diff --git a/libguile/ports-internal.h b/libguile/ports-internal.h
index 14d00c2a6..862d85800 100644
--- a/libguile/ports-internal.h
+++ b/libguile/ports-internal.h
@@ -36,51 +36,57 @@ scm_port_buffer_size (scm_t_port_buffer *buf)
 static inline void
 scm_port_buffer_reset (scm_t_port_buffer *buf)
 {
-  buf->cur = buf->end = 0;
+  buf->cur = buf->end = SCM_INUM0;
 }
 
 static inline void
 scm_port_buffer_reset_end (scm_t_port_buffer *buf)
 {
-  buf->cur = buf->end = scm_port_buffer_size (buf);
+  buf->cur = buf->end = scm_from_size_t (scm_port_buffer_size (buf));
 }
 
 static inline size_t
 scm_port_buffer_can_take (scm_t_port_buffer *buf)
 {
-  return buf->end - buf->cur;
+  return scm_to_size_t (buf->end) - scm_to_size_t (buf->cur);
 }
 
 static inline size_t
 scm_port_buffer_can_put (scm_t_port_buffer *buf)
 {
-  return scm_port_buffer_size (buf) - buf->end;
+  return scm_port_buffer_size (buf) - scm_to_size_t (buf->end);
+}
+
+static inline size_t
+scm_port_buffer_can_putback (scm_t_port_buffer *buf)
+{
+  return scm_to_size_t (buf->cur);
 }
 
 static inline void
 scm_port_buffer_did_take (scm_t_port_buffer *buf, size_t count)
 {
-  buf->cur += count;
+  buf->cur = scm_from_size_t (scm_to_size_t (buf->cur) + count);
 }
 
 static inline void
 scm_port_buffer_did_put (scm_t_port_buffer *buf, size_t count)
 {
-  buf->end += count;
+  buf->end = scm_from_size_t (scm_to_size_t (buf->end) + count);
 }
 
 static inline const scm_t_uint8 *
 scm_port_buffer_take_pointer (scm_t_port_buffer *buf)
 {
   signed char *ret = SCM_BYTEVECTOR_CONTENTS (buf->bytevector);
-  return ((scm_t_uint8 *) ret) + buf->cur;
+  return ((scm_t_uint8 *) ret) + scm_to_size_t (buf->cur);
 }
 
 static inline scm_t_uint8 *
 scm_port_buffer_put_pointer (scm_t_port_buffer *buf)
 {
   signed char *ret = SCM_BYTEVECTOR_CONTENTS (buf->bytevector);
-  return ((scm_t_uint8 *) ret) + buf->end;
+  return ((scm_t_uint8 *) ret) + scm_to_size_t (buf->end);
 }
 
 static inline size_t
@@ -108,12 +114,13 @@ static inline void
 scm_port_buffer_putback (scm_t_port_buffer *buf, const scm_t_uint8 *src,
                          size_t count)
 {
-  assert (count <= buf->cur);
+  assert (count <= scm_to_size_t (buf->cur));
 
   /* Sometimes used to move around data within a buffer, so we must use
      memmove.  */
-  buf->cur -= count;
-  memmove (SCM_BYTEVECTOR_CONTENTS (buf->bytevector) + buf->cur, src, count);
+  buf->cur = scm_from_size_t (scm_to_size_t (buf->cur) - count);
+  memmove (SCM_BYTEVECTOR_CONTENTS (buf->bytevector) + scm_to_size_t (buf->cur),
+           src, count);
 }
 
 enum scm_port_encoding_mode {
diff --git a/libguile/ports.c b/libguile/ports.c
index a433e28fb..b8b6dbf92 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -518,6 +518,7 @@ scm_c_make_port_buffer (size_t size)
   scm_t_port_buffer *ret = scm_gc_typed_calloc (scm_t_port_buffer);
 
   ret->bytevector = scm_c_make_bytevector (size);
+  ret->cur = ret->end = SCM_INUM0;
   ret->has_eof_p = SCM_BOOL_F;
 
   return ret;
@@ -1000,7 +1001,7 @@ looking_at_bytes (SCM port, const unsigned char *bytes, int len)
 
   while (i < len && scm_peek_byte_or_eof_unlocked (port) == bytes[i])
     {
-      pt->read_buf->cur++;
+      scm_port_buffer_did_take (pt->read_buf, 1);
       i++;
     }
   scm_i_unget_bytes_unlocked (bytes, i, port);
@@ -1413,7 +1414,8 @@ scm_i_read_unlocked (SCM port, scm_t_port_buffer *buf)
 {
   size_t count;
 
-  count = scm_i_read_bytes_unlocked (port, buf->bytevector, buf->end,
+  count = scm_i_read_bytes_unlocked (port, buf->bytevector,
+                                     scm_to_size_t (buf->end),
                                      scm_port_buffer_can_put (buf));
   scm_port_buffer_did_put (buf, count);
   buf->has_eof_p = scm_from_bool (count == 0);
@@ -1636,7 +1638,7 @@ get_utf8_codepoint (SCM port, scm_t_wchar *codepoint,
   if (SCM_UNLIKELY ((b) == EOF))		\
     goto invalid_seq
 #define CONSUME_PEEKED_BYTE()				\
-  pt->read_buf->cur++
+  scm_port_buffer_did_take (pt->read_buf, 1)
 
   int byte;
   scm_t_port *pt;
@@ -1985,7 +1987,7 @@ scm_i_unget_bytes_unlocked (const scm_t_uint8 *buf, size_t len, SCM port)
       pt->rw_active = SCM_PORT_READ;
     }
 
-  if (read_buf->cur < len)
+  if (scm_port_buffer_can_putback (read_buf) < len)
     {
       /* The bytes don't fit directly in the read_buf.  */
       size_t buffered, size;
@@ -2562,8 +2564,8 @@ scm_i_write_unlocked (SCM port, scm_t_port_buffer *src)
 {
   size_t start, count;
 
-  assert (src->cur < src->end);
-  assert (src->end <= scm_port_buffer_size (src));
+  assert (scm_to_size_t (src->cur) < scm_to_size_t (src->end));
+  assert (scm_to_size_t (src->end) <= scm_port_buffer_size (src));
 
   /* Update cursors before attempting to write, assuming that I/O errors
      are sticky.  That way if the write throws an error, causing the
@@ -2571,8 +2573,8 @@ scm_i_write_unlocked (SCM port, scm_t_port_buffer *src)
      by GC when it's open, any subsequent close-port / force-output
      won't signal *another* error.  */
 
-  start = src->cur;
-  count = src->end - src->cur;
+  start = scm_to_size_t (src->cur);
+  count = scm_port_buffer_can_take (src);
   scm_port_buffer_reset (src);
   scm_i_write_bytes_unlocked (port, src->bytevector, start, count);
 }
diff --git a/libguile/ports.h b/libguile/ports.h
index 12a67ec99..173562324 100644
--- a/libguile/ports.h
+++ b/libguile/ports.h
@@ -78,8 +78,8 @@ typedef struct
   SCM bytevector;
 
   /* Offsets into the buffer.  Invariant: cur <= end <= size(buf).  */
-  size_t cur;
-  size_t end;
+  SCM cur;
+  SCM end;
 
   /* For read buffers, flag indicating whether the last read() returned
      zero bytes.  Note that in the case of pushback, there could still
@@ -429,16 +429,24 @@ SCM_INLINE_IMPLEMENTATION int
 scm_get_byte_or_eof_unlocked (SCM port)
 {
   scm_t_port_buffer *buf = SCM_PTAB_ENTRY (port)->read_buf;
-  scm_t_uint8 *ptr;
+  size_t cur;
 
-  ptr = (scm_t_uint8 *) SCM_BYTEVECTOR_CONTENTS (buf->bytevector);
-  if (SCM_LIKELY (buf->cur < buf->end))
-    return ptr[buf->cur++];
+  cur = scm_to_size_t (buf->cur);
+  if (SCM_LIKELY (cur < scm_to_size_t (buf->end)))
+    {
+      scm_t_uint8 ret = SCM_BYTEVECTOR_CONTENTS (buf->bytevector)[cur];
+      buf->cur = scm_from_size_t (cur + 1);
+      return ret;
+    }
 
   buf = scm_fill_input_unlocked (port);
-  ptr = (scm_t_uint8 *) SCM_BYTEVECTOR_CONTENTS (buf->bytevector);
-  if (buf->cur < buf->end)
-    return ptr[buf->cur++];
+  cur = scm_to_size_t (buf->cur);
+  if (cur < scm_to_size_t (buf->end))
+    {
+      scm_t_uint8 ret = SCM_BYTEVECTOR_CONTENTS (buf->bytevector)[cur];
+      buf->cur = scm_from_size_t (cur + 1);
+      return ret;
+    }
 
   /* The next peek or get should cause the read() function to be called
      to see if we still have EOF.  */
@@ -451,14 +459,22 @@ SCM_INLINE_IMPLEMENTATION int
 scm_peek_byte_or_eof_unlocked (SCM port)
 {
   scm_t_port_buffer *buf = SCM_PTAB_ENTRY (port)->read_buf;
-  scm_t_uint8 *ptr = (scm_t_uint8 *) SCM_BYTEVECTOR_CONTENTS (buf->bytevector);
+  size_t cur;
 
-  if (SCM_LIKELY (buf->cur < buf->end))
-    return ptr[buf->cur];
+  cur = scm_to_size_t (buf->cur);
+  if (SCM_LIKELY (cur < scm_to_size_t (buf->end)))
+    {
+      scm_t_uint8 ret = SCM_BYTEVECTOR_CONTENTS (buf->bytevector)[cur];
+      return ret;
+    }
 
   buf = scm_fill_input_unlocked (port);
-  if (buf->cur < buf->end)
-    return ptr[buf->cur];
+  cur = scm_to_size_t (buf->cur);
+  if (cur < scm_to_size_t (buf->end))
+    {
+      scm_t_uint8 ret = SCM_BYTEVECTOR_CONTENTS (buf->bytevector)[cur];
+      return ret;
+    }
 
   return EOF;
 }
diff --git a/libguile/read.c b/libguile/read.c
index a4357d179..bc5c3c179 100644
--- a/libguile/read.c
+++ b/libguile/read.c
@@ -2074,11 +2074,11 @@ scm_i_scan_for_encoding (SCM port)
       pt->rw_active = SCM_PORT_READ;
     }
 
-  if (buf->cur == buf->end)
+  if (scm_port_buffer_can_take (buf) == 0)
     {
       /* We can use the read buffer, and thus avoid a seek. */
       buf = scm_fill_input_unlocked (port);
-      bytes_read = buf->end - buf->cur;
+      bytes_read = scm_port_buffer_can_take (buf);
       if (bytes_read > SCM_ENCODING_SEARCH_SIZE)
         bytes_read = SCM_ENCODING_SEARCH_SIZE;
 
diff --git a/libguile/rw.c b/libguile/rw.c
index 76467a90e..d6437e96a 100644
--- a/libguile/rw.c
+++ b/libguile/rw.c
@@ -240,7 +240,7 @@ SCM_DEFINE (scm_write_string_partial, "write-string/partial", 1, 3, 0,
 
       /* Filling the last character in the buffer would require a
          flush.  */
-      if (write_len < scm_port_buffer_size (write_buf) - write_buf->end)
+      if (write_len < scm_port_buffer_can_put (write_buf))
 	{
           scm_c_write_unlocked (port, src, write_len);
 	  return scm_from_long (write_len);