.bp .P1 "ENVIRONMENT INTERACTIONS" .PP EM programs can interact with their environment in three ways. Two, starting/stopping and monitor calls, are dealt with in this chapter. The remaining way to interact, interrupts, will be treated together with traps in chapter 9. .P2 "Program starting and stopping" .PP EM user programs start with a call to a procedure called _m_a_i_n. The assembler and backends look for the definition of a procedure with this name in their input. The call passes three parameters to the procedure. The parameters are similar to the parameters supplied by the .UX operating system to C programs. These parameters are often called \fBargc\fP, \fBargv\fP and \fBenvp\fP. Argc is the parameter nearest to LB and is a wordsized integer. The other two are pointers to the first element of an array of string pointers. The \fBargv\fP array contains \fBargc\fP strings, the first of which contains the program call name. The other strings in the \fBargv\fP array are the program parameters. .PP The \fBenvp\fP array contains strings in the form "name=string", where 'name' is the name of an environment variable and string its value. The \fBenvp\fP is terminated by a zero pointer. .PP An EM user program stops if the program returns from the first invocation of _m_a_i_n. The contents of the function return area are used to procure a wordsized program return code. EM programs also stop when traps and interrupts occur that are not caught and when the exit monitor call is executed. .P2 "Input/Output and other monitor calls" .PP EM differs from most conventional machines in that it has high level i/o instructions. Typical instructions are OPEN FILE and READ FROM FILE instead of low level instructions such as setting and clearing bits in device registers. By providing such high level i/o primitives, the task of implementing EM on various non EM machines is made considerably easier. .PP I/O is initiated by the MON instruction, which expects an iocode on top of the stack. Often there are also parameters which are pushed on the stack in reverse order, that is: last parameter first. Some i/o functions also provide results, which are returned on the stack. In the list of monitor calls we use several types of parameters and results, these types consist of integers and unsigneds of varying sizes, but never smaller than the wordsize, and the two pointer types. .LP The names of the types used are: .DS .TS tab(:); l l. int:an integer of wordsize int2:an integer whose size is the maximum of the wordsize and 2 bytes int4:an integer whose size is the maximum of the wordsize and 4 bytes intp:an integer with the size of a pointer uns2:an unsigned integer whose size is the maximum of the wordsize and 2 unsp:an unsigned integer with the size of a pointer ptr:a pointer into data space .TE .DE .LP The table below lists the i/o codes with their results and parameters. This list is similar to the system calls of the UNIX Version 7 operating system. .QQ To execute a monitor call, proceed as follows: .IP a) Stack the parameters, in reverse order, last parameter first. .IP b) Push the monitor call number (iocode) onto the stack. .IP c) Execute the MON instruction. .LP An error code is present on the top of the stack after execution of most monitor calls. If this error code is zero, the call performed the action requested and the results are available on top of the stack. Non-zero error codes indicate a failure, in this case no results are available and the error code has been pushed twice. This construction enables programs to test for failure with a single instruction (~TEQ or TNE~) and still find out the cause of the failure. The result name 'e' is reserved for the error code. .ne 5 .LP List of monitor calls. .LP .nf .na .ta 4n 13n 29n 52n nr name parameters results function 1 Exit status:int Terminate this process 2 Fork e,flag,pid:int Spawn new process 3 Read fildes:int;buf:ptr;nbytes:unsp e:int;rbytes:unsp Read from file 4 Write fildes:int;buf:ptr;nbytes:unsp e:int;wbytes:unsp Write on a file 5 Open string:ptr;flag:int e,fildes:int Open file for read and/or write 6 Close fildes:int e:int Close a file 7 Wait e:int;status,pid:int2 Wait for child 8 Creat string:ptr;mode:int e,fildes:int Create a new file 9 Link string1,string2:ptr e:int Link to a file 10 Unlink string:ptr e:int Remove directory entry 12 Chdir string:ptr e:int Change default directory 14 Mknod string:ptr;mode,addr:int2 e:int Make a special file 15 Chmod string:ptr;mode:int2 e:int Change mode of file 16 Chown string:ptr;owner,group:int2 e:int Change owner/group of a file 18 Stat string,statbuf:ptr e:int Get file status 19 Lseek fildes:int;off:int4;whence:int e:int;oldoff:int4 Move read/write pointer 20 Getpid pid:int2 Get process identification 21 Mount special,string:ptr;rwflag:int e:int Mount file system 22 Umount special:ptr e:int Unmount file system 23 Setuid userid:int2 e:int Set user ID 24 Getuid e_uid,r_uid:int2 Get user ID 25 Stime time:int4 e:int Set time and date 26 Ptrace request:int;pid:int2;addr:ptr;data:int e,value:int Process trace 27 Alarm seconds:uns2 previous:uns2 Schedule signal 28 Fstat fildes:int;statbuf:ptr e:int Get file status 29 Pause Stop until signal 30 Utime string,timep:ptr e:int Set file times 33 Access string:ptr;mode:int e:int Determine file accessibility 34 Nice incr:int Set program priority 35 Ftime bufp:ptr e:int Get date and time 36 Sync Update filesystem 37 Kill pid:int2;sig:int e:int Send signal to a process 41 Dup fildes,newfildes:int e,fildes:int Duplicate a file descriptor 42 Pipe e,w_des,r_des:int Create a pipe 43 Times buffer:ptr Get process times 44 Profil buff:ptr;bufsiz,offset,scale:intp Execution time profile 46 Setgid gid:int2 e:int Set group ID 47 Getgid e_gid,r_gid:int Get group ID 48 Sigtrp trapno,signo:int e,prevtrap:int See below 51 Acct file:ptr e:int Turn accounting on or off 53 Lock flag:int e:int Lock a process 54 Ioctl fildes,request:int;argp:ptr e:int Control device 56 Mpxcall cmd:int;vec:ptr e:int Multiplexed file handling 59 Exece name,argv,envp:ptr e:int Execute a file 60 Umask mask:int2 oldmask:int2 Set file creation mode mask 61 Chroot string:ptr e:int Change root directory .fi .ad .LP Codes 0, 11, 13, 17, 31, 32, 38, 39, 40, 45, 49, 50, 52, 55, 57, 58, 62, and 63 are not used. .PP All monitor calls, except fork and sigtrp are the same as the UNIX version 7 system calls. .PP The sigtrp entry maps UNIX signals onto EM interrupts. Normally, trapno is in the range 0 to 252. In that case it requests that signal signo will cause trap trapno to occur. When given trap number \-2, default signal handling is reset, and when given trap number \-3, the signal is ignored. .PP The flag returned by fork is 1 in the child process and 0 in the parent. The pid returned is the process-id of the other process.