How Does a Microprocessor Use Digital Logic to Execute Commands?

[jaydnul]

Tell me if I'm on the right track. Any command in a computer program when you break it down is really just a combination of different digital logic configurations (AND, OR, NAND, etc.). Then the microprocessor activates some of its transistors that are in the right configuration to perform that logical operation?

 

[CWatters]

That will do as a start.

It's possible to quibble with "the microprocessor activates some of its transistors that are in the right configuration". Perhaps better to say that the "microprocessor configures logic gates to perform that logical operation".

In many processor designs groups of bits (called fields) in the instruction word directly control the logical operation. For example suppose the microprocessor has 8 registers and you want to perform an operation on registers A and B and put the answer in C. Then 3 bits of the instruction might select Register A, another 3 bits select register B and another 3 the destination register C. A further 3+ bits might select the operation (AND, OR etc). When the instruction arrives in the microprocessor it might be held in an instruction register and the outputs of the register may literally be used to select which registers are connected to the arithmetic and logic unit and what logical function that performs.

 

[syhprum1]

I too was puzzled by this until I went on a computer course to Siemens Hell as part of my employment and we studied the R30 computers in detail, these were not micro processors but were built with separate plug in boards one of which was a 128 bit wide memory board that held the instruction set and I suddenly realized how the things worked.

 

[berkeman]

Tell me if I'm on the right track. Any command in a computer program when you break it down is really just a combination of different digital logic configurations (AND, OR, NAND, etc.). Then the microprocessor activates some of its transistors that are in the right configuration to perform that logical operation?

Be sure to include memory cells in your list of fundamental building blocks of computers. You can make memory cells out of logic gates (static memory cells), but more often they are made out of specialized cells (dynamic RAM cells).

 

[phinds]

Tell me if I'm on the right track. Any command in a computer program when you break it down is really just a combination of different digital logic configurations (AND, OR, NAND, etc.). Then the microprocessor activates some of its transistors that are in the right configuration to perform that logical operation?

The specific complement of logic gates is irrelevant, as long as they can perform all of the requisite operations. Your combination of AND, OR, and NAND could, for example, be reduced to just NAND alone since combinations of NAND gates can perform OR and AND, so it would just take more transistors total.