Background review and context of the problem that we are discussing
 

Previously, we studied a circuit that switch the registers' value (= outputs) to the inputs of the ALU:

Background review and context of the problem that we are discussing
 

In this set of slides, we will study a circuit that stores the ALU output into the correct destination register:

Introduction
 

  • In this slide set, I will show you a circuit used to select the destination register that will be updated by a computer instruction

  • In order to understand this material, you need knowledge on how a register (or computer memory) is "written" (= updated)

Operation of a one-bit memory cell
 

A one-bit memory cell has 2 inputs (data input and write control) and 1 outputs:

When write=0, the output will remain unchanged (= remembers)
When write=1, the output become equal to the input (= updated)
DEMO: /home/cs355001/demo/circuits/1-bit-memory

Registers and computer memory cells
 

A register (or memory cell) consists of N one-bit memories under control by a write signal:

DEMO: /home/cs355001/demo/circuits/4-bit-memory

(The demo shows a 4 bit memory cell, it's easy to make an 8 bit or 32 bit memory cell)
Note: we will study the internals of memory cells later

What we will do next
 

Now that you have seen how memory cells are written (= updated), I will next:

  • Show you how the CPU selects the correct register to update according to the instruction code

    I.e.:

      • How does the CPU know that it needs to update register 1 when it executes add r1, r0, r1 ???

      • How does the CPU know that it needs to update register 3 when it executes add r3, r0, r1 ???

Instruction encoding: how a computer instruction is expressed
 

Each CPU has its own instruction encoding format.

This is the ARM instruction encoding format (each ARM instruction is 32 bits long)

How the CPU uses the different fields in the ARM instruction
 

The Rd field is used to select the destination operand for output of the ALU:

 

Destination register field in the ARM instruction
 

Sample ARM add assembler instructions: 

   3                    main:
   4 0000 081083E0              add r1, r3, r8
   5 0004 085083E0              add r5, r3, r8
   6 0008 089083E0              add r9, r3, r8

Can you spot the place of the destination register number field in the ARM's "add" instruction ???

Destination register field in the instruction
 

Here they are: (bytes stored in little endian ordering !) 

   3                    main:
   4 0000 081083E0              add r1, r3, r8
   5 0004 085083E0              add r5, r3, r8
   6 0008 089083E0              add r9, r3, r8

Can you spot the place of the destination register number field in the ARM's "add" instruction ??? (Did you find the field location ?)

Prelude: we omit the selection of registers for the ALU input

Selecting the (outputs of) registers to the (input of the) ALU has been discussed:

Prelude: we omit the selection of registers for the ALU input

I will only present this switching circuit in the lecture material (for brevity):

How does the CPU use the destination register field to select a register

Circuit used to update the destination register with the values (= output) of the ALU:

I will explain this circuit in the next few slides...

Circuit to store the ALU output to the destination register in instruction

Problem description:

Circuit to store the ALU output to the destination register in instruction

Connect the output bit of ALU to the corresponding input bit of each register:

Circuit to store the ALU output to the destination register in instruction

What have we achieved:

Circuit to store the ALU output to the destination register in instruction

The CPU has a write signal used to control the time it wants to update registers:

Circuit to store the ALU output to the destination register in instruction

Use a decoder to select the destination register to write:

Circuit to store the ALU output to the destination register in instruction

This AND gate will allow the CPU to control sending a write signal to R0:

Circuit to store the ALU output to the destination register in instruction

This AND gate will allow the CPU to control sending a write signal to R1:

Circuit to store the ALU output to the destination register in instruction

This AND gate will allow the CPU to control sending a write signal to R2:

Circuit to store the ALU output to the destination register in instruction

This AND gate will allow the CPU to control sending a write signal to R3:

DEMO

Demo circuit looks like this:

DEMO file: /home/cs355001/demo/circuits/alu-reg