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How the Super Mario Bros. extra lives system works  (I think)
MEGAߥTE  05.24.2000  megabyte@kontek.net  http://kontek.net

##h represents Hexadecimal.
The extra lives information is stored in byte offset 106Ah
(Add 10h for ROM header and 200h if the trainer is present)

Value     Mario x      Extra life
00h       1            0
01h-08h   2-9          1-8
09h-12h   [crown]0-9   9-18

The crown appears after Mario x 9
The CHR data for the numbers 0-9 are stored in 00h-08h of the
second CHR table.
After this comes letters (09h-23h) and the rest of the second
CHR table.
Therefore, to find out which character will appear for each
value,
subtract 0Ah (because 0-9 are repeated) and look on the table.

Value     Mario x                 Extra life
13h-2Dh   [crown]A-Z              19-45
2Eh-7Fh   [crown][tile 23h-75h]   46-127

This continues up until 7Fh.  However, after you obtain 128
1-ups,
you will get Game Over if you lose a life.  The reason for this
is that
the extra life data is stored as a signed byte.  This means
that for values
where the first bit is 0, the value is positive (00h-79h)
Where the first bit is 1, the value is negative (80h-FFh)
When you obtain a 1-up, the game simply adds 1 to the byte
value.
However, when the game checks to see if your extra life count
is greater than 0,
values past 127 will result in a value less than 0. (FFh=-1,
80h=-127)
This results in Game Over.
If you managed to get 257 1-ups, the counter resets to 00h,
and if you get 258+ 1-ups, the counter will be at 01h+,
and you won't get Game Over after you die.
Since 00h resulted in tile 01h, FFh +1 results in tile 00h,
so Mario x 0 would result.

Value     Mario x                 Extra life
80h-FEh   [crown][tile 76h-F4h]   0 (-127 - -2)
FFh       0                       0 (-1)

Of course you could not get a reading of greater than 7Fh since
you would
automatically die, so to get this result, an emulator or game
enhancer must be
used.

To change the extra life value with the Game Genie,
you must use the base code: 12TOZ6
0 extra lives is AATOZA
To add     Change value
1          1 to P
2          1 to Z
3          1 to L
4          1 to G
5          1 to I
6          1 to T
7          1 to Y
8          6 to E
16         2 to P
32         2 to Z
48         2 to L
64         2 to G
80         2 to I
96         2 to T
112        2 to Y
128 (-127) 1 to E
129 (-126) 1 to O
130 (-125) 1 to X
131 (-124) 1 to U
132 (-123) 1 to K
133 (-122) 1 to S
134 (-121) 1 to V
135 (-120) 1 to N
etc.
255 (-1) = NYTOZE

I think that covers it.
EoF.
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