diff --git a/doc/sim65.sgml b/doc/sim65.sgml
index 8a7329b0b..075d95849 100644
--- a/doc/sim65.sgml
+++ b/doc/sim65.sgml
@@ -115,22 +115,12 @@ The <tt/exit/ function may also be used to terminate with an exit code.
  
 Exit codes are limited to 8 bits.
 
-The standard C library high level file input and output is functional,
-and can be used like a command line application in sim65.
+The standard C library high level file input and output is functional.
+A sim65 application can be written like a command line application,
+providing arguments to <tt/main/ and using the <tt/stdio.h/ interfaces.
 
-Lower level file input and output is provided by
-a set of built-in paravirtualization functions:
-
-<tscreen><verb>
-  int open (const char* name, int flags, ...);
-  int __fastcall__ close (int fd);
-  int __fastcall__ read (int fd, void* buf, unsigned count);
-  int __fastcall__ write (int fd, const void* buf, unsigned count);
-</verb></tscreen>
-
-These built-in functions can be used with
-<tt/STDIN_FILENO/, <tt/STDOUT_FILENO/ and <tt/STDERR_FILENO/
-which are mapped to sim65's corresponding file descriptors.
+Internally, file input and output is provided at a lower level by
+a set of built-in paravirtualization functions (<ref id="paravirt-internal" name="see below">).
 
 
 <sect>Creating a Test in Assembly<p>
@@ -169,6 +159,11 @@ pre-loaded with the given <bf/reset address/.
 <item>The <tt/exit/ address is <tt/$FFF9/.
 Jumping to this address will terminate execution with the A register value as an exit code.
 
+<label id="paravirt-internal">
+<item>Several bytes immediately below the vector table are reserved for paravirtualization functions.
+Except for <tt/exit/, a <tt/JSR/ to one of these addresses will return immediately after performing a special function.
+These use cc65 calling conventions, and are intended for use with the sim65 target C library.
+
 <item><tt/IRQ/ and <tt/NMI/ events will not be generated, though <tt/BRK/
 can be used if the IRQ vector at <tt/$FFFE/ is manually prepared by the test code.
 
diff --git a/src/sim65/main.c b/src/sim65/main.c
index bcd436a13..f2daf9295 100644
--- a/src/sim65/main.c
+++ b/src/sim65/main.c
@@ -200,8 +200,8 @@ static unsigned char ReadProgramFile (void)
     /* Read the file body into memory */
     Addr = Load;
     while ((Val = fgetc(F)) != EOF) {
-        if (Addr == 0xFF00) {
-            Error ("'%s': To large to fit into $%04X-$FFF0", ProgramFile, Addr);
+        if (Addr >= PARAVIRT_BASE) {
+            Error ("'%s': To large to fit into $%04X-$%04X", ProgramFile, Addr, PARAVIRT_BASE);
         }
         MemWriteByte (Addr++, (unsigned char) Val);
     }
diff --git a/src/sim65/paravirt.c b/src/sim65/paravirt.c
index c9f6e61b0..603a07e9a 100644
--- a/src/sim65/paravirt.c
+++ b/src/sim65/paravirt.c
@@ -318,13 +318,13 @@ void ParaVirtHooks (CPURegs* Regs)
 /* Potentially execute paravirtualization hooks */
 {
     /* Check for paravirtualization address range */
-    if (Regs->PC <  0xFFF4 ||
-        Regs->PC >= 0xFFF4 + sizeof (Hooks) / sizeof (Hooks[0])) {
+    if (Regs->PC <  PARAVIRT_BASE ||
+        Regs->PC >= PARAVIRT_BASE + sizeof (Hooks) / sizeof (Hooks[0])) {
         return;
     }
 
     /* Call paravirtualization hook */
-    Hooks[Regs->PC - 0xFFF4] (Regs);
+    Hooks[Regs->PC - PARAVIRT_BASE] (Regs);
 
     /* Simulate RTS */
     Regs->PC = Pop(Regs) + (Pop(Regs) << 8) + 1;
diff --git a/src/sim65/paravirt.h b/src/sim65/paravirt.h
index cd4915398..99c28fa02 100644
--- a/src/sim65/paravirt.h
+++ b/src/sim65/paravirt.h
@@ -38,6 +38,17 @@
 
 
 
+/*****************************************************************************/
+/*                                   Data                                    */
+/*****************************************************************************/
+
+
+
+#define PARAVIRT_BASE        0xFFF4
+/* Lowest address used by a paravirtualization hook */
+
+
+
 /*****************************************************************************/
 /*                                   Code                                    */
 /*****************************************************************************/