Bloody Paws Snow Bug Fix
                        ========================
Author: Flavio M. Matsumoto
        reikainosuke-tk@yahoo.com.br, http://cantinhotk90x.blogspot.com.br
Date:   08/20/2012

Bloody Paws has a bug that cause the 'snow' effect in the screen. When IM 2 is
activated with I register in the range 64-127, ULA is confused and shows noises
in the screen. This bug does not affect machine code execution and game can
be played normally, but this effect is visually very annoying. 

The program disassembly reveals forbidden value (93 or #5D) being attributed for 
I register:

24000   LD SP,24000
24003   DI
24004   LD A,#5D
24006   LD I,A
24008   IM 2
24010   XOR A

The value of assembly instruction operand at 24004 must be replaced by any value
greater than #7F. The chosen value was #FB because the RAM in the #FB00-#FC00
range was empty. The instruction at 24004 has been altered to:

24004   LD A,#FB

In IM 2, the most significant byte (MSB) of the interrupt vector is the value of
I register and the least significant byte (LSB) is read from data bus. Owing to 
the push-up resistors present in the data bus, the LSB is usually 255 (#FF), 
but it can return any other values because noise can be left by some device 
(mainly ULA). Programmer of Bloody Paws erroneously assumed that LSB will always
be 255, but this is wrong! The proper way to handle interrupts would be making 
an interrupt vector table in the RAM, containing all possible values of LSB.
Moreover, the interrupt routine address must be symmetrical because LSB can 
assume even or odd value. 

To prepare interrupt vector table, the RAM located at #FB00-#FC00 range was
chosen because it was empty. Address #FAFA was also empty and was chosen to 
hold the interrupt routine that consists only by 'JP #FEF6' instruction (#FEF6 
is the beginning of program's real interrupt routine).