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.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 2n 8n 16n 32n 48n
number 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,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 complmode: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.