Pipelining instructions: Chapter 4 of B&H

srcA, srcB, valE, and valM are intermediate registers, used
by the hardware.  (There are other registers, too, but these
are the most important.)

PC holds the address of the "next" instruction.

The 5-Stage B&H Instruction Pipeline:

Fetch:   Using the program counter (PC), get the next instruction.

Decode:  Determine what instruction (add, mov, etc.) this is
         and put the operands into registers srcA and srcB.

Execute: Using the operands srcA and srcB and the opcode, 
         determine the result valE and condition codes 
         (SF, LF, etc.).  
         The result of a jump is computed here, too, and
         is used to change the PC if necessary.

Memory:  Read from memory (into valM) or write to memory if 
         necessary, depending on the opcode and the result in valE.

Writeback: Put valE or valM into a register (%eax, %ebx, etc.).