.SN 8
.VS 1 0
.BP
.S1 "ENVIRONMENT INTERACTIONS"
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.
.S2 "Program starting and stopping"
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
UNIX
.FS
UNIX is a Trademark of Bell Laboratories.
.FE
operating system to C programs.
These parameters are often called
.BW argc ,
.B argv
and
.BW envp .
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.
.N
The
.B argv
array contains
.B argc
strings, the first of which contains the program call name.
The other strings in the
.B argv
array are the program parameters.
.P
The
.B envp
array contains strings in the form "name=string", where 'name'
is the name of an environment variable and string its value.
The
.B envp
is terminated by a zero pointer.
.P
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.
.S2 "Input/Output and other monitor calls"
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.
.P
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.
.N 1
The names of the types used are:
.IS 4
.PS - 10
.PT int
an integer of wordsize
.PT int2
an integer whose size is the maximum of the wordsize and 2
bytes
.PT int4
an integer whose size is the maximum of the wordsize and 4
bytes
.PT intp
an integer with the size of a pointer
.PT uns2
an unsigned integer whose size is the maximum of the wordsize and 2
.PT unsp
an unsigned integer with the size of a pointer
.PT ptr
a pointer into data space
.PE 1
.IE 0
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.
.A
To execute a monitor call, proceed as follows:
.IS 2
.N 1
.PS a 4 "" )
.PT
Stack the parameters, in reverse order, last parameter first.
.PT
Push the monitor call number (iocode) onto the stack.
.PT
Execute the MON instruction.
.PE 1
.IE
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.
.N 1
List of monitor calls.
.DS B
.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
.DE 1
Codes 0, 11, 13, 17, 31, 32, 38, 39, 40, 45, 49, 50, 52,
55, 57, 58, 62, and 63 are
not used.
.P
All monitor calls, except fork and sigtrp
are the same as the UNIX version 7 system calls.
.P
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.
.P
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.
.VS