diff --git a/py/lexer.c b/py/lexer.c
index 8732d64..dab7515 100644
--- a/py/lexer.c
+++ b/py/lexer.c
@@ -50,25 +50,25 @@
 
     unichar chr0, chr1, chr2;   // current cached characters from source
 
-    uint line;                  // source line
-    uint column;                // source column
+    mp_uint_t line;             // source line
+    mp_uint_t column;           // source column
 
-    int emit_dent;              // non-zero when there are INDENT/DEDENT tokens to emit
-    int nested_bracket_level;   // >0 when there are nested brackets over multiple lines
+    mp_int_t emit_dent;             // non-zero when there are INDENT/DEDENT tokens to emit
+    mp_int_t nested_bracket_level;  // >0 when there are nested brackets over multiple lines
 
-    uint alloc_indent_level;
-    uint num_indent_level;
+    mp_uint_t alloc_indent_level;
+    mp_uint_t num_indent_level;
     uint16_t *indent_level;
 
     vstr_t vstr;
     mp_token_t tok_cur;
 };
 
-uint mp_optimise_value;
+mp_uint_t mp_optimise_value;
 
 // TODO replace with a call to a standard function
-bool str_strn_equal(const char *str, const char *strn, int len) {
-    uint i = 0;
+bool str_strn_equal(const char *str, const char *strn, mp_uint_t len) {
+    mp_uint_t i = 0;
 
     while (i < len && *str == *strn) {
         ++i;
@@ -81,7 +81,7 @@
 
 #ifdef MICROPY_DEBUG_PRINTERS
 void mp_token_show(const mp_token_t *tok) {
-    printf("(%d:%d) kind:%d str:%p len:%d", tok->src_line, tok->src_column, tok->kind, tok->str, tok->len);
+    printf("(" UINT_FMT ":" UINT_FMT ") kind:%u str:%p len:" UINT_FMT, tok->src_line, tok->src_column, tok->kind, tok->str, tok->len);
     if (tok->str != NULL && tok->len > 0) {
         const byte *i = (const byte *)tok->str;
         const byte *j = (const byte *)i + tok->len;
@@ -175,7 +175,7 @@
         return;
     }
 
-    int advance = 1;
+    mp_uint_t advance = 1;
 
     if (lex->chr0 == '\n') {
         // LF is a new line
@@ -210,7 +210,7 @@
     }
 }
 
-void indent_push(mp_lexer_t *lex, uint indent) {
+void indent_push(mp_lexer_t *lex, mp_uint_t indent) {
     if (lex->num_indent_level >= lex->alloc_indent_level) {
         // TODO use m_renew_maybe and somehow indicate an error if it fails... probably by using MP_TOKEN_MEMORY_ERROR
         lex->indent_level = m_renew(uint16_t, lex->indent_level, lex->alloc_indent_level, lex->alloc_indent_level + MICROPY_ALLOC_LEXEL_INDENT_INC);
@@ -219,7 +219,7 @@
     lex->indent_level[lex->num_indent_level++] = indent;
 }
 
-uint indent_top(mp_lexer_t *lex) {
+mp_uint_t indent_top(mp_lexer_t *lex) {
     return lex->indent_level[lex->num_indent_level - 1];
 }
 
@@ -308,9 +308,9 @@
     "__debug__",
 };
 
-STATIC int hex_digit(unichar c) {
+STATIC mp_uint_t hex_digit(unichar c) {
     // c is assumed to be hex digit
-    int n = c - '0';
+    mp_uint_t n = c - '0';
     if (n > 9) {
         n &= ~('a' - 'A');
         n -= ('A' - ('9' + 1));
@@ -320,8 +320,9 @@
 
 // This is called with CUR_CHAR() before first hex digit, and should return with
 // it pointing to last hex digit
-STATIC bool get_hex(mp_lexer_t *lex, int num_digits, uint *result) {
-    uint num = 0;
+// num_digits must be greater than zero
+STATIC bool get_hex(mp_lexer_t *lex, mp_uint_t num_digits, mp_uint_t *result) {
+    mp_uint_t num = 0;
     while (num_digits-- != 0) {
         next_char(lex);
         unichar c = CUR_CHAR(lex);
@@ -394,7 +395,7 @@
     } else if (had_physical_newline && lex->nested_bracket_level == 0) {
         tok->kind = MP_TOKEN_NEWLINE;
 
-        uint num_spaces = lex->column - 1;
+        mp_uint_t num_spaces = lex->column - 1;
         lex->emit_dent = 0;
         if (num_spaces == indent_top(lex)) {
         } else if (num_spaces > indent_top(lex)) {
@@ -463,7 +464,7 @@
         next_char(lex);
 
         // work out if it's a single or triple quoted literal
-        int num_quotes;
+        mp_uint_t num_quotes;
         if (is_char_and(lex, quote_char, quote_char)) {
             // triple quotes
             next_char(lex);
@@ -475,7 +476,7 @@
         }
 
         // parse the literal
-        int n_closing = 0;
+        mp_uint_t n_closing = 0;
         while (!is_end(lex) && (num_quotes > 1 || !is_char(lex, '\n')) && n_closing < num_quotes) {
             if (is_char(lex, quote_char)) {
                 n_closing += 1;
@@ -512,7 +513,7 @@
                                 // Otherwise fall through.
                             case 'x':
                             {
-                                uint num = 0;
+                                mp_uint_t num = 0;
                                 if (!get_hex(lex, (c == 'x' ? 2 : c == 'u' ? 4 : 8), &num)) {
                                     // TODO error message
                                     assert(0);
@@ -531,8 +532,8 @@
                             default:
                                 if (c >= '0' && c <= '7') {
                                     // Octal sequence, 1-3 chars
-                                    int digits = 3;
-                                    int num = c - '0';
+                                    mp_uint_t digits = 3;
+                                    mp_uint_t num = c - '0';
                                     while (is_following_odigit(lex) && --digits != 0) {
                                         next_char(lex);
                                         num = num * 8 + (CUR_CHAR(lex) - '0');
@@ -627,7 +628,7 @@
         // search for encoded delimiter or operator
 
         const char *t = tok_enc;
-        uint tok_enc_index = 0;
+        mp_uint_t tok_enc_index = 0;
         for (; *t != 0 && !is_char(lex, *t); t += 1) {
             if (*t == 'e' || *t == 'c') {
                 t += 1;
@@ -649,7 +650,7 @@
 
             // get the maximum characters for a valid token
             t += 1;
-            uint t_index = tok_enc_index;
+            mp_uint_t t_index = tok_enc_index;
             for (;;) {
                 for (; *t == 'e'; t += 1) {
                     t += 1;
@@ -712,8 +713,8 @@
         // the parser gives a syntax error on, eg, x.__debug__.  Otherwise, we
         // need to check for this special token in many places in the compiler.
         // TODO improve speed of these string comparisons
-        //for (int i = 0; tok_kw[i] != NULL; i++) {
-        for (int i = 0; i < MP_ARRAY_SIZE(tok_kw); i++) {
+        //for (mp_int_t i = 0; tok_kw[i] != NULL; i++) {
+        for (mp_int_t i = 0; i < MP_ARRAY_SIZE(tok_kw); i++) {
             if (str_strn_equal(tok_kw[i], tok->str, tok->len)) {
                 if (i == MP_ARRAY_SIZE(tok_kw) - 1) {
                     // tok_kw[MP_ARRAY_SIZE(tok_kw) - 1] == "__debug__"