diff --git a/scheme.base.scm b/scheme.base.scm
index 763e50c..35e90a0 100644
--- a/scheme.base.scm
+++ b/scheme.base.scm
@@ -43,7 +43,7 @@
 (import (rename (only srfi-4 make-u8vector subu8vector u8vector
                       u8vector? u8vector-length u8vector-ref
                       u8vector-set! read-u8vector read-u8vector!
-                      write-u8vector)
+                      write-u8vector u8vector->blob/shared)
                 (u8vector bytevector)
                 (u8vector-length bytevector-length)
                 (u8vector-ref bytevector-u8-ref)
@@ -51,7 +51,8 @@
                 (u8vector? bytevector?)
                 (make-u8vector make-bytevector)
                 (read-u8vector read-bytevector)
-                (write-u8vector write-bytevector)))
+                (write-u8vector write-bytevector)
+                (u8vector->blob/shared %u8vector->blob/shared)))
 
 ;; u8-ready?
 (import (rename (only scheme char-ready?)
@@ -597,10 +598,8 @@
                   (##sys#check-range start 0 (fx+ end 1) 'utf8->string)
                   (##sys#check-range end start (fx+ len 1) 'utf8->string)
                   (let ((s (##sys#make-string (fx- end start))))
-                    (do ((si 0 (fx+ si 1))
-                         (vi start (fx+ vi 1)))
-                        ((fx= vi end) s)
-                      (##sys#setbyte s si (bytevector-u8-ref bv vi))))))))
+                    (##sys#copy-bytes (%u8vector->blob/shared bv) s start 0 (fx- end start))
+                    s)))))
     (case-lambda
       ((bv) (bv->s bv 0))
       ((bv start) (bv->s bv start))
@@ -614,10 +613,8 @@
                   (##sys#check-range start 0 (fx+ end 1) 'string->utf8)
                   (##sys#check-range end start (fx+ len 1) 'string->utf8)
                   (let ((bv (make-bytevector (fx- end start))))
-                    (do ((vi 0 (fx+ vi 1))
-                         (si start (fx+ si 1)))
-                        ((fx= si end) bv)
-                      (bytevector-u8-set! bv vi (##sys#byte s si))))))))
+                    (##sys#copy-bytes s (%u8vector->blob/shared bv) start 0 (fx- end start))
+                    bv)))))
     (case-lambda
       ((s) (s->bv s 0))
       ((s start) (s->bv s start))