This algorithm is used to decrease the counter by 1... I keep the rightmost values (the least significant bits), which, if I ignore the carry, results in the register decreasing by 1 at each iteration...

Amazing what a small piece of paper and the Windows calculator (in scientific mode) can do for you. :-)

Here's the breakdown:

1F  =   0001 1111 = 1F
FF  =   1111 1111
----------------- <- sum
11E = 1 0000 1110
 1E =   0000 1110 <- keep 8 least significant bits
 FF =   1111 1111
----------------- <- sum
11D = 1 0000 1101
 1D =   0000 1101
... etc ...




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