Standard loading system notes
-----------------------------

Files are split into blocks. Each block has a header and data block.

The header block is 64 bytes long, and has a sync byte of &2c.
The header block is written using the cassette block structure described
below.

The header has the following form:

offset		length		description

0			16			filename (padded with "0"s)
16			1			block number (normally 1)
17			1			last block flag (!=0 means this is last block
						in file)
18			1			file type 
							bit 0 = 1: file is protected
							bit 1..3 = contents
									0 = basic
									1 = binary
									2 = screen image
									3 = ASCII
									4..7 not defined
							bit 4..7 = version
									ASCII files are version 1,
									all other files are version 0

The data block stores up to 2K of data and has a sync byte of &16.
The data block is written using the cassette block structure described 
below.

cassette block structure:

Leader/pilot, sync bit (0), sync byte, 1 or more cassette data structures (see below), trailer

cassette data structure:

256 bytes, CRC high byte, CRC low byte

notes:
1. Leader/pilot is 2048 "1" bits (4096 pulses)
2. sync bit is timing for a 0 bit pulse
3. timing for 0 and 1 bits in data is derived from leader/pilot
4. see details above for standard sync byte values
6. CRC is calculated using polynomial with initial seed of &ffff. 
7. CRC is stored NOTed on the cassette!
8. Trailer is 32 "1" bits at end