riscemu/docs/syscalls.md

Syscalls

Performing a syscall is quite simple:

    ; set syscall code:
    addi a7, zero, 93   ; or SCALL_EXIT if syscall symbols are mapped
    ; set syscall args:
    addi a0, zero, 1    ; exit with code 1
    ; invode syscall handler
    scall

The global symbols (e.g. SCALL_READ) are loaded by default. If you specify the option no_syscall_symbols, they will be omitted.

Read (63) SCALL_READ

• a0: source file descriptor
• a1: addr at which to write the input
• a2: number of bytes to read (at most)
• return in a0: number of bytes read or -1

Write (64) SCALL_WRITE

• a0: target file descriptor
• a1: addr at which the data to be written is located
• a2: number of bytes to write
• return in a0: number of bytes written or -1

Exit (93) SCALL_EXIT

• a0: exit code

Open (1024) SCALL_OPEN

• a0: open mode:
  • 0: read
  • 1: write (truncate)
  • 2: read/write (no truncate)
  • 3: only create
  • 4: append
• a1: addr where path is stored
• a2: length of path
• return in a0: file descriptor of opened file or -1

Requires flag --scall-fs to be set to True

Close (1025) SCALL_CLOSE

• a0: file descriptor to close
• return in a0: 0 if closed correctly or -1

Extending these syscalls

You can implement your own syscall by adding its code to the SYSCALLS dict in the riscemu/syscalls.py file, creating a mapping of a syscall code to a name, and then implementing that syscall name in the SyscallInterface class further down that same file. Each syscall method should have the same signature: read(self, scall: Syscall). The Syscall object gives you access to the cpu, through which you can access registers and memory. You can look at the read or write syscalls for further examples.