Introduction

The datapath is the piece of circuitry in the CPU responsible for data manipulation, registers storage and flags. The datapath is steered by the control unit to perform the CPU instructions.

Datapath in more detail

As we saw earlier the datapath is responsible to provide data operations for the control unit, now we're going to analyse it in greater detail. By describing each main component.

The signals that control the datapath operation, forms a word called Control Word, that will be generated by the Control unit, for each instruction.

Multiplexer

This element is responsible to select the signals from the external world, from the registers, or from the imediate operand.

Register File

This element will hold the 16 16-bit registers used in our CPU

ALU

This combinational circuit is responsible to provide the arithimetic and logic operations of the datapath. The table bellow will list all ALU operations.

ALU operations
code operation Description
op_pass ALUOutput = A Does nothing
op_and ALUOutput = A and B And operation
op_or ALUOutput = A or B Or operation
op_not ALUOutput = not A Not operation
op_add ALUOutput = A + B Addition
op_sub ALUOutput = A - B Subtraction
op_inc ALUOutput = A++ Increment
op_dec ALUOutput = A-- Decrement

Shifter

This element will provide the shift (left,right) and rotate operations.

Shifter operations
code operation Description
op_pass ALUOutput = A Does nothing
op_shift_left ALUOutput = A << 1 Shift left
op_shift_right ALUOutput = A >> 1 Shift right
op_rotate_right ALUOutput = A ror 1 Rotate right

Flags circuit

This combinational circuit is responsible to provide tha flags (Zero, Carry, Sign and overflow) based on the datapath results.

Tristate Buffer

This buffer is used to apply logic levels high, low, impedance on the external line, this is used to allow a single external interface act as a input/output port.

Control Word

Each datapath module has is own control signals, all of those signals form the control word, the idea now is to find the control word that executes all instructions from our instruction set. In our case this is our Control Word.

Control Word bits
SelMux RFWriteAdd RFReadAAdd RFReadBAdd RFReadAE RFReadBE AluOP ShifterOP WriteRF OE

sel_External

sel_Imediate

sel_AluOut

 r0..r15 r0..r15 r0..r15 Enable port A reading Enable port B reading

op_pass

op_and

op_or

op_not

op_add

op_sub

op_inc

op_dec

op_pass

op_shift_right

op_shift_left

op_rotate_right

 Write value in Register File Output Enable

Simple Algorithm sample

Now we're going to execute a simple algorithm generating all ControWords needed. Here is all simple algorithm ....

     r1 = 5;
     input r2;       // Consider the external input to be 1, so this while will only run once (r2=1)
     while (r2 != 0)
     {
       r1 = r1 + r2;
       r2--;
     }
     output r1;      // Output should be 6
     output r2;      // Output should be 0

Now let's derive all control word bits needed to execute this algorithm.

Line 1 (r1 = 5)

Line 2 (input r2)

Line 3 (r1 = r1 + r2)

Line 4 (r2--)

Line 5 (output r1)

Line 6 (output r2)

Bellow we have a figure of the datapath been simulated with ModelSim

Timming Issues

Before we see the Control Unit we must know one of the most important problems when designing digital circuits. When the control unit is interfacing with the Datapath it generates control words for every instruction and the Datapath respond with flags results. Some care must be taken to ensure that the datapath generated the correct result. For exemple when you write a value to some register the proper value will be avaible only on the next clock cycle.

In order to correct timming issues you can add an extra state to wait for the valid result, or to place your flag circuits in other places. Let's see a real exemple to ilustrate this.

Imagine this scenario that we write to the register A the value '5' and in the next instruction it tests the zero flag.

A = 5
if (A == 5)
//Do something

Our Datapath look like this:

In the exact moment that we write in register A the value 5 the (A=5) flag will not present the correct result, actually the value 5 will be avaible in the register in the next clock cycle. The solution for this problem could be add some wait cycle in the control unit in order to get things settle down. Other solution is to replace the flag circuit directly in the input.

Next topic:

Now that we defined our Datapath we're going to see how the Control Unit works

Control Unit