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+How to program the cartrige?
+
+Basically you have a 5 bit shift register connected to the high address
+bits of the EEPROM IC. So you need to write these 5 bits one by one.
+Every time you write a bit the ROM is changed. The ROM is paged
+at $0000-$3FFF, so you need place the paging routine in the RAM area.
+
+There is also a LOCK bit. When you write a 1 in the LOCK bit the paging
+is locked. The cartridge has a LED indicating LOCK bit. To unlock paging
+you must push the button in the cartridge, or power off the spectrum.
+This also reset the page to 00000, where is the game selection menu.
+
+There are 4 address to change these bits: $3FFC, $3FFD, $3FFE, $3FFF.
+Both reads and writes to this address will work and data bus is
+discarded. In this example I will use writes with LD (N), A instruction.
+The LSB of address (A0) is inserted into the shift register from the right
+to the left and the second bit (A1) is the LOCK bit.
+
+So for example you want to go to the page 13 (pages are between 0 and 31)
+by activating the LOCK bit in the last bit. In binary (5 bits) is 01101.
+So you need to write 0, then 1, then 1, then 0 and finally 1. With the
+last 1 you need also to set LOCK=1.
+
+  LD  ($3FFC), A    ; Write 0. Now page is 00000 and LOCK=0
+  LD  ($3FFD), A    ; Write 1. Now page is 00001 and LOCK=0
+  LD  ($3FFD), A    ; Write 1. Now page is 00011 and LOCK=0
+  LD  ($3FFC), A    ; Write 0. Now page is 00110 and LOCK=0
+  LD  ($3FFF), A    ; Write 1. Now page is 01101 and LOCK=1
+
+The locking mechanism is good for a game selection menu, when you want
+to disable future accidental paging once game is selected. But if
+you want create your own game (it can be up to 512K) is better avoid
+locking.