Syscalls & I/O

To do anything useful (print, exit), the simulated program needs to talk to the host.

ECALL is the bridge:

li a7, 64       # syscall number: write
li a0, 1        # fd: stdout
la a1, msg      # buffer
li a2, 14       # length
ecall

li a7, 93       # syscall: exit
li a0, 0        # exit code
ecall

Linux RISC-V syscalls (a7):

• 64: write(fd, buf, count)
• 63: read(fd, buf, count)
• 93: exit(status)
• 56: openat(...)
• 57: close(fd)
• 80: fstat(fd, statbuf)
• 214: brk(addr)

Simulator implements a subset:

For embedded targets:

• No OS. ECALL might trap to firmware.
• Memory-mapped I/O: writing to specific address sends byte to UART.

For toy debugging:

• EBREAK: pause + dump state.
• Custom syscall like print_int: easier than buffered write.

Memory-mapped UART (alternative to syscalls):

Now sb a0, 0x10000000(x0) prints a character.

Real systems combine both: syscalls for OS, MMIO for hardware control.