Nezha : Using syscalls for IO

 It was back in July I first posted about my Nezha RISC-V SBC.  At the time I was pleased to have a solid, reliable Debian build and excited with the ability to write native RV assembly programs and run them  at the command line without the usual inconvenience of cross-compilation and loading.

I purchased Anthony Reis' (AR) beginners guide to RV assembly which provides a gentle learning curve for me.  It starts with a chapter on machine language which is interesting as it describes the format of some types of instructions ().  I am hoping not to do too much low level debugging but this info provides an insight into the RV design, in which instructions are standardised to facilitate their implementation on a variety of hardware platforms.

One of the problems with an instructional RV book is that people have a range of environments in which they write programs.  Many would use a QEMU RV VM, some would have a smaller RV machine to load executables onto and a lucky few like me have an RV linux SBCfor native compilation. As AR starts on the assembly language tutorials he provides his own program which functions as an RV-32 assembler and simulation environment.  This provides input output to the screen using his own bespoke functions.  For example two instructions sin and sout are provided to input / output a string to / from a program.  A lot of assembly programming deals with peripherals so this isn't very helpful (although it is the correct approach for a general purpose tutorial / textbook).

The first thing I need to do is to work out how to implement the same functionality in my linux / gcc environment so that I don't need to use AR's assembler / io functions.  Previously I was able to use Stephen Smith's hello world example which uses a linux system call (ecall) to output a string.  Instead of using ecall it is possible to to the call statement, so that you can use the C function name rather than a numeric syscall number.  RV registers a0, a1, a2, ... registers are used for the parameters.  I was very pleased with my first attempt, shown below, which calls printf to display a string containing an integer parameter.  I needed to link it with a linker option "-no-pie" to include the necessary library modules for terminal output.

Once we have this mechanism it is easy to provide simple macros with the same name and parameter as those used in Anthony Reis book.  Now my first program example, which adds two numbers together looks just like the one in the book.