long __svc_indirect_r7(0) SVC_write(unsigned, int fd, const char * buf, size_t count);
#define write(fd, buf, count) SVC_write(4, (fd), (buf), (count))
Calling:
write(fd, buf, count);
compiles to SVC #0 with r0 = fd, r1 = buf, r2 = count, and r7 = 4.