Merge pull request #120 from philbudne/handle-zero-word-write

a7out: handle zero word write; src/cmd/p0.s now part of op.s

src/cmd/p0.s

@@ -1,37 +0,0 @@
-"** 10-36-55.pdf page 1
-osc = 0640001
-omq = 0640002
-otq = 0642000
-cmq = 0640004
-lmq = 0652000
-ecla = 0641000
-
-i = 020000
-
-save = 1
-getuid = 2
-open = 3
-read = 4
-write = 5
-creat = 6
-seek = 7
-tell = 8
-close = 9
-link = 10
-unlink = 11
-setuid = 12
-rename = 13
-exit = 14
-time = 15
-intrp = 16
-chdir = 17
-chmode = 18
-chowner = 19
-sysloc = 21
-capt = 23
-rele = 24
-status = 25
-sleep = 26
-smes = 27
-rmes = 28
-fork = 29

tools/a7out

@@ -1124,7 +1124,7 @@ sub sys_write {
     my $count = $Mem[ $PC + 2 ];
     my $end   = ( $start + $count - 1 ) & MAXADDR;
     die("sys_write: bad start/end addresses $start $end\n")
-      if ( $end < $start );
+      if ( $count != 0 && $end < $start );
     printf( "write: %d words from %s to fd %d\n", $count, addr($start), $fd )
 					if ( ($debug) || ($singlestep) );