Hypothesized Official Name: SignExtendDE
Syscall Address: 021D7Ch
This was originally a broken call, and still is in OS 5.0.0.0089 (and others, perchance?) But after sending in information, TI did an amazing job and fixed this call.
It is known to work in OS 5.1.0.0110.
DE - 16-bit signed value
DE - 24-bit signed value