diff --git a/src/errors.h b/src/errors.h
index 0c61340..fc815ee 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3262,4 +3262,6 @@
 #ifdef FEAT_EVAL
 EXTERN char e_string_or_function_required_for_arrow_parens_expr[]
 	INIT(= N_("E1275: String or function required for ->(expr)"));
+EXTERN char e_illegal_map_mode_string_str[]
+	INIT(= N_("E1276: Illegal map mode string: '%s'"));
 #endif
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 4456f69..f065026 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -422,6 +422,21 @@
 }
 
 /*
+ * Check "type" is a string or a dict of 'any'
+ */
+    static int
+arg_string_or_dict_any(type_T *type, type_T *decl_type UNUSED, argcontext_T *context)
+{
+    if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING
+	    || type->tt_type == VAR_DICT)
+	return OK;
+    arg_type_mismatch(&t_string, type, context->arg_idx + 1);
+    return FAIL;
+}
+
+/*
  * Check "type" is a string or a blob
  */
     static int
@@ -998,8 +1013,8 @@
 static argcheck_T arg3_string_any_dict[] = {arg_string, NULL, arg_dict_any};
 static argcheck_T arg3_string_any_string[] = {arg_string, NULL, arg_string};
 static argcheck_T arg3_string_bool_bool[] = {arg_string, arg_bool, arg_bool};
-static argcheck_T arg3_string_bool_dict[] = {arg_string, arg_bool, arg_dict_any};
 static argcheck_T arg3_string_number_bool[] = {arg_string, arg_number, arg_bool};
+static argcheck_T arg3_string_or_dict_bool_dict[] = {arg_string_or_dict_any, arg_bool, arg_dict_any};
 static argcheck_T arg3_string_string_bool[] = {arg_string, arg_string, arg_bool};
 static argcheck_T arg3_string_string_dict[] = {arg_string, arg_string, arg_dict_any};
 static argcheck_T arg3_string_string_number[] = {arg_string, arg_string, arg_number};
@@ -2053,7 +2068,7 @@
 			ret_list_dict_any,  f_maplist},
     {"mapnew",		2, 2, FEARG_1,	    arg2_mapnew,
 			ret_first_cont,	    f_mapnew},
-    {"mapset",		3, 3, FEARG_1,	    arg3_string_bool_dict,
+    {"mapset",		1, 3, FEARG_1,	    arg3_string_or_dict_bool_dict,
 			ret_void,	    f_mapset},
     {"match",		2, 4, FEARG_1,	    arg24_match_func,
 			ret_any,	    f_match},
diff --git a/src/map.c b/src/map.c
index ad2e831..6ea73eb 100644
--- a/src/map.c
+++ b/src/map.c
@@ -2283,7 +2283,8 @@
 	mapblock_T  *mp,
 	dict_T	    *dict,
 	char_u	    *lhsrawalt,	    // may be NULL
-	int	    buffer_local)   // false if not buffer local mapping
+	int	    buffer_local,   // false if not buffer local mapping
+	int	    abbr)	    // true if abbreviation
 {
     char_u	    *lhs = str2special_save(mp->m_keys, TRUE);
     char_u	    *mapmode = map_mode_to_chars(mp->m_mode);
@@ -2307,6 +2308,7 @@
     dict_add_number(dict, "buffer", (long)buffer_local);
     dict_add_number(dict, "nowait", mp->m_nowait ? 1L : 0L);
     dict_add_string(dict, "mode", mapmode);
+    dict_add_number(dict, "abbr", abbr ? 1L : 0L);
 
     vim_free(mapmode);
 }
@@ -2381,7 +2383,8 @@
     }
     else if (rettv_dict_alloc(rettv) != FAIL && rhs != NULL)
 	mapblock2dict(mp, rettv->vval.v_dict,
-			  did_simplify ? keys_simplified : NULL, buffer_local);
+			  did_simplify ? keys_simplified : NULL,
+			  buffer_local, abbr);
 
     vim_free(keys_buf);
     vim_free(alt_keys_buf);
@@ -2448,7 +2451,8 @@
 		vim_free(lhs);
 
 		mapblock2dict(mp, d,
-				 did_simplify ? keys_buf : NULL, buffer_local);
+				 did_simplify ? keys_buf : NULL,
+				 buffer_local, abbr);
 		vim_free(keys_buf);
 	    }
 	}
@@ -2490,6 +2494,56 @@
 }
 
 /*
+ * Get the mapping mode from the mode string.
+ * It may contain multiple characters, eg "nox", or "!", or ' '
+ * Return 0 if there is an error.
+ */
+    static int
+get_map_mode_string(char_u *mode_string, int abbr)
+{
+    char_u	*p = mode_string;
+    int		mode = 0;
+    int		tmode;
+    int		modec;
+    const int	MASK_V = VISUAL + SELECTMODE;
+    const int	MASK_MAP = VISUAL + SELECTMODE + NORMAL + OP_PENDING;
+    const int	MASK_BANG = INSERT + CMDLINE;
+
+    if (*p == NUL)
+	p = (char_u *)" ";	// compatibility
+    while ((modec = *p++))
+    {
+	switch (modec)
+	{
+	    case 'i': tmode = INSERT;		break;
+	    case 'l': tmode = LANGMAP;		break;
+	    case 'c': tmode = CMDLINE;		break;
+	    case 'n': tmode = NORMAL;		break;
+	    case 'x': tmode = VISUAL;		break;
+	    case 's': tmode = SELECTMODE;	break;
+	    case 'o': tmode = OP_PENDING;	break;
+	    case 't': tmode = TERMINAL;		break;
+	    case 'v': tmode = MASK_V;		break;
+	    case '!': tmode = MASK_BANG;	break;
+	    case ' ': tmode = MASK_MAP;		break;
+	    default:
+		      return 0; // error, unknown mode character
+	}
+	mode |= tmode;
+    }
+    if ((abbr && (mode & ~MASK_BANG) != 0)
+	|| (!abbr && (mode & (mode-1)) != 0 // more than one bit set
+	    && (
+		// false if multiple bits set in mode and mode is fully
+		// contained in one mask
+		!(((mode & MASK_BANG) != 0 && (mode & ~MASK_BANG) == 0)
+		    || ((mode & MASK_MAP) != 0 && (mode & ~MASK_MAP) == 0)))))
+	return 0;
+
+    return mode;
+}
+
+/*
  * "mapset()" function
  */
     void
@@ -2518,25 +2572,51 @@
     mapblock_T  **abbr_table = &first_abbr;
     int		nowait;
     char_u	*arg;
+    int		dict_only;
 
+    // If first arg is a dict, then that's the only arg permitted.
+    dict_only = argvars[0].v_type == VAR_DICT;
     if (in_vim9script()
-	    && (check_for_string_arg(argvars, 0) == FAIL
-		|| check_for_bool_arg(argvars, 1) == FAIL
-		|| check_for_dict_arg(argvars, 2) == FAIL))
+	    && (check_for_string_or_dict_arg(argvars, 0) == FAIL
+		|| (dict_only && check_for_unknown_arg(argvars, 1) == FAIL)
+		|| (!dict_only
+		    && (check_for_string_arg(argvars, 0) == FAIL
+			|| check_for_bool_arg(argvars, 1) == FAIL
+			|| check_for_dict_arg(argvars, 2) == FAIL))))
 	return;
 
-    which = tv_get_string_buf_chk(&argvars[0], buf);
-    if (which == NULL)
-	return;
-    mode = get_map_mode(&which, 0);
-    is_abbr = (int)tv_get_bool(&argvars[1]);
-
-    if (argvars[2].v_type != VAR_DICT)
+    if (dict_only)
     {
-	emsg(_(e_key_not_present_in_dictionary));
+	d = argvars[0].vval.v_dict;
+	which = dict_get_string(d, (char_u *)"mode", FALSE);
+	is_abbr = dict_get_bool(d, (char_u *)"abbr", -1);
+	if (which == NULL || is_abbr < 0)
+	{
+	    emsg(_(e_entries_missing_in_mapset_dict_argument));
+	    return;
+	}
+    }
+    else
+    {
+	which = tv_get_string_buf_chk(&argvars[0], buf);
+	if (which == NULL)
+	    return;
+	is_abbr = (int)tv_get_bool(&argvars[1]);
+
+	if (argvars[2].v_type != VAR_DICT)
+	{
+	    emsg(_(e_dictionary_required));
+	    return;
+	}
+	d = argvars[2].vval.v_dict;
+    }
+    mode = get_map_mode_string(which, is_abbr);
+    if (mode == 0)
+    {
+	semsg(_(e_illegal_map_mode_string_str), which);
 	return;
     }
-    d = argvars[2].vval.v_dict;
+
 
     // Get the values in the same order as above in get_maparg().
     lhs = dict_get_string(d, (char_u *)"lhs", FALSE);
diff --git a/src/proto/typval.pro b/src/proto/typval.pro
index b744bbc..459cf7c 100644
--- a/src/proto/typval.pro
+++ b/src/proto/typval.pro
@@ -9,6 +9,7 @@
 varnumber_T tv_get_bool(typval_T *varp);
 varnumber_T tv_get_bool_chk(typval_T *varp, int *denote);
 float_T tv_get_float(typval_T *varp);
+int check_for_unknown_arg(typval_T *args, int idx);
 int check_for_string_arg(typval_T *args, int idx);
 int check_for_nonempty_string_arg(typval_T *args, int idx);
 int check_for_opt_string_arg(typval_T *args, int idx);
diff --git a/src/testdir/test_map_functions.vim b/src/testdir/test_map_functions.vim
index 66f3479..21c8990 100644
--- a/src/testdir/test_map_functions.vim
+++ b/src/testdir/test_map_functions.vim
@@ -19,13 +19,13 @@
         \ 'lhsraw': "foo\x80\xfc\x04V", 'lhsrawalt': "foo\x16",
         \ 'mode': ' ', 'nowait': 0, 'expr': 0, 'sid': sid, 'scriptversion': 1,
         \ 'lnum': lnum + 1, 
-	\ 'rhs': 'is<F4>foo', 'buffer': 0},
+	\ 'rhs': 'is<F4>foo', 'buffer': 0, 'abbr': 0},
 	\ maparg('foo<C-V>', '', 0, 1))
   call assert_equal({'silent': 1, 'noremap': 1, 'script': 1, 'lhs': 'bar',
         \ 'lhsraw': 'bar', 'mode': 'v',
         \ 'nowait': 0, 'expr': 1, 'sid': sid, 'scriptversion': 1,
         \ 'lnum': lnum + 2,
-	\ 'rhs': 'isbar', 'buffer': 1},
+	\ 'rhs': 'isbar', 'buffer': 1, 'abbr': 0},
         \ 'bar'->maparg('', 0, 1))
   let lnum = expand('<sflnum>')
   map <buffer> <nowait> foo bar
@@ -33,7 +33,7 @@
         \ 'lhsraw': 'foo', 'mode': ' ',
         \ 'nowait': 1, 'expr': 0, 'sid': sid, 'scriptversion': 1,
         \ 'lnum': lnum + 1, 'rhs': 'bar',
-	\ 'buffer': 1},
+	\ 'buffer': 1, 'abbr': 0},
         \ maparg('foo', '', 0, 1))
   let lnum = expand('<sflnum>')
   tmap baz foo
@@ -41,8 +41,17 @@
         \ 'lhsraw': 'baz', 'mode': 't',
         \ 'nowait': 0, 'expr': 0, 'sid': sid, 'scriptversion': 1,
         \ 'lnum': lnum + 1, 'rhs': 'foo',
-	\ 'buffer': 0},
+	\ 'buffer': 0, 'abbr': 0},
         \ maparg('baz', 't', 0, 1))
+  let lnum = expand('<sflnum>')
+  iab A B
+  call assert_equal({'silent': 0, 'noremap': 0, 'script': 0, 'lhs': 'A',
+        \ 'lhsraw': 'A', 'mode': 'i',
+        \ 'nowait': 0, 'expr': 0, 'sid': sid, 'scriptversion': 1,
+        \ 'lnum': lnum + 1, 'rhs': 'B',
+	\ 'buffer': 0, 'abbr': 1},
+        \ maparg('A', 'i', 1, 1))
+  iuna A
 
   map abc x<char-114>x
   call assert_equal("xrx", maparg('abc'))
@@ -250,10 +259,156 @@
   bwipe!
 
   call assert_fails('call mapset([], v:false, {})', 'E730:')
-  call assert_fails('call mapset("i", 0, "")', 'E716:')
+  call assert_fails('call mapset("i", 0, "")', 'E715:')
   call assert_fails('call mapset("i", 0, {})', 'E460:')
 endfunc
 
+def Test_mapset_arg1_dir()
+  # This test is mostly about get_map_mode_string.
+  # Once the code gets past that, it's common with the 3 arg mapset.
+
+  # GetModes() return list of modes for 'XZ' lhs using maplist.
+  # There is one list item per mapping
+  def GetModes(abbr: bool = false): list<string>
+    return maplist(abbr)->filter((_, m) => m.lhs == 'XZ')
+                ->mapnew((_, m) => m.mode)
+  enddef
+
+  const unmap_cmds = [ 'unmap', 'unmap!', 'tunmap', 'lunmap' ]
+  def UnmapAll(lhs: string)
+    for cmd in unmap_cmds
+      try | execute(cmd .. ' ' .. lhs) | catch /E31/ | endtry
+    endfor
+  enddef
+
+  var tmap: dict<any>
+
+  # some mapset(mode, abbr, dict) tests using get_map_mode_str
+  map XZ x
+  tmap = maplist()->filter((_, m) => m.lhs == 'XZ')[0]->copy()
+  # this splits the mapping into 2 mappings
+  mapset('ox', false, tmap)
+  assert_equal(2, len(GetModes()))
+  mapset('o', false, tmap)
+  assert_equal(3, len(GetModes()))
+  # test that '' acts like ' ', and that the 3 mappings become 1
+  mapset('', false, tmap)
+  assert_equal([' '], GetModes())
+  # dict's mode/abbr are ignored
+  UnmapAll('XZ')
+  tmap.mode = '!'
+  tmap.abbr = true
+  mapset('o', false, tmap)
+  assert_equal(['o'], GetModes())
+
+  # test the 3 arg version handles bad mode string, dict not used
+  assert_fails("mapset('vi', false, {})", 'E1276:')
+
+
+  # get the abbreviations out of the way
+  abbreviate XZ ZX
+  tmap = maplist(true)->filter((_, m) => m.lhs == 'XZ')[0]->copy()
+
+  abclear
+  # 'ic' is the default ab command, shows up as '!'
+  tmap.mode = 'ic'
+  mapset(tmap)
+  assert_equal(['!'], GetModes(true))
+
+  abclear
+  tmap.mode = 'i'
+  mapset(tmap)
+  assert_equal(['i'], GetModes(true))
+
+  abclear
+  tmap.mode = 'c'
+  mapset(tmap)
+  assert_equal(['c'], GetModes(true))
+
+  abclear
+  tmap.mode = '!'
+  mapset(tmap)
+  assert_equal(['!'], GetModes(true))
+
+  assert_fails("mapset({mode: ' !', abbr: 1})", 'E1276:')
+  assert_fails("mapset({mode: 'cl', abbr: 1})", 'E1276:')
+  assert_fails("mapset({mode: 'in', abbr: 1})", 'E1276:')
+
+  # the map commands
+  map XZ x
+  tmap = maplist()->filter((_, m) => m.lhs == 'XZ')[0]->copy()
+
+  # try the combos
+  UnmapAll('XZ')
+  # 'nxso' is ' ', the unadorned :map
+  tmap.mode = 'nxso'
+  mapset(tmap)
+  assert_equal([' '], GetModes())
+
+  UnmapAll('XZ')
+  # 'ic' is '!'
+  tmap.mode = 'ic'
+  mapset(tmap)
+  assert_equal(['!'], GetModes())
+
+  UnmapAll('XZ')
+  # 'xs' is really 'v'
+  tmap.mode = 'xs'
+  mapset(tmap)
+  assert_equal(['v'], GetModes())
+
+  # try the individual modes
+  UnmapAll('XZ')
+  tmap.mode = 'n'
+  mapset(tmap)
+  assert_equal(['n'], GetModes())
+
+  UnmapAll('XZ')
+  tmap.mode = 'x'
+  mapset(tmap)
+  assert_equal(['x'], GetModes())
+
+  UnmapAll('XZ')
+  tmap.mode = 's'
+  mapset(tmap)
+  assert_equal(['s'], GetModes())
+
+  UnmapAll('XZ')
+  tmap.mode = 'o'
+  mapset(tmap)
+  assert_equal(['o'], GetModes())
+
+  UnmapAll('XZ')
+  tmap.mode = 'i'
+  mapset(tmap)
+  assert_equal(['i'], GetModes())
+
+  UnmapAll('XZ')
+  tmap.mode = 'c'
+  mapset(tmap)
+  assert_equal(['c'], GetModes())
+
+  UnmapAll('XZ')
+  tmap.mode = 't'
+  mapset(tmap)
+  assert_equal(['t'], GetModes())
+
+  UnmapAll('XZ')
+  tmap.mode = 'l'
+  mapset(tmap)
+  assert_equal(['l'], GetModes())
+
+  UnmapAll('XZ')
+
+  # get errors for modes that can't be in one mapping
+  assert_fails("mapset({mode: 'nxsoi', abbr: 0})", 'E1276:')
+  assert_fails("mapset({mode: ' !', abbr: 0})", 'E1276:')
+  assert_fails("mapset({mode: 'ix', abbr: 0})", 'E1276:')
+  assert_fails("mapset({mode: 'tl', abbr: 0})", 'E1276:')
+  assert_fails("mapset({mode: ' l', abbr: 0})", 'E1276:')
+  assert_fails("mapset({mode: ' t', abbr: 0})", 'E1276:')
+enddef
+
 func Check_ctrlb_map(d, check_alt)
   call assert_equal('<C-B>', a:d.lhs)
   if a:check_alt
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index e15993d..7b64fc4 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -2463,7 +2463,8 @@
         sid: SID(),
         scriptversion: 999999,
         rhs: 'bar',
-        buffer: 0})
+        buffer: 0,
+        abbr: 0})
   unmap foo
   v9.CheckDefAndScriptFailure(['maparg(1)'], ['E1013: Argument 1: type mismatch, expected string but got number', 'E1174: String required for argument 1'])
   v9.CheckDefAndScriptFailure(['maparg("a", 2)'], ['E1013: Argument 2: type mismatch, expected string but got number', 'E1174: String required for argument 2'])
@@ -2545,7 +2546,7 @@
 enddef
 
 def Test_mapset()
-  v9.CheckDefAndScriptFailure(['mapset(1, true, {})'], ['E1013: Argument 1: type mismatch, expected string but got number', 'E1174: String required for argument 1'])
+  v9.CheckDefAndScriptFailure(['mapset(1, true, {})'], ['E1013: Argument 1: type mismatch, expected string but got number', 'E1223: String or Dictionary required for argument 1'])
   v9.CheckDefAndScriptFailure(['mapset("a", 2, {})'], ['E1013: Argument 2: type mismatch, expected bool but got number', 'E1212: Bool required for argument 2'])
   v9.CheckDefAndScriptFailure(['mapset("a", false, [])'], ['E1013: Argument 3: type mismatch, expected dict<any> but got list<unknown>', 'E1206: Dictionary required for argument 3'])
 enddef
diff --git a/src/typval.c b/src/typval.c
index 4f6e41d..b131ba2 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -360,6 +360,20 @@
 #endif
 
 /*
+ * Give an error and return FAIL unless "args[idx]" is unknown
+ */
+    int
+check_for_unknown_arg(typval_T *args, int idx)
+{
+    if (args[idx].v_type != VAR_UNKNOWN)
+    {
+	semsg(_(e_too_many_arguments), idx + 1);
+	return FAIL;
+    }
+    return OK;
+}
+
+/*
  * Give an error and return FAIL unless "args[idx]" is a string.
  */
     int
diff --git a/src/version.c b/src/version.c
index c3ecb62..51db458 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4861,
+/**/
     4860,
 /**/
     4859,