RISC-V General Purpose Registers

https://cmput229.github.io/229-labs-RISCV/RISC-V-Examples_Public/01-Basics/01c-System_Calls.html

"Two registers and a single instruction are involved in a system call in RISC-V. The idea is to store important pieces of information into registers a0and a7, and then run the instruction ecall to perform the system call. The register a7 specifies the type of system call. Each type of system call has a number associated with it. To specify the type of system call the assembly program simply puts the corresponding number into a7. If the call needs a value, this value must be put into a0. For instance, to print an integer value, the value of the integer must be put into a0. For a system call that allocates memory, the number of bytes that will be allocated is put into a0."