Processor main parts
datapath
control
datapath
caries out arithmetic and other operations, moves operands and results around
control
task to determine the appropriate value of each control unit within the datapath
datapath components include
ALU, registers, cache memory, multiplexors, plus others
time for one step of a machine language instructions to execute
one clock cycle
delimiting steps
use storage elements that are stored in such a way that the stored values can change only at the end of a clock cycle
combinational circuits
output is only a function of the current input
sequential circuits
output depends both on the current input and on the current circuit “state” (value stored), as set by previous inputs
S-R latch
S=1, R=0
store a 1 in the circuit (the output Q=1)
S-R latch
S=0, R=1
store 0 in the circuit
the output Q=0
S-R latch
S=0, R=0
Cause no change to stored value
D latch
C input = 0
no change to the stored value
D latch
C input = 1
D input value is stored
D flip-flop
The D input value is stored when
only when the clock input C changes from 1 to 0
Using D flip-flops we can build
storage elements whose stored values can change only at the end of a clock cycle
Simple single clock cycle datapath
each instruction executes in a single clock cycle
contains only
* PC register
* adder for incrementing PC by 4
* instruction memory component
* register file
* an ALUThe clock cycle must be long enough so
the adder, register file, memory, ALU, all have time to do their work
Control units within the datapath includes
- read/write enable control inputs for storage elements
- selector inputs for multiplexors
- ALU control units
Control for single clock cycle datapath
uses op and funct fields from instruction
