Added files to the pet library that support the overlay demo sample program.

cbm_load() is needed because the Pet/CBM Kernals don't have a LOAD function that can be used by machine code programs.

libsrc/pet/cbm_load.c

@@ -0,0 +1,59 @@
+/*
+** 2020-10-15, Greg King
+**
+** unsigned int __fastcall__ cbm_load (const char* name,
+**                                     unsigned char device,
+**                                     void* data);
+*/
+
+#include <cbm.h>
+#include <limits.h>
+
+/* Loads file "name" from the given device to the given address, or to the load
+** address of the file if "data" is the null pointer (like load"name",8,1
+** in BASIC).
+** Returns the number of bytes that were loaded if loading was successful;
+** otherwise 0; "_oserror" contains an error-code, then.
+*/
+unsigned int __fastcall__ cbm_load (const char* name, unsigned char device, void* data)
+{
+    void* load;
+    int length;
+    unsigned int size = 0;
+
+    if (cbm_open (1, device, CBM_READ, name) != 0) {
+        /* Can't load from a file that can't be openned. */
+        return 0;
+    }
+
+    /* Get the file's load address. */
+    if (cbm_read (1, &load, sizeof load) != sizeof load) {
+        /* Either the file wasn't found, or it was too short.  (Note:
+        ** the computer openned a file even if the drive couldn't open one.)
+        */
+        cbm_close (1);
+        return 0;
+    }
+
+    /* If "data" doesn't hold an address, then use the file's address. */
+    if (data == (void*)0x0000) {
+        data = load;
+    }
+
+    /* Pull the file into RAM.  [Note that, if cbm_read() grabbed more
+    ** than 32767 bytes at a time, then its result would look negative,
+    ** which would cancel the load.]
+    */
+    do {
+        size += (length = cbm_read (1, data, INT_MAX));
+        data = (unsigned char*)data + length;
+    } while (length == INT_MAX && cbm_k_readst() == 0);
+    cbm_close (1);
+
+    /* "length" is -1 if there was an error. */
+    if (length < 0) {
+        size = 0;
+    }
+
+    return size;
+}