Table 16 - MODIFY DATA POINTER
================================================================================
| Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|Byte | | | | | | | | |
|==============================================================================|
| 0 | Extended message (01h) |
|------------------------------------------------------------------------------|
| 1 | Extended message length (05h) |
|------------------------------------------------------------------------------|
| 2 | MODIFY DATA POINTER (00h) |
|------------------------------------------------------------------------------|
| 3 | (MSB) |
|---------- Argument ---|
| 6 | (LSB) |
================================================================================
The MODIFY DATA POINTER message (see table 16) is sent from the
target to the initiator and requests that the signed argument be
added (two's complement) to the value of the current data pointer.
