AsirPad--a computer algebra system with a pen-based interface on PDA. In 2003, I developed AsirPad, a computer algebra system (CAS) on Linux PDA Zaurus [1]. It had a handwriting interface for mathematical expressions and could communicate with a CAS engine Risa/Asir through the OpenXM protocol. It was used to present a lecture on RSA cryptography at a junior high school. Ordinary calculator is not available because encryption and decryption in RSA use division of large numbers. The students learned how to encrypt/decrypt their messages through calculations by AsirPad. They could input mathematical expressions and calculate without any special training. This experience encouraged us to explore the possibility of mobile devices in math classroom. In 2010, we started a project to develop a Math e-Learning system Mathellan for pen-based mobile devices [2]. We are planning to rebuild AsirPad for a client of Mathellan. However, the mainstream of the current mobile devices is shifting from PDA to smartphones or tablet devices. Therefore, we needed a new development environment. AsirPad consists of two main components: a CAS engine and a handwriting interface. We used a cross-build environment by QEMU and chroot to make an executable binary of Risa/Asir for the Android platform. We can build C/C++ source code in this environment as if we are in a self -build environment. It means that we do not need to modify source code for cross-build. Furthermore, we adopted a cross-platform application framework Qt to build the handwriting interface of AsirPad. Qt can be used to build applications for various operating systems: Windows, MacOS X, Linux, Android and iOS. We can develop GUI for various mobile devices with the same source code by Qt.