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+Overview
+--------
+
+This is a small project to implement a connection such that binary files can
+be downloaded to an Amstrad CPC from a Raspberry Pi via the joystick port.
+
+The circuitry is such that GPIO pins are connected via transistors and mapped
+to joystick directions:
+
+ GPIO 2 - Fire
+ GPIO 3 - Up
+ GPIO 4 - Down
+ GPIO 5 - Left
+ GPIO 6 - Right
+
+The idea is that a nibble is placed on the direction pins. Once these are set
+the fire pin is activated and held for a period. Then the fire is released.
+The next nibble is fired in the same way and this finishes the process for
+sending a byte.
+
+On the CPC side when reading the joystick via KM_GET_JOYSTICK(0xbb24) the
+firmware docs reveal this bit layout:
+
+Bit Direction/Fire
+0 Up
+1 Down
+2 Left
+3 Right
+4 Fire2
+5 Fire1
+6 Spare
+7 Always zero
+
+However KM_GET_JOYSTICK only reads the joystick every 1/50th of seconds, so this
+is too slow for our purposes. As such the joystick has to be read directly.
+When read directly the bits are reversed (i.e. a bit is set if that direction
+isn't pressed) so the results have to be xored with 255 to get the value we
+want.
+
+As the 4 directions are already arranged in the low nibble this layout is
+maintained for the protocol, so that this is the mapping between the GPIO
+pins and the info to send:
+
+Bit GPIO Pin
+0 3
+1 4
+2 5
+3 6
+4 2
+
+
+Z80 code
+--------
+
+The z80 directory contains code to read the information sent over the joystick
+port. xfer.z80 is for the basic file transfer. copydisk.z80 is a more
+complicated process used to transfer and write disk images.
+
+Included is the text of a BASIC loader (loadxfer.bas) to poke in the results of
+assembling xfer.z80. This should be used to get the initial code across to the
+CPC, which will involve some typing. Needless to say, I'd save the code before
+running it.
+
+
+Pi code
+-------
+
+This comprises two command line tools. cpcxfer is to transfer a file.
+cpcdiskxfer is to transfer a disk image. Usage is as follows:
+
+cpcxfer file start_address milliseconds
+
+ file is the file to send.
+
+ start_address is the start address of the file. Hex numbers can be
+ preceded with 0x.
+
+ milliseconds is the number of milliseconds to hold the fire button high
+ and then the number of milliseconds to hold it off. Hence one nibble is
+ transferred every 2 * milliseconds.
+
+cpcdiskxfer disk_image milliseconds
+
+ disk_image is the DSK image to send.
+
+ milliseconds is the number of milliseconds to hold the fire button high
+ and then the number of milliseconds to hold it off. Hence one nibble is
+ transferred every 2 * milliseconds.
+
+
+File transfer protocol
+----------------------
+
+This is a simple protocol using the following format:
+
+Byte 0 - Destination address low byte.
+Byte 1 - Destination address high byte.
+Byte 2 - File length low byte.
+Byte 3 - File length high byte.
+Byte n - File length bytes.
+
+
+Disk image transfer protocol
+----------------------------
+
+This protocol follows the following format. Note that double sided disk
+images are not supported.
+
+First an overview packet is sent.
+
+TODO