We have now wired up the Control Section of our CPU. As a result, we can place a series of instructions in RAM, and the Clock, Stepper, Instruction Register and wiring will fetch and execute those instructions. Here is the entire control section:
Yes, this looks pretty complicated, but we have l...
There is one annoying detail that we need to have here. When you do addition or shifting, you have the possibility of getting the carry flag turned on by the operation. This is necessary, we use it for the Jump If instruction as in the previous chapter.
The Carry Flag is also used as an input to ...
Here is the third, and last, invention that makes a computer a computer.
This is just like the Jump Instruction, but sometimes it jumps, and sometimes it doesn't. Of course, to jump or not to jump is just two possibilities, so it only takes one bit to determine which will happen. Mostly what we a...
This is another type of Jump instruction. It is similar to the Data instruction in that it uses two bytes. It replaces the IAR with the byte that is in RAM immediately following the instruction byte, thus changing where in RAM the next and subsequent instructions will come from. Here is the Instr...
The first great invention is this idea of having a string of instructions in RAM that get executed one by one by the CPU. But our clock is very fast, and the amount of RAM we have is limited. What will happen, in far less than a second, when we have executed every instruction in RAM?
Fortunately,...
Now here is an interesting instruction. All it does is load a byte from RAM into a Register like the Load instruction, above. The thing that is different about it though, is where in RAM it will get that byte.
In the Data instruction, the data comes from where the next instruction ought to be. So...
The Load and Store instructions are pretty simple. They move a byte between RAM and a register. They are very similar to each other so we will cover both of them in one chapter.
We'll get to the details of these instructions in a moment, but first we need to have something that tells us when we h...
We now have this new register, called the Instruction Register, which contains a byte that is going to tell the Control Section what to do. The patterns that are put into this register have a meaning. Sounds like another code, and indeed, it is. This code will be called the "Instruction Code."
Si...
This first type of instruction is the type that uses the ALU like our ADD operation earlier. As you recall, the ALU has eight things it can do, and for some of those things it uses two bytes of input, for other things it only uses one byte of input. And in seven of those cases, it has one byte of...
What we need is some way to do different operations from one stepper sequence to the next. How could we have it wired up one way for one sequence, and then a different way for the next sequence? The answer, of course, is to use more gates. The wiring for one operation can be connected or disconne...