The circuit below shows two latches of memory at the bottom, latch 0000 and latch 1111. The other fourteen latches are not shown. It also shows a processor above the memory. The processor is mainly made of latches. Latches in a processor are called registers. Register 001 is not a latch, however, because it doesn't have loops.

Writing to a latch will not clear any bits that were previously 1, so always clear a latch before writing data to it. Therefore, to write to a latch, do the following.

1. Press the correct address keys (A3, A2, A1, and A0) and data keys (D3, D2, D1, and D0).
2. Press the clear key, CL, to clear the latch.
3. Release the clear key, CL.
4. Press the enable key, EN, to copy data to the latch.
5. Release the enable key, EN.
6. Release the address keys, A3, A2, A1 and A0, and the data keys, D3, D2, D1 and D0.

To read data from a latch, do the following.

1. Press the correct address keys, A3, A2, A1 and A0, to select the latch to read.
2. Press the enable key, EN, to send the latch's values to the lights, D3, D2, D1, and D0.
3. Release the enable key EN.
4. Release the address keys, A3, A2, A1 and A0.

Notice that reading a latch connects the loops of the latch to the data bus wires, D3, D2, D1, and D0.

Similarly, writing to a register will not clear any bits that were previously 1, so always clear a register before writing data to it. Therefore, to write to a register (except 'register' 001, which has no loops to write to), do the following.

1. Press the correct register address keys (RA2, RA1, and RA0) and data keys (D3, D2, D1, and D0).
2. Press the clear register key, CLR, to clear the register.
3. Release the clear register key, CLR.
4. Press the enable register key, ENR, to copy data to the register.
5. Release the enable register key, ENR.
6. Release the register address keys RA2, RA1 and RA0, and the data keys, D3, D2, D1 and D0.

To read register data, just do the following.

1. Press the correct register address keys, RA2, RA1 and RA0, to select the register to read.
2. Press the enable register key, ENR, to copy the register's values to the lights D3, D2, D1, and D0.
3. Release the enable register key, ENR.
4. Release the register address keys, RA2, RA1 and RA0.
Notice that reading a register connects the loops of the register to the data bus wires, D3, D2, D1, and D0.

Do the following to copy data from a latch to a register (except for register 001).

1. Select the register with RA2, RA1, and RA0.
2. Temporarily press CLR to clear the register.
3. Select the latch in memory with A3, A2, A1, and A0.
4. Temporarily press ENR and EN to connect the register loops and latch loops to the data bus wires, D3, D2, D1, and D0, and so to each other.
5. Release all keys.

Do the following to copy data from a register to a latch.

1. Select the latch with A3, A2, A1, and A0.
2. Temporarily press CL to clear the latch.
3. Select the register with RA2, RA1, and RA0.
4. Temporarily press ENR and EN to connect the register loops and latch loops to the data bus wires, D3, D2, D1, and D0, and so to each other.
5. Release all keys.

Processor and Memory Without Clock


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