/usr/src/include/minix/callnr.h

 This file contains the integer codes for each system call. 

 ...
 #define FSTAT  28
 #define PAUSE  29
 #define UTIME  30
 #define ACCESS 33
 ...

• Note that 31 and 32 are not defined.

• These numbers are not currently assigned for system routines by any server process.

• So 31 and 32 and are available for new system calls directed to any server.