After instruction_2 is executed, the memory has the following bit values.

               latch  bit
             address  values
                0000  1100 <-----|   address of instruction
                0001  0000 <--|  |   value of a
                0010  0001    |  |   not 0 = 1
                0011  0000 ---|  |   not 1 = 0
instruction_1   0100  0001       |   from address
                0101  1000       |   to address
                0110  0001       |   'to' bits to change
                0111  1000       |   instr.addr.and rot.amount
instruction_2   1000  0011       |   from address
                1001  0001       |   to address
                1010  0001       |   'to' bits to change
                1011  1100 ------|   instr.addr.and rot.amount
instruction_3   1100  0000           from address
                1101  0000           to address
                1110  0000           'to' bits to change
                1111  1100           instr.addr.and rot.amount

The italics show from where data was copied. The underlining shows to where data was copied. The arrows show how data was copied.

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Latch 0000 now has value 1100, so that the next instruction to be executed is instruction_3 in latches 1100, 1101, 1110, and 1111.

1. Latch 1100 holds 0000, so data will be copied from latch 0000.
2. Latch 1101 holds 0000, so data is copied to latch 0000.
3. Latch 1110 holds 0000, so no data bits are copied (to latch 0000).
4. Latch 1111 holds 1100, so 1100 is copied to latch 0000.

This results in the following bit values in memory.

               latch  bit
             address  values
                0000  1100 <-----|   address of instruction
                0001  0000       |   value of a
                0010  0001       |   not 0 = 1
                0011  0000       |   not 1 = 0
instruction_1   0100  0001       |   from address
                0101  1000       |   to address
                0110  0001       |   'to' bits to change
                0111  1000       |   instr.addr.and rot.amount
instruction_2   1000  0011       |   from address
                1001  0001       |   to address
                1010  0001       |   'to' bits to change
                1011  1100       |   instr.addr.and rot.amount
instruction_3   1100  0000       |   from address
                1101  0000       |   to address
                1110  0000       |   'to' bits to change
                1111  1100 ------|    instr.addr.and rot.amount

The underlined bits have been copied to from the italic bits.

Thus, instruction_3 changes nothing (because latch 0000 already held 1100) and leads to instruction_3 being executed again and again.

Ending a program with an instruction like instruction_3 ensures that nothing else will happen after the desired instructions (instruction_1 and instruction_2) are executed. It's just something for the computer to do until we stop the processor and look in memory for the results.

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