All the info I can find on this chip is that it was a 24bit fixed-point DSP introduced in 1995 by TI Japan. It seems to have had a very short lifetime and TI don't seem to have any information online about. I'm interested in seeing datasheets or programming manuals if anyone has any material.
edit: not my question, but it seems TI support have never heard of the part
http://e2e.ti.com/support/microcontrollers/hercules/f/312/p/375000/1319391Further quote found online:
"We are sorry to inform you that TMS57070FFT DSP has been obsoleted
since 2000. This part was not released to the public and was only
available to OEM's a public data sheet was not available. Additionally
the part was only released in Japan and due to export restrictions the
part was not available in any other country. Since any support for this
device is non existing and we do not have any information on this device
in our system. I am sorry that we could not be of more help."