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Increment and Decrement (Class 9)

Up to two registers can be incremented or decremented at the same time (in a single clock
cycle) by using the following instructions:

INC r           Increment register r
DEC r           Decrement register r
INCM  r1, r2    Increment registers r1 and r2 simultaneously
DECM  r1, r2    Decrement registers r1 and r2 simultaneously
INDEC r1, r2    Increment register r1 and decrement r2 simultaneously

All Increment/Decrement instructions are encoded the same way:

[9][mode][r2][r1]

The field [mode] determines what to do with registers r2, r1 according to the following table:

D11  D10  D9  D8                                     Example
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
0    0    0    0    Do nothing                    
0    0    0    1    Increment r2                     INC   B
0    0    1    0    Decrement r2                     DEC   B
0    0    1    1    (Illegal)
0    1    0    0    Increment r1                     INC   A
0    1    0    1    Increment both r2 and r1         INCM  B, A
0    1    1    0    Increment r2 and Decrement r1    INDEC B, A
0    1    1    1    (Illegal)
1    0    0    0    Decrement r2                     DEC   B
1    0    0    1    Decrement r2 and Increment r1    INDEC A, B
1    0    1    0    Decrement both r2 and r1         DECM  B, A
1    0    1    1    (Illegal)
1    1    0    0    (Illegal)
1    1    0    1    (Illegal)
1    1    1    0    (Illegal)
1    1    1    1    (Illegal)