Recently the study and implementation of elliptic curve cryptosystems (ECC) have developed rapidly and its achievements have become a center of attraction. ECC has the advantage of high-speed processing in software even on restricted environments such as smart cards. In this paper, we concentrate on implementation of ECC over a field of prime characteristic on a 16-bit microcomputer M16C (10MHz). We report a practical software implementation of a cryptographic library which supports 160-bit elliptic curve DSA (ECDSA) signature generation, verification and SHA-1 on the processor. This library also includes general integer arithmetic routines for applicability to other cryptographic algorithms. We successfully implemented the library in 4Kbyte code/data size including SHA-1, and confirmed a speed of 150msec for generating an ECDSA signature and 630msec for verifying an ECDSA signature on M16C.