Add NOTES.md

Author: t-w

NOTES.md

@@ -0,0 +1,17 @@
+
+# Devices with non-standard (or rather - non-valid) size
+
+Many existing devices (HDFs) have sizes not rounded to 512-byte blocks (like
+2000000 or 5000000). Access to such devices using ADFlib is through 512-byte
+blocks and, therefore, limited to:
+
+    floor(size / 512) * 512
+
+meaning that remaining, smaller than 512-byte part after the last full block
+cannot be accessed.
+
+Note that in such cases the size calculated from device's geometry:
+
+    cylinders * heads * sectors * sector_size (512)
+
+is different from the size of HDF file.