dec0.cpp: restore comment that was removed (nw)

src/mame/drivers/dec0.cpp

@@ -56,6 +56,30 @@ ToDo:
 - background pen in Birdie Try is presumably wrong.
 - Pixel clock frequency isn't verified
 
+Bad Dudes MCU implements a command to calculate a program ROM checksum and
+compare the low byte of the result to a value supplied by the host CPU, but it
+doesn't work.  Here's the code in question:
+
+0AB0: 51 50    acall   $0A50
+0AB2: C3       clr     c
+0AB3: 48       orl     a,r0
+0AB4: 70 02    jnz     $0AB8
+0AB6: 80 89    sjmp    $0A41
+0AB8: 21 F0    ajmp    $09F0
+
+The function at $0A50 reads the expected value from the host, $0A41 is the
+normal command response, and $09F0 is the error response.  The orl instruction
+doesn't make sense here.  However, changing it from 48 to 60 makes it an xrl
+instruction which would work as expected.  The game doesn't issue this command.
+The checksum function can't be used to verify that the program is good because
+the expected value mod 256 has to be supplied by the host.
+
+Bad Dudes only seems to use commands $0B (sync), $01 (reset if parameter is not
+$3B), $07 (return table index if parameter matches table, otherwise reset), and
+$09 (set table index to zero).  Dragonninja only seems to use commands $03 (on
+startup), $07 (same function as Bad Dudes) and $09 (same function as Bad Dudes).
+Most of the MCU program isn't utilised.
+
 
 Guru-Readme for Data East 16 bit games (Updated 7-Feb-2017)