one segment must exist, you may try the following sequence:

delete one segment. Choose one with bad segment base value. Do not disable addresses occupied by the segment being deleted.
change bounds of another segment. Note that the create segment command changes the boundaries of the overlapping segment automatically.

Segments with the 'debugger' attribute are the segments whose memory contents are not saved in the database. Usually, these segments are created by the debugger to reflect the current memory state of the program.

However, the user can modify this attribute.

If it is cleared, then the segment will permanently stay in the database after closing the debugger session. The database will reflect the state of the segment which was at the time when the status is changed.

If it is set, then the segment will become a temporary segment and will be deleted at the end of the debugging session.

The "debugger segment" checbkox is available only during debugging sessions.

The 'loader' segments are the segment created by the file loader. The segment having this attribute are considered to be part of the input file.

A segment with the 'debugger' attribute set and the 'loader' attribute not set is considered to be an ephemeral segment. Such segments are not analyzed automatically by IDA.

"Segment permissions" group box can be used to modify Segment access permissions (Read/Write/Execute)

Change Segment Name

Enter a new name for the segment. A segment name is up to 8 characters long. IDA does check if the length is ok. Try to give mnemonic names for the segments.

Segment Class Name

The segment class name identifies the segment with a class name (such as CODE, FAR_DATA, or STACK). The linker places segments with the same class name into a contiguous range of memory in the runtime memory map.

Changing the segment class changes only the segment definition on the screen. There are the following predefined segment class names:

        CODE    -       Pure code
        DATA    -       Pure data
        CONST   -       Pure data
        BSS     -       Uninitialized data
        STACK   -       Uninitialized data
        XTRN    -       Extern definitions segment

If you change segment class and the segment type is "Regular", then the segment type will be changed accordingly.

In order to set the segment type "Regular", you should change the segment class to "UNK".

Segment class names are never deleted. Once you define a segment class name, you cannot reuse it as a name of another object.

Change Segment Addressing

You can choose between 16-bit and 32-bit segment addressing.

IDA will delete all instructions and data in the segment if the segment address is changed.

Never do it if you are not sure. It may have irreversible consequences, all instructions/data will be converted to undefined bytes.

Change Segment Alignment

Alignment: select between abs,byte,word,dword,para,page

You can specify the segment alignment for the selected segment. By default, IDA assumes 'byte' alignment.

Changing the alignment changes only the segment definition on the screen. Nothing else will happen.

Change Segment Combination

Combination

A field that describes how the linker can combine the segment with other segments. Under MS-DOS, segments with the same name and class can be combined in two ways: they can be concatenated to form one logical segment, or they can be overlapped. In the latter case, they have either the same start address or the same end address, and they describe a common range in memory. Values for the field are: