Chapter 3 Flashcards
What is a computer’s most elementary sequential element?
What is this element’s interface?
How is a register implemented?
what is the basic mechanism for remembering a single bit over time?
A 1 bit register.
What does the interface of a RAM chip look like?
see figure 3.3
A classical RAM device accepts three inputs: a data input, an address input, and a load bit. The address specifies which RAM register should be accessed in the current time unit. In the case of a read operation (load=0), the RAM’s output immediately emits the value of the selected register. In the case of a write operation (load=1), the selected memory register commits to the input value in the next time unit, at which point the RAM’s output will start emitting it.
How does a counter work?
What is it’s interface?
A counter chip is designed to contain the address of the instruction that the computer should fetch and execute next.
In most cases, the counter has to simply increment itself by 1 in each clock cycle, thus causing the computer to fetch the next instruction in the program. In other cases, for example, in ‘‘jump to execute instruction number n,’’ we want to be able to set the counter to n, then have it continue its default counting behavior. Finally, the program’s execution can be restarted anytime by resetting the counter to 0, assuming that that’s the address of the program’s first instruction. In short, we need a loadable and resettable counter.
With that in mind, the interface of our Counter chip is similar to that of a register, except that it has two additional control bits labeled reset and inc. When inc=1, the counter increments its state in every clock cycle, emitting the value out(t)= out(t-1)+1. If we want to reset the counter to 0, we assert the reset bit; if we want to set the counter to a specific number, we put that number in the input and assert the load bit.
