I've added fork() to a7out after adding some comments to src/cmd/init.s.

There is a test program to demonstrate fork working.

src/cmd/init.s

@@ -2,46 +2,46 @@
 
    -1
    sys intrp
-   jms init1
-   jms init2
+   jms init1			" Fork the first child connected to ttyin/ttyout
+   jms init2			" Fork the second child connected to keyboard/display
 1:
-   sys rmes
+   sys rmes			" Wait for a child to exit
    sad pid1
-   jmp 1f
+     jmp 1f			" It was child 1, so jump to 1f and restart it
    sad pid2
-   jms init2
-   jmp 1
+     jms init2			" It was child 2, so restart it
+   jmp 1			" and loop back. XXX: weird use of 1: not 1b. I don't like it!
 1:
    jms init1
-   jmp 1
+   jmp 1			" Weird use of 1: not 1b. I don't like it!
 
 init1: 0
-   sys fork
+   sys fork			" Fork a child process
      jmp 1f
-   sys open; ttyin; 0
-   sys open; ttyout; 1
-   jmp login
+   sys open; ttyin; 0		" which opens the ttyin
+   sys open; ttyout; 1		" and ttyout files, and
+   jmp login			" waits for a user to log in
 1:
-   dac pid1
-   jmp init1 i
+   dac pid1			" Parent stores childs pid in pid1
+   jmp init1 i			" and returns
 
 init2: 0
-   sys fork
+   sys fork			" Fork a child process
      jmp 1f
-   sys open; keybd; 0
-   sys open; displ; 1
-   jmp login
+   sys open; keybd; 0		" which opens the keyboard
+   sys open; displ; 1		" and display files, and
+   jmp login			" waits for a user to log in
 1:
-   dac pid2
-   jmp init2 i
+   dac pid2			" Parent stores childs pid in pid2
+   jmp init2 i			" and returns
 
 login:
    -1
    sys intrp
-   sys open; password; 0
+   sys open; password; 0	" Open the passwd file
    lac d1
-   sys write; m1; m1s
-   jms rline
+   sys write; m1; m1s		" Write "\nlogin:" on the terminal
+   jms rline			" and read the user's username
    lac ebufp
    dac tal
 1:
@@ -67,8 +67,8 @@ login:
    tad 9
    dac 9
    lac d1
-   sys write; m3; m3s
-   jms rline
+   sys write; m3; m3s	" Write "password: " on the terminal
+   jms rline		" and read the user's password
    law ibuf-1
    dac 8
 2:
@@ -123,13 +123,13 @@ login:
    jmp 1b
 2:
    lac nchar
-   sys setuid
-   sys chdir; dd
+   sys setuid		" Set the user's user-id
+   sys chdir; dd	" and change into the "dd" directory
    sys chdir; dir
 
-   lac d2
+   lac d2		" Close file descriptor 2
    sys close
-   sys open; sh; 0
+   sys open; sh; 0	" Open the shell executable file (we get fd 2)
    sma
      jmp 1f
    sys link; system; sh; sh
@@ -140,25 +140,26 @@ login:
      jmp error
    sys unlink; sh
 1:
-   law 017700
-   dac 9
+   law 017700		" Copy the code at the boot label below
+   dac 9		" up to location 017700
    law boot-1
    dac 8
 1:
    lac 8 i
    dac 9 i
-   sza
+   sza			" Stop copying when we hit the 0 marker
      jmp 1b
-   jmp 017701
+   jmp 017701		" and then jump to the code
 
 boot:
    lac d2
-   lmq
-   sys read; 4096; 07700
-   lacq
+   lmq				" Save the fd into MQ
+   sys read; 4096; 07700	" Read 4,032 words into locations 4096 onwards
+				" leaving the top 64 words for this boot code.
+   lacq				" Get the fd back and close the file
    sys close	
-   jmp 4096
-   0
+   jmp 4096			" and jump to the beginning of that executable
+   0				" 0 marks the end of the code, used by the copy routine above
 
 rline: 0
    law ibuf-1

src/other/fork_test.s

@@ -0,0 +1,20 @@
+" Fork text code
+
+main:
+   sys fork
+   jmp parent
+
+child:
+   lac d1
+   sys write; childmsg;  3
+   sys exit
+
+parent:
+   lac d1
+   sys write; parentmsg; 4
+   sys exit
+
+d1: 1		" stdout fd
+
+parentmsg: <pa>; <re>; <nt>; 012000
+childmsg:  <ch>; <il>; <d 012

tools/a7out

@@ -533,9 +533,11 @@ sub cal {
         9  => \&sys_close,
         11 => \&sys_unlink,
         14 => \&sys_exit,
+        16 => \&sys_intrp,
         17 => \&sys_chdir,
         18 => \&sys_chmod,
         19 => \&sys_chown,
+        29 => \&sys_fork,
     );
 
     # Simulate the syscall. Each syscall updates the $PC
@@ -553,6 +555,26 @@ sub sys_exit {
     exit(0);
 }
 
+# Intrp system call
+sub sys_intrp {
+    # For now, do nothing
+    dprint("intrp system call\n");
+    $PC += 1;
+    return;
+}
+
+# Fork system call
+sub sys_fork {
+
+    # Fork and get the child's process-id back, or zero if we are the child
+    my $pid= fork();
+    $AC= $pid;
+
+    # The parent returns back to PC+1, the child returns to PC+2
+    $PC += ($pid) ? 1 : 2;
+    return;
+}
+
 # Close system call
 sub sys_close {