diff --git a/lib/chibi/io-test.sld b/lib/chibi/io-test.sld
index 7ef8ec74..08572f46 100644
--- a/lib/chibi/io-test.sld
+++ b/lib/chibi/io-test.sld
@@ -134,6 +134,20 @@
             (read-string 4096 in)
             (read-line in)))
 
+      (let ((bv (string->utf8 "日本語")))
+        (test #\日 (utf8-ref bv 0))
+        (test #\本 (utf8-ref bv 3))
+        (test #\語 (utf8-ref bv 6))
+        (test 3 (utf8-next bv 0 9))
+        (test 6 (utf8-next bv 3 9))
+        (test 9 (utf8-next bv 6 9))
+        (test #f (utf8-next bv 9 9))
+        (test 6 (utf8-prev bv 9 0))
+        (test 3 (utf8-prev bv 6 0))
+        (test 0 (utf8-prev bv 3 0))
+        (test #f (utf8-prev bv 0 0))
+        )
+
       (test #u8(0 1 2)
         (let ((in (bytevectors->input-port (list #u8(0 1 2)))))
           (read-bytevector 3 in)))
diff --git a/lib/chibi/io.sld b/lib/chibi/io.sld
index f056258a..b756588a 100644
--- a/lib/chibi/io.sld
+++ b/lib/chibi/io.sld
@@ -14,7 +14,8 @@
           make-filtered-output-port make-filtered-input-port
           string-count-chars
           open-input-bytevector open-output-bytevector get-output-bytevector
-          string->utf8 utf8->string
+          string->utf8 string->utf8! string-offset utf8->string utf8->string!
+          utf8-ref utf8-next utf8-prev
           write-string write-u8 read-u8 peek-u8 send-file
           is-a-socket?
           call-with-input-file call-with-output-file)
diff --git a/lib/chibi/io/io.scm b/lib/chibi/io/io.scm
index 49112657..1e5542db 100644
--- a/lib/chibi/io/io.scm
+++ b/lib/chibi/io/io.scm
@@ -9,25 +9,10 @@
   (call-with-input-string " "
     (lambda (in) (read-char in) (read-char in))))
 
-;; Copy whole characters from the given cursor positions.
-;; Return the src cursor position of the next unwritten char,
-;; which may be before `to' if the char would overflow.
-;; Now provided as a primitive from (chibi ast).
-;; (define (string-cursor-copy! dst start src from to)
-;;   (let lp ((i from)
-;;            (j (string-cursor->index dst start)))
-;;     (let ((i2 (string-cursor-next src i)))
-;;       (cond ((> i2 to) i)
-;;             (else
-;;              (string-set! dst j (string-cursor-ref src i))
-;;              (lp i2 (+ j 1)))))))
-
 (define (utf8->string vec . o)
-  (if (pair? o)
-      (let ((start (car o))
-            (end (if (pair? (cdr o)) (cadr o) (bytevector-length vec))))
-        (utf8->string (subbytes vec start end)))
-      (string-copy (utf8->string! vec))))
+  (let ((start (if (pair? o) (car o) 0))
+        (end (if (and (pair? o) (pair? (cdr o))) (cadr o) (bytevector-length vec))))
+    (string-copy (utf8->string! vec start end))))
 
 (define (string->utf8 str . o)
   (if (pair? o)
diff --git a/lib/chibi/io/io.stub b/lib/chibi/io/io.stub
index c0c331b6..593ab9b8 100644
--- a/lib/chibi/io/io.stub
+++ b/lib/chibi/io/io.stub
@@ -50,8 +50,19 @@
   ((value ctx sexp) (value self sexp) sexp sexp sexp (default NULL sexp)))
 (define-c sexp (%string->utf8 "sexp_string_to_utf8")
   ((value ctx sexp) (value self sexp) sexp))
-(define-c sexp (utf8->string! "sexp_utf8_to_string_x")
+(define-c sexp (string->utf8! "sexp_string_to_utf8_x")
   ((value ctx sexp) (value self sexp) sexp))
+(define-c sexp (string-offset "sexp_string_offset_op")
+  ((value ctx sexp) (value self sexp) sexp))
+(define-c sexp (utf8->string! "sexp_utf8_to_string_x")
+  ((value ctx sexp) (value self sexp) sexp sexp sexp))
+
+(define-c sexp (utf8-ref "sexp_utf8_ref")
+  ((value ctx sexp) (value self sexp) sexp sexp))
+(define-c sexp (utf8-next "sexp_utf8_next")
+  ((value ctx sexp) (value self sexp) sexp sexp sexp))
+(define-c sexp (utf8-prev "sexp_utf8_prev")
+  ((value ctx sexp) (value self sexp) sexp sexp sexp))
 
 (define-c sexp (write-u8 "sexp_write_u8")
   ((value ctx sexp) (value self sexp) sexp (default (current-output-port) sexp)))
diff --git a/lib/chibi/io/port.c b/lib/chibi/io/port.c
index c6956fee..391cd8ea 100644
--- a/lib/chibi/io/port.c
+++ b/lib/chibi/io/port.c
@@ -258,15 +258,15 @@ sexp sexp_make_custom_binary_output_port (sexp ctx, sexp self,
   return res;
 }
 
-sexp sexp_bytes_to_string (sexp ctx, sexp vec) {
+sexp sexp_bytes_to_string (sexp ctx, sexp vec, sexp_uint_t offset, sexp_uint_t size) {
   sexp res;
 #if SEXP_USE_PACKED_STRINGS
-  res = sexp_c_string(ctx, sexp_bytes_data(vec), sexp_bytes_length(vec));
+  res = sexp_c_string(ctx, sexp_bytes_data(vec) + offset, size);
 #else
   res = sexp_alloc_type(ctx, string, SEXP_STRING);
   sexp_string_bytes(res) = vec;
-  sexp_string_offset(res) = 0;
-  sexp_string_size(res) = sexp_bytes_length(vec);
+  sexp_string_offset(res) = offset;
+  sexp_string_size(res) = size - offset;
 #endif
   return res;
 }
@@ -275,7 +275,7 @@ sexp sexp_open_input_bytevector (sexp ctx, sexp self, sexp vec) {
   sexp_gc_var2(str, res);
   sexp_assert_type(ctx, sexp_bytesp, SEXP_BYTES, vec);
   sexp_gc_preserve2(ctx, str, res);
-  str = sexp_bytes_to_string(ctx, vec);
+  str = sexp_bytes_to_string(ctx, vec, 0, sexp_bytes_length(vec));
   res = sexp_open_input_string(ctx, str);
   sexp_port_binaryp(res) = 1;
   sexp_gc_release2(ctx);
@@ -341,10 +341,72 @@ sexp sexp_string_to_utf8 (sexp ctx, sexp self, sexp str) {
   return sexp_string_to_bytes(ctx, res);
 }
 
-/* TODO: add validation */
-sexp sexp_utf8_to_string_x (sexp ctx, sexp self, sexp vec) {
+sexp sexp_string_to_utf8_x (sexp ctx, sexp self, sexp str) {
+  sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, str);
+#if SEXP_USE_PACKED_STRINGS
+  return sexp_string_to_utf8(ctx, self, str);
+#else
+  return sexp_string_bytes(str);
+#endif
+}
+
+sexp sexp_string_offset_op (sexp ctx, sexp self, sexp str) {
+  sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, str);
+#if SEXP_USE_PACKED_STRINGS
+  return SEXP_ZERO;
+#else
+  return sexp_make_fixnum(sexp_string_offset(str));
+#endif
+}
+
+sexp sexp_utf8_ref (sexp ctx, sexp self, sexp bv, sexp offset) {
+  sexp_assert_type(ctx, sexp_bytesp, SEXP_BYTES, bv);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, offset);
+  unsigned char *p=(unsigned char*)sexp_bytes_data(bv) + sexp_unbox_fixnum(offset);
+  if (*p < 0x80)
+    return sexp_make_character(*p);
+  else if ((*p < 0xC0) || (*p > 0xF7))
+    return sexp_user_exception(ctx, NULL, "utf8-ref: invalid utf8 byte", offset);
+  else if (*p < 0xE0)
+    return sexp_make_character(((p[0]&0x3F)<<6) + (p[1]&0x3F));
+  else if (*p < 0xF0)
+    return sexp_make_character(((p[0]&0x1F)<<12) + ((p[1]&0x3F)<<6) + (p[2]&0x3F));
+  else
+    return sexp_make_character(((p[0]&0x0F)<<18) + ((p[1]&0x3F)<<12) + ((p[2]&0x3F)<<6) + (p[3]&0x3F));
+}
+
+/* computes length, consider scanning permissively */
+sexp sexp_utf8_next (sexp ctx, sexp self, sexp bv, sexp offset, sexp end) {
+  sexp_sint_t initial, res;
+  sexp_assert_type(ctx, sexp_bytesp, SEXP_BYTES, bv);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, offset);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, end);
+  if (sexp_unbox_fixnum(offset) >= sexp_unbox_fixnum(end)) return SEXP_FALSE;
+  initial = ((unsigned char*)sexp_bytes_data(bv) + sexp_unbox_fixnum(offset))[0];
+  res = sexp_unbox_fixnum(offset) + (initial < 0xC0 ? 1 : initial < 0xE0 ? 2 : 3 + ((initial>>4)&1));
+  return res > sexp_unbox_fixnum(end) ? SEXP_FALSE : sexp_make_fixnum(res);
+}
+
+/* scans backwards permissively */
+sexp sexp_utf8_prev (sexp ctx, sexp self, sexp bv, sexp offset, sexp start) {
+  sexp_sint_t i, limit;
+  sexp_assert_type(ctx, sexp_bytesp, SEXP_BYTES, bv);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, offset);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, start);
+  unsigned char *p=(unsigned char*)sexp_bytes_data(bv);
+  i = sexp_unbox_fixnum(offset) - 1;
+  limit = sexp_unbox_fixnum(start);
+  while (i >= limit && ((p[i]>>6) == 2))
+    --i;
+  return i < limit ? SEXP_FALSE : sexp_make_fixnum(i);
+}
+
+/* TODO: add optional encoding validation */
+sexp sexp_utf8_to_string_x (sexp ctx, sexp self, sexp vec, sexp offset, sexp size) {
   sexp_assert_type(ctx, sexp_bytesp, SEXP_BYTES, vec);
-  return sexp_bytes_to_string(ctx, vec);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, offset);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, size);
+  return sexp_bytes_to_string(ctx, vec, sexp_unbox_fixnum(offset), sexp_unbox_fixnum(size));
 }
 
 sexp sexp_write_u8 (sexp ctx, sexp self, sexp u8, sexp out) {