diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 021296d..5c92e09 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3445,7 +3445,7 @@
     // Vi compatible: ":z!" uses display height, without a count uses
     // 'scroll'
     if (eap->forceit)
-	bigness = curwin->w_height;
+	bigness = Rows - 1;
     else if (!ONE_WINDOW)
 	bigness = curwin->w_height - 3;
     else
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 480bf3c..1ea7442 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1824,7 +1824,7 @@
 	EX_RANGE|EX_WHOLEFOLD|EX_REGSTR|EX_COUNT|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_LINES),
 EXCMD(CMD_z,		"z",		ex_z,
-	EX_RANGE|EX_WHOLEFOLD|EX_EXTRA|EX_FLAGS|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
+	EX_RANGE|EX_WHOLEFOLD|EX_BANG|EX_EXTRA|EX_FLAGS|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_LINES),
 
 // commands that don't start with a letter
diff --git a/src/testdir/test_ex_z.vim b/src/testdir/test_ex_z.vim
index fcb82b3..4607d96 100644
--- a/src/testdir/test_ex_z.vim
+++ b/src/testdir/test_ex_z.vim
@@ -16,8 +16,9 @@
   call assert_equal(23, line('.'))
 
   let a = execute('20z+3')
-  " FIXME: I would expect the same result as '20z3' but it
-  " gives "\n21\n22\n23" instead. Bug in Vim or in ":help :z"?
+  " FIXME: I would expect the same result as '20z3' since 'help z'
+  " says: Specifying no mark at all is the same as "+".
+  " However it " gives "\n21\n22\n23" instead. Bug in Vim or in ":help :z"?
   "call assert_equal("\n20\n21\n22", a)
   "call assert_equal(22, line('.'))
 
@@ -55,19 +56,48 @@
   call assert_equal(100, line('.'))
 
   let a = execute('20z-1000')
-  call assert_match("^\n1\n2\n.*\n19\n20$", a)
   call assert_equal(20, line('.'))
 
   let a = execute('20z=1000')
   call assert_match("^\n1\n.*\n-\\+\n20\n-\\\+\n.*\n100$", a)
   call assert_equal(20, line('.'))
 
+  " Tests with multiple windows.
+  5split
+  call setline(1, range(1, 100))
+  " Without a count, the number line is window height - 3.
+  let a = execute('20z')
+  call assert_equal("\n20\n21", a)
+  call assert_equal(21, line('.'))
+  " If window height - 3 is less than 1, it should be clamped to 1.
+  resize 2
+  let a = execute('20z')
+  call assert_equal("\n20", a)
+  call assert_equal(20, line('.'))
+
   call assert_fails('20z=a', 'E144:')
 
   set window& scroll&
   bw!
 endfunc
 
+" :z! is the same as :z but count uses the Vim window height when not specified.
+func Test_z_bang()
+  4split
+  call setline(1, range(1, 20))
+
+  let a = execute('10z!')
+  call assert_equal("\n10\n11\n12\n13\n14\n15\n16\n17\n18\n19\n20", a)
+
+  let a = execute('10z!#')
+  call assert_equal("\n 10 10\n 11 11\n 12 12\n 13 13\n 14 14\n 15 15\n 16 16\n 17 17\n 18 18\n 19 19\n 20 20", a)
+
+  let a = execute('10z!3')
+  call assert_equal("\n10\n11\n12", a)
+
+  %bwipe!
+endfunc
+
 func Test_z_overflow()
   " This used to access invalid memory as a result of an integer overflow
   " and freeze vim.
diff --git a/src/version.c b/src/version.c
index a9219db..7482d3f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3400,
+/**/
     3399,
 /**/
     3398,