Hypothesized Official Name: SignExtendHL
Syscall Address: 021D80h
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.
HL - 16-bit signed value
HL - 24-bit signed value