Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [72]
At the time I showed this circuit to you in Chapter 14, the only latches that I had introduced so far were level triggered. In a level-triggered latch, the Clock input has to go to 1 and then back to 0 in order for the latch to store something. During the time the Clock input is 1, the data inputs of the latch can change and these changes will affect the stored output. Later in that chapter, I introduced edge-triggered latches. These latches save their values in the brief moment that the Clock input goes from 0 to 1. Edge-triggered latches are often somewhat easier to use, so I want to assume that all the latches in this chapter are edge triggered.
A latch used to accumulate a running total of numbers is called an accumulator. But we'll see later in this chapter that an accumulator need not simply accumulate. An accumulator is often a latch that holds first one number and then that number plus or minus another number.
The big problem with the adding machine shown above is fairly obvious: Say you have a list of 100 binary numbers you want to add together. You sit down at the adding machine and doggedly enter each and every number and accumulate the sum. But when you're finished, you discover that a couple of the numbers on the list were incorrect. Now you have to do the whole thing over again.
But maybe not. In the preceding chapter, we used almost 5 million relays to build a RAM array containing 64 KB of memory. We also wired a control panel (shown on page 204) that let us close a switch labeled Takeover and literally take over all the writing and reading of this RAM array using switches.
If you had typed all 100 binary numbers into this RAM array rather than directly into the adding machine, making a few corrections would be a lot easier.
So now we face the challenge of connecting the RAM array to the accumulating adder. It's pretty obvious that the RAM Data Out signals replace the switches to the adder, but it's perhaps not so obvious that a 16-bit counter (such as we built in Chapter 14) can control the address signals of the RAM array. The Data Input and Write signals to the RAM aren't needed in this circuit:
This is certainly not the easiest piece of calculating equipment ever invented. To use it, you first must close the switch labeled Clear. This clears the contents of the latch and sets the output of the 16-bit counter to 0000h. Then you close the Takeover switch on the RAM control panel. You can then enter a set of 8-bit numbers that you want to add beginning at RAM address 0000h. If you have 100 numbers, you'll store these numbers at addresses 0000h through 0063h. (You should also set all the unused entries in the RAM array to 00h.) You can then open the Takeover switch of the RAM control panel (so that the control panel no longer has control over the RAM array) and open the Clear switch. Then just sit back and watch the flashing lightbulbs.
Here's how it works: When the Clear switch is first opened, the address of the RAM array is 0000h. The 8-bit value stored in the RAM array at that address is an input to the adder. The other input to the adder is 00h because the latch is also cleared.
The oscillator provides a clock signal—a signal that alternates between 0 and 1 very quickly. After the Clear switch is opened, whenever the clock changes from a 0 to a 1, two things happen simultaneously: The latch stores the sum from the adder, and the 16-bit counter increments, thus addressing the next value in the RAM array. The first time the clock changes from 0 to 1 after the Clear switch is opened, the latch stores the first value and the counter increments to 0001h. The second time, the latch stores the sum of the first and second values, and the counter increments to 0002h. And so on.
Of course, I'm making some assumptions here. Above all, I'm assuming that the oscillator is slow enough to allow all the rest of the circuitry to