diff --git a/asminc/rp6502.inc b/asminc/rp6502.inc
index 7dd1b8fcd..816712abb 100644
--- a/asminc/rp6502.inc
+++ b/asminc/rp6502.inc
@@ -37,6 +37,7 @@ RIA_OP_PHI2               := $02
 RIA_OP_CODEPAGE           := $03
 RIA_OP_LRAND              := $04
 RIA_OP_STDIN_OPT          := $05
+RIA_OP_CLOCK              := $0F
 RIA_OP_CLOCK_GETRES       := $10
 RIA_OP_CLOCK_GETTIME      := $11
 RIA_OP_CLOCK_SETTIME      := $12
diff --git a/include/rp6502.h b/include/rp6502.h
index 2b40cfc71..7deeebc4c 100644
--- a/include/rp6502.h
+++ b/include/rp6502.h
@@ -47,8 +47,7 @@ struct __RP6502
     unsigned char step1;
     unsigned int addr1;
     unsigned char xstack;
-    unsigned char errno_lo;
-    unsigned char errno_hi;
+    unsigned int errno;
     unsigned char op;
     unsigned char irq;
     const unsigned char spin;
@@ -101,6 +100,7 @@ long __fastcall__ ria_call_long_errno (unsigned char op);
 #define RIA_OP_CODEPAGE 0x03
 #define RIA_OP_LRAND 0x04
 #define RIA_OP_STDIN_OPT 0x05
+#define RIA_OP_CLOCK 0x0F
 #define RIA_OP_CLOCK_GETRES 0x10
 #define RIA_OP_CLOCK_GETTIME 0x11
 #define RIA_OP_CLOCK_SETTIME 0x12
@@ -117,6 +117,8 @@ long __fastcall__ ria_call_long_errno (unsigned char op);
 
 /* C API for the operating system. */
 
+int __cdecl__ xregn (char device, char channel, unsigned char address, unsigned count,
+    ...);
 int __cdecl__ xreg (char device, char channel, unsigned char address, ...);
 int phi2 (void);
 int codepage (void);
diff --git a/include/time.h b/include/time.h
index f8977ab0c..5eb6f144a 100644
--- a/include/time.h
+++ b/include/time.h
@@ -86,6 +86,8 @@ struct tm {
 #  define CLOCKS_PER_SEC        135   /* FIXME */
 #elif defined(__GEOS__)
 #  define CLOCKS_PER_SEC        1
+#elif defined (__RP6502__)
+#  define CLOCKS_PER_SEC        100
 #elif defined(__TELESTRAT__)
 #  define CLOCKS_PER_SEC        10
 #elif defined(__ATARI__) || defined (__LYNX__)
diff --git a/libsrc/rp6502/clock.c b/libsrc/rp6502/clock.c
new file mode 100644
index 000000000..f8756f553
--- /dev/null
+++ b/libsrc/rp6502/clock.c
@@ -0,0 +1,7 @@
+#include <rp6502.h>
+#include <time.h>
+
+clock_t __fastcall__ clock (void)
+{
+    return ria_call_long (RIA_OP_CLOCK);
+}
diff --git a/libsrc/rp6502/read.c b/libsrc/rp6502/read.c
index eb96f779c..87a9bbf7d 100644
--- a/libsrc/rp6502/read.c
+++ b/libsrc/rp6502/read.c
@@ -5,7 +5,7 @@ int __fastcall__ read (int fildes, void* buf, unsigned count)
 {
     int total = 0;
     while (count) {
-        unsigned blockcount = (count > 256) ? 256 : count;
+        unsigned blockcount = (count > 512) ? 512 : count;
         int bytes_read = read_xstack (&((char*)buf)[total], blockcount, fildes);
         if (bytes_read < 0) {
             return bytes_read;
diff --git a/libsrc/rp6502/sysrename.c b/libsrc/rp6502/sysrename.c
index 96eca24cf..46bdd8b31 100644
--- a/libsrc/rp6502/sysrename.c
+++ b/libsrc/rp6502/sysrename.c
@@ -7,7 +7,7 @@ unsigned char __fastcall__ _sysrename (const char* oldpath, const char* newpath)
     size_t oldpathlen, newpathlen;
     oldpathlen = strlen (oldpath);
     newpathlen = strlen (newpath);
-    if (oldpathlen + newpathlen > 254) {
+    if (oldpathlen + newpathlen > 510) {
         return _mappederrno (EINVAL);
     }
     while (oldpathlen) {
diff --git a/libsrc/rp6502/write.c b/libsrc/rp6502/write.c
index 11241dab5..23877b6e8 100644
--- a/libsrc/rp6502/write.c
+++ b/libsrc/rp6502/write.c
@@ -5,7 +5,7 @@ int __fastcall__ write (int fildes, const void* buf, unsigned count)
 {
     int ax, total = 0;
     while (count) {
-        int blockcount = (count > 256) ? 256 : count;
+        int blockcount = (count > 512) ? 512 : count;
         ax = write_xstack (&((char*)buf)[total], blockcount, fildes);
         if (ax < 0) {
             return ax;
diff --git a/libsrc/rp6502/write_xstack.c b/libsrc/rp6502/write_xstack.c
index b53aa95e7..29285a87e 100644
--- a/libsrc/rp6502/write_xstack.c
+++ b/libsrc/rp6502/write_xstack.c
@@ -1,8 +1,12 @@
 #include <rp6502.h>
+#include <errno.h>
 
 int __fastcall__ write_xstack (const void* buf, unsigned count, int fildes)
 {
     unsigned i;
+    if (count > 512) {
+        return _mappederrno (EINVAL);
+    }
     for (i = count; i;) {
         ria_push_char (((char*)buf)[--i]);
     }
diff --git a/libsrc/rp6502/xregn.c b/libsrc/rp6502/xregn.c
new file mode 100644
index 000000000..ec040be20
--- /dev/null
+++ b/libsrc/rp6502/xregn.c
@@ -0,0 +1,19 @@
+#include <rp6502.h>
+#include <stdarg.h>
+
+int __cdecl__ xregn (char device, char channel, unsigned char address, unsigned count,
+    ...)
+{
+    va_list args;
+    va_start (args, count);
+    RIA.xstack = device;
+    RIA.xstack = channel;
+    RIA.xstack = address;
+    while (count--) {
+        unsigned v = va_arg (args, unsigned);
+        RIA.xstack = v >> 8;
+        RIA.xstack = v;
+    }
+    va_end (args);
+    return ria_call_int_errno (RIA_OP_XREG);
+}