377 lines
14 KiB
Plaintext
377 lines
14 KiB
Plaintext
.SN 8
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.VS 1 0
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.BP
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.S1 "ENVIRONMENT INTERACTIONS"
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EM programs can interact with their environment in three ways.
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Two, starting/stopping and monitor calls, are dealt with in this chapter.
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The remaining way to interact, interrupts, will be treated
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together with traps in chapter 9.
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.S2 "Program starting and stopping"
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EM user programs start with a call to a procedure called
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m_a_i_n.
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The assembler and backends look for the definition of a procedure
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with this name in their input.
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The call passes three parameters to the procedure.
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The parameters are similar to the parameters supplied by the
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UNIX
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.FS
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UNIX is a Trademark of Bell Laboratories.
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.FE
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operating system to C programs.
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These parameters are often called
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.BW argc ,
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.B argv
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and
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.BW envp .
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Argc is the parameter nearest to LB and is a wordsized integer.
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The other two are pointers to the first element of an array of
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string pointers.
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.N
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The
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.B argv
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array contains
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.B argc
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strings, the first of which contains the program call name.
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The other strings in the
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.B argv
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array are the program parameters.
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.P
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The
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.B envp
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array contains strings in the form "name=string", where 'name'
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is the name of an environment variable and string its value.
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The
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.B envp
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is terminated by a zero pointer.
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.P
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An EM user program stops if the program returns from the first
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invocation of m_a_i_n.
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The contents of the function return area are used to procure a
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wordsized program return code.
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EM programs also stop when traps and interrupts occur that are
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not caught and when the exit monitor call is executed.
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.S2 "Input/Output and other monitor calls"
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EM differs from most conventional machines in that it has high level i/o
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instructions.
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Typical instructions are OPEN FILE and READ FROM FILE instead
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of low level instructions such as setting and clearing
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bits in device registers.
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By providing such high level i/o primitives, the task of implementing
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EM on various non EM machines is made considerably easier.
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.P
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I/O is initiated by the MON instruction, which expects an iocode on top
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of the stack.
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Often there are also parameters which are pushed on the
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stack in reverse order, that is: last
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parameter first.
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Some i/o functions also provide results, which are returned on the stack.
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In the list of monitor calls we use several types of parameters and results,
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these types consist of integers and unsigneds of varying sizes, but never
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smaller than the wordsize, and the two pointer types.
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.N 1
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The names of the types used are:
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.IS 4
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.PS - 10
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.PT int
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an integer of wordsize
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.PT int2
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an integer whose size is the maximum of the wordsize and 2
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bytes
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.PT int4
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an integer whose size is the maximum of the wordsize and 4
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bytes
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.PT intp
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an integer with the size of a pointer
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.PT uns2
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an unsigned integer whose size is the maximum of the wordsize and 2
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.PT unsp
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an unsigned integer with the size of a pointer
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.PT ptr
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a pointer into data space
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.PE 1
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.IE 0
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The table below lists the i/o codes with their results and
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parameters.
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This list is similar to the system calls of the UNIX Version 7
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operating system.
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.BP
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.A
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To execute a monitor call, proceed as follows:
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.IS 2
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.N 1
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.PS a 4 "" )
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.PT
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Stack the parameters, in reverse order, last parameter first.
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.PT
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Push the monitor call number (iocode) onto the stack.
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.PT
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Execute the MON instruction.
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.PE 1
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.IE
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An error code is present on the top of the stack after
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execution of most monitor calls.
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If this error code is zero, the call performed the action
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requested and the results are available on top of the stack.
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Non-zero error codes indicate a failure, in this case no
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results are available and the error code has been pushed twice.
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This construction enables programs to test for failure with a
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single instruction (~TEQ or TNE~) and still find out the cause of
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the failure.
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The result name 'e' is reserved for the error code.
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.N 1
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List of monitor calls.
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.DS B
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number name parameters results function
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1 Exit status:int Terminate this process
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2 Fork e,flag,pid:int Spawn new process
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3 Read fildes:int;buf:ptr;nbytes:unsp
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e:int;rbytes:unsp Read from file
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4 Write fildes:int;buf:ptr;nbytes:unsp
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e:int;wbytes:unsp Write on a file
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5 Open string:ptr;flag:int
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e,fildes:int Open file for read and/or write
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6 Close fildes:int e:int Close a file
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7 Wait e:int;status,pid:int2
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Wait for child
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8 Creat string:ptr;mode:int
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e,fildes:int Create a new file
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9 Link string1,string2:ptr
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e:int Link to a file
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10 Unlink string:ptr e:int Remove directory entry
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12 Chdir string:ptr e:int Change default directory
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14 Mknod string:ptr;mode,addr:int2
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e:int Make a special file
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15 Chmod string:ptr;mode:int2
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e:int Change mode of file
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16 Chown string:ptr;owner,group:int2
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e:int Change owner/group of a file
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18 Stat string,statbuf:ptr
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e:int Get file status
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19 Lseek fildes:int;off:int4;whence:int
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e:int;oldoff:int4 Move read/write pointer
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20 Getpid pid:int2 Get process identification
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21 Mount special,string:ptr;rwflag:int
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e:int Mount file system
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22 Umount special:ptr e:int Unmount file system
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23 Setuid userid:int2 e:int Set user ID
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24 Getuid e_uid,r_uid:int2 Get user ID
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25 Stime time:int4 e:int Set time and date
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26 Ptrace request:int;pid:int2;addr:ptr;data:int
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e,value:int Process trace
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27 Alarm seconds:uns2 previous:uns2 Schedule signal
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28 Fstat fildes:int;statbuf:ptr
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e:int Get file status
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29 Pause Stop until signal
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30 Utime string,timep:ptr
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e:int Set file times
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33 Access string,mode:int e:int Determine file accessibility
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34 Nice incr:int Set program priority
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35 Ftime bufp:ptr e:int Get date and time
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36 Sync Update filesystem
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37 Kill pid:int2;sig:int
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e:int Send signal to a process
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41 Dup fildes,newfildes:int
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e,fildes:int Duplicate a file descriptor
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42 Pipe e,w_des,r_des:int Create a pipe
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43 Times buffer:ptr Get process times
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44 Profil buff:ptr;bufsiz,offset,scale:intp Execution time profile
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46 Setgid gid:int2 e:int Set group ID
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47 Getgid e_gid,r_gid:int Get group ID
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48 Sigtrp trapno,signo:int
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e,prevtrap:int See below
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51 Acct file:ptr e:int Turn accounting on or off
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53 Lock flag:int e:int Lock a process
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54 Ioctl fildes,request:int;argp:ptr
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e:int Control device
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56 Mpxcall cmd:int;vec:ptr e:int Multiplexed file handling
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59 Exece name,argv,envp:ptr
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e:int Execute a file
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60 Umask complmode:int2 oldmask:int2 Set file creation mode mask
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61 Chroot string:ptr e:int Change root directory
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.DE 1
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Codes 0, 11, 13, 17, 31, 32, 38, 39, 40, 45, 49, 50, 52,
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55, 57, 58, 62, and 63 are
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not used.
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.P
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All monitor calls, except fork and sigtrp
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are the same as the UNIX version 7 system calls.
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.P
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The sigtrp entry maps UNIX signals onto EM interrupts.
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Normally, trapno is in the range 0 to 252.
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In that case it requests that signal signo
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will cause trap trapno to occur.
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When given trap number -2, default signal handling is reset, and when given
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trap number -3, the signal is ignored.
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.P
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The flag returned by fork is 1 in the child process and 0 in
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the parent.
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The pid returned is the process-id of the other process.
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.BP
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.S1 "TRAPS AND INTERRUPTS"
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EM provides a means for the user program to catch all traps
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generated by the program itself, the hardware, or external conditions.
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This mechanism uses five instructions: LIM, SIM, SIG, TRP and RTT.
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This section of the manual may be omitted on the first reading since it
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presupposes knowledge of the EM instruction set.
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.P
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The action taken when a trap occures is determined by the value
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of an internal EM trap register.
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This register contains a pointer to a procedure.
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Initially the pointer used is zero and all traps halt the
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program with, hopefully, a useful message to the outside world.
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The SIG instruction can be used to alter the trap register,
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it pops a procedure pointer from the
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stack into the trap register.
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When a trap occurs after storing a nonzero value in the trap
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register, the procedure pointed to by the trap register
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is called with the trap number
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as the only parameter (see below).
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SIG returns the previous value of the trap register on the
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stack.
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Two consecutive SIGs are a no-op.
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When a trap occurs, the trap register is reset to its initial
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condition, to prevent recursive traps from hanging the machine up,
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e.g. stack overflow in the stack overflow handling procedure.
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.P
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The runtime systems for some languages need to ignore some EM
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traps.
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EM offers a feature called the ignore mask.
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It contains one bit for each of the lowest 16 trap numbers.
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The bits are numbered 0 to 15, with the least significant bit
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having number 0.
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If a certain bit is 1 the corresponding trap never
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occurs and processing simply continues.
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The actions performed by the offending instruction are
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described by the Pascal program in appendix A.
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.N
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If the bit is 0, traps are not ignored.
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The instructions LIM and SIM allow copying and replacement of
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the ignore mask.~
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.P
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The TRP instruction generates a trap, the trap number being found on the
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stack.
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This is, among other things,
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useful for library procedures and runtime systems.
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It can also be used by a low level trap procedure to pass the trap to a
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higher level one (see example below).
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.P
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The RTT instruction returns from the trap procedure and continues after the
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trap.
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In the list below all traps marked with an asterisk ('*') are
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considered to be fatal and it is explicitly undefined what happens if
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you try to restart after the trap.
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.P
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The way a trap procedure is called is completely compatible
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with normal calling conventions. The only way a trap procedure
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differs from normal procedures is the return. It has to use RTT instead
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of RET. This is necessary because the complete runtime status is saved on the
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stack before calling the procedure and all this status has to be reloaded.
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Error numbers are in the range 0 to 252.
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The trap numbers are divided into three categories:
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.IS 4
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.N 1
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.PS - 10
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.PT ~~0-~63
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EM machine errors, e.g. illegal instruction.
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.PS - 8
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.PT ~0-15
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maskable
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.PT 16-63
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not maskable
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.PE
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.PT ~64-127
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Reserved for use by compilers, run time systems, etc.
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.PT 128-252
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Available for user programs.
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.PE 1
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.IE
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EM machine errors are numbered as follows:
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.DS I 5
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.TS
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tab(@);
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n l l.
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0@EARRAY@Array bound error
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1@ERANGE@Range bound error
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2@ESET@Set bound error
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3@EIOVFL@Integer overflow
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4@EFOVFL@Floating overflow
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5@EFUNFL@Floating underflow
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6@EIDIVZ@Divide by 0
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7@EFDIVZ@Divide by 0.0
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8@EIUND@Undefined integer
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9@EFUND@Undefined float
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10@ECONV@Conversion error
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16*@ESTACK@Stack overflow
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17*@EHEAP@Heap overflow
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18*@EILLINS@Illegal instruction
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19*@EODDZ@Illegal size argument
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20*@ECASE@Case error
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21*@EMEMFLT@Addressing non existent memory
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22*@EBADPTR@Bad pointer used
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23*@EBADPC@Program counter out of range
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24@EBADLAE@Bad argument of LAE
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25@EBADMON@Bad monitor call
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26@EBADLIN@Argument of LIN too high
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27@EBADGTO@GTO descriptor error
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.TE
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.DE 0
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.P
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As an example,
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suppose a subprocedure has to be written to do a numeric
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calculation.
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When an overflow occurs the computation has to be stopped and
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the higher level procedure must be resumed.
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This can be programmed as follows using the mechanism described above:
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.DS B
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mes 2,2,2 ; set sizes
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ersave
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bss 2,0,0 ; Room to save previous value of trap procedure
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msave
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bss 2,0,0 ; Room to save previous value of trap mask
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pro calcule,0 ; entry point
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lxl 0 ; fill in non-local goto descriptor with LB
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ste jmpbuf+4
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lor 1 ; and SP
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ste jmpbuf+2
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lim ; get current ignore mask
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ste msave ; save it
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lim
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loc 16 ; bit for EFOVFL
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ior 2 ; set in mask
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sim ; ignore EFOVFL from now on
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lpi $catch ; load procedure identifier
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sig ; catch wil get all traps now
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ste ersave ; save previous trap procedure identifier
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; perform calculation now, possibly generating overflow
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1 ; label jumped to by catch procedure
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loe ersave ; get old trap procedure
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sig ; refer all following trap to old procedure
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asp 2 ; remove result of sig
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loe msave ; restore previous mask
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sim ; done now
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; load result of calculation
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ret 2 ; return result
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jmpbuf
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con *1,0,0
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end
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.DE 0
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.VS 1 1
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.DS
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Example of catch procedure
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pro catch,0 ; Local procedure that must catch the overflow trap
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lol 2 ; Load trap number
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loc 4 ; check for overflow
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bne *1 ; if other trap, call higher trap procedure
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gto jmpbuf ; return to procedure calcule
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1 ; other trap has occurred
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loe ersave ; previous trap procedure
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sig ; other procedure will get the traps now
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asp 2 ; remove the result of sig
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lol 2 ; stack trap number
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trp ; call other trap procedure
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rtt ; if other procedure returns, do the same
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end
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.DE
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