diff --git a/include/rp6502.h b/include/rp6502.h
index 61664c78f..53028c35a 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;
diff --git a/libsrc/rp6502/write_xstack.c b/libsrc/rp6502/write_xstack.c
index b53aa95e7..ff979899d 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 > 256) {
+        return _mappederrno (EINVAL);
+    }
     for (i = count; i;) {
         ria_push_char (((char*)buf)[--i]);
     }