diff --git a/ueforth/common/boot.fs b/ueforth/common/boot.fs
index 008b415..5e677f1 100644
--- a/ueforth/common/boot.fs
+++ b/ueforth/common/boot.fs
@@ -52,7 +52,7 @@
 : ] -1 state ! ; immediate
 
 ( Quoting Words )
-: ' bl parse find ;
+: ' bl parse find dup 0= 'throw @ execute ;
 : ['] ' aliteral ; immediate
 : char bl parse drop c@ ;
 : [char] char aliteral ; immediate
@@ -110,8 +110,10 @@ rp@ constant rp0
 
 ( Exceptions )
 variable handler
-: catch   sp@ >r handler @ >r rp@ handler ! execute r> handler ! r> drop 0 ;
-: throw   handler @ rp! r> handler ! r> swap >r sp! drop r> ;
+: catch ( xt -- n )
+  sp@ >r handler @ >r rp@ handler ! execute r> handler ! r> drop 0 ;
+: throw ( n -- )
+  dup if handler @ rp! r> handler !  r> swap >r sp! drop r> else drop then ;
 ' throw 'throw !
 
 ( Values )
@@ -120,7 +122,7 @@ variable handler
                       else ' >body ! then ; immediate
 
 ( Deferred Words )
-: defer ( "name" -- ) create 0 , does> @ execute ;
+: defer ( "name" -- ) create 0 , does> @ dup 0= throw execute ;
 : is ( xt "name -- ) postpone to ; immediate
 
 ( Defer I/O to platform specific )
@@ -185,4 +187,3 @@ create input-buffer   input-limit allot
 : query   begin ['] eval-buffer catch
           if ." ERROR" cr then prompt refill drop again ;
 : ok   ." uEForth" cr prompt refill drop query ;
-
diff --git a/ueforth/common/core.h b/ueforth/common/core.h
index 4303a2a..6283194 100644
--- a/ueforth/common/core.h
+++ b/ueforth/common/core.h
@@ -111,6 +111,7 @@ static void ueforth(const char *src, cell_t src_len) {
   g_sys.last[-1] = 1;  // Make ; IMMEDIATE
   g_sys.DOLIT_XT = FIND("DOLIT");
   g_sys.DOEXIT_XT = FIND("EXIT");
+  g_sys.tthrow = FIND("DROP");
   ip = g_sys.heap;
   *g_sys.heap++ = FIND("EVAL1");
   *g_sys.heap++ = FIND("BRANCH");
diff --git a/ueforth/posix/posix.fs b/ueforth/posix/posix.fs
index a811698..17d39bf 100644
--- a/ueforth/posix/posix.fs
+++ b/ueforth/posix/posix.fs
@@ -5,24 +5,33 @@
 create calls
 ' call0 , ' call1 , ' call2 , ' call3 , ' call4 ,
 ' call5 , ' call6 , ' call7 , ' call8 , ' call9 ,
-: sofunc ( z n a "name" -- ) swap >r swap dlsym create , r> cells calls + @ ,
-                      does> dup @ swap cell+ @ execute ;
+: sofunc ( z n a "name" -- )
+   swap >r swap dlsym dup 0= throw create , r> cells calls + @ ,
+   does> dup @ swap cell+ @ execute ;
 : sysfunc ( z n "name" -- ) 0 sofunc ;
-: shared-library ( z "name" -- ) RTLD_NOW dlopen create , does> @ sofunc ;
+: shared-library ( z "name" -- )
+   RTLD_NOW dlopen 0= throw create , does> @ sofunc ;
 
 ( Major Syscalls )
+z" open" 3 sysfunc open
+z" creat" 2 sysfunc creat
 z" close" 1 sysfunc close
 z" read" 3 sysfunc read
 z" write" 3 sysfunc write
-z" open" 3 sysfunc open
-z" creat" 2 sysfunc creat
+z" lseek" 3 sysfunc lseek
 z" exit" 1 sysfunc sysexit
+z" fork" 0 sysfunc fork
 
 ( Default Pipes )
 0 constant stdin
 1 constant stdout
 2 constant stderr
 
+( Seek )
+0 constant SEEK_SET
+1 constant SEEK_CUR
+2 constant SEEK_END
+
 ( Terminal handling )
 : n. ( n -- ) base @ swap decimal <# #s #> type base ! ;
 : esc   27 emit ;