diff --git a/boot.c b/boot.c
index a7b16de..1b51e0e 100644
--- a/boot.c
+++ b/boot.c
@@ -521,16 +521,16 @@ const char* reg_name(int reg) {
     if (reg == 25) return "s9";
     if (reg == 26) return "s10";
     if (reg == 27) return "s11";
-    // overflow begin
     if (reg == 28) return "t3";
     if (reg == 29) return "t4";
     if (reg == 30) return "t5";
     if (reg == 31) return "t6";
+    // overflow begin
     return 0;
 }
 
 int is_overflow(int reg) {
-    return reg > REG_S11;
+    return reg > REG_T6;
 }
 
 void reset_local() {
@@ -1017,7 +1017,17 @@ int parse_function_call(int id) {
     for (int i = 0; i < arg; ++i) {
         asm_mv(i + REG_A0, args[i]);
     }
+    for (int i = REG_T3; i <= REG_T6; ++i) {
+        if (i < max_reg_id) {
+            asm_sd(reg_name(i), (REG_S2 - i) * 8 - 24, "fp");
+        }
+    }
     printf("  call %s\n", name);
+    for (int i = REG_T3; i <= REG_T6; ++i) {
+        if (i < max_reg_id) {
+            asm_ld(reg_name(i), (REG_S2 - i) * 8 - 24, "fp");
+        }
+    }
     int type = global_type[id];
     if (type != TYPE_VOID) {
         int rd = next_reg(type);
@@ -1355,7 +1365,7 @@ int parse_expr() {
 
 void parse_local_variable(int type) {
     if (type == TYPE_VOID) {
-        eprintf("local variable of void type is not supported\n");
+        eprintf("variable cannot be of void type\n");
         exit(1);
     }
     expect_token(TOKEN_ID);
@@ -1556,6 +1566,10 @@ void parse_function(const char* name) {
             }
             arg_type = arg_type | TYPE_PTR_MASK;
         }
+        if (arg >= 8) {
+            eprintf("too many arguments\n");
+            exit(1);
+        }
         args[arg++] = declare_local(token_data, arg_type);
         if (token_type == TOKEN_COMMA) {
             // continue;
@@ -1590,8 +1604,9 @@ void parse_function(const char* name) {
     }
     asm_j(epilog_label);
     int reg_used = max_reg_id - REG_S2;
-    if (reg_used > 10) reg_used = 10;
+    if (reg_used > 14) reg_used = 14;
     int frame_size = (max_local_id - 1 + reg_used + 2) * 8;
+    if (reg_used > 10) reg_used = 10;
     if (frame_size % 16 != 0) {
         frame_size = frame_size + 8;
     }
@@ -1623,7 +1638,7 @@ void parse_function(const char* name) {
 
 void parse_global_variable(int id, const char* name, int type) {
     if (type == TYPE_VOID) {
-        eprintf("global variable of void type is not supported\n");
+        eprintf("variable cannot be of void type\n");
         exit(1);
     }
     printf(".data\n");