The Spartan-3 Family Configuration User Guide states that the Device DNA for the Spartan-3A family of devices can be read out via JTAG.
Can this be performed in iMPACT?
"readDna -p <position>" is the batch command that reads the Device DNA from the device.
This option requires the use of iMPACT 10.1 or later as algorithm changes were required in the 9.1 and 9.1 software.
This feature is also only supported on a blank FPGA. If the device is programmed and DONE has gone High, there are other design considerations which will need to be taken into account.
The software does not support this method and requires that the device be blank to function properly.
If a post-configuration read of the DNA via JTAG is required, please contact Technical Support.
Typically, post-configuration reading of the DNA is done from Fabric, and this might be the best option.
The read DNA function in iMPACT is only intended for prototyping and requires a blank device.