CS61CL Machine Structures Lec 8 – State and Register Transfers David Culler Electrical Engineering and Computer Sciences University of California, Berkeley

CS61CL Road Map 10/14/09 CS61CL F09 2 Hardware Software Machine Lang. pgm Instruction Set Architecture Machine Organization HLL Program Asm Lang. Pgm Compiler Assembler foo.c foo.s foo.o I/O systemInstr. Set Proc. Digital Design Circuit Design Datapath & Control Layout & fab Semiconductor Materials foo.exe Linker

Review: Combinational Logic Any boolean function can be expressed as an acyclic connection of gates Often specified by a truth table Outputs are purely a function of the inputs –no history, no state 10/14/09 CS61CL F09 3 Combinational Logic inputs outputs inputs outputs k 2k2k

Examples: Logical Operations 10/14/09 CS61CL F09 4 C = A & B A B C A 31 B 31 C 31 A 30 B 30 C 30 A0A0 B0B0 C0C0 °°° A 31:0 B 31:0 C 31:0 A 31:0 B 31:0 C 31:0 A 31:0 B 31:0 C 31:0 A 31:0 C 31:0

Example: Multiplexor 10/14/09 CS61CL F09 5 C = S ? A : B A B C S C = (S & A) | (~S & B) S A 31:0 B 31:0 C 31:0

Example: Adder 10/14/09 CS61CL F09 6 A B Ci Co S A B Ci Co S A B Ci Co S A B Ci Co S A B Ci Co S A 31:0 B 31:0 C 31:0

Example: Arithmetic Logic Unit 10/14/09 CS61CL F09 7 A 31:0 B 31:0 C 31:0 S 1:0

ALU 10/14/09 CS61CL F09 8

10/14/09 CS61CL F09 9 Element of Time Logical change is not instantaneous Broader digital design methodology has to make it appears as such –Clocking, delay estimation, glitch avoidance V out +3 0 T Propagation delay

10/14/09 CS61CL F09 10 What makes Digital Systems tick? Combinational Logic time clk

Administrative Issues HW 6 due tonight Project 2 dues Monday 10/26 –bimodal check-off –testing tools available tomorrow –they are really picky Project 1 grading almost done –Friday HW 7 – discuss Midterm 2 on 11/9 as in original schedule –11/11 is holiday 10/14/09 CS61CL F09 11

8/30/ A Bit of state: D-type edge-triggered flip-flop The edge of the clock is used to sample the "D" input & send it to "Q” (positive edge triggering). –At all other times the output Q is independent of the input D (just stores previously sampled value). –The input must be stable for a short time before the clock edge

9/18/07 13 RSRSRS DQDQDQDQ OUT1OUT2OUT3OUT4 CLK IN1IN2IN3IN4 RS "0" Registers Collections of flip-flops with similar controls and logic –Stored values somehow related (e.g., form binary value) –Share clock, reset, and set lines –Similar logic at each stage

What “registers” do we need? “read” vs use the output “write” on the clock edge => Load Load Control 10/14/09 CS61CL F r0 r1 ° r31 PC lo hi Programmable storage 2^32 x bytes 31 x 32-bit GPRs (R0=0) 32 x 32-bit FP regs (paired DP) HI, LO, PC

Register with Load Control 10/14/09 CS61CL F09 15 R0R0 R 31 clock load 1 0

Register File 10/14/09 CS61CL F09 16 R0 °°° R1 R2 R31 Asel Bsel Bout Aout Dsel 1 ld Din decoder

Towards a Data Path 10/14/09 CS61CL F09 17 °°° Asel Bsel Dsel ld aluOP

Exercise a Data Path 10/14/09 CS61CL F09 18 °°° Asel Bsel Dsel ld aluOP

What about RAM - Randomly Accessed Memory? Like a HUGE register file –dense, slower, low-cost storage cell (6T) –fewer ports –wider address lines –accessed over a “bus” Bus: means of composition in hardware system –logically related collection of wires –interfacing one or more sources to one or more destinations 10/14/09 CS61CL F09 19 RAM data address

Recall: Instruction Cycle 9/16/09 UCB CS61CL F09 Lec 4 20 °°° : FFF..F: n: 0B20 0B20: Instruction Fetch Execute PC “add $1,$2,$3” Operand Result Next Decode + 0B24 main:

Register Transfers 10/14/09 CS61CL F09 21

10/14/09 CS61CL F09 22 Synchronous Circuit Design clock –distributed to all flip-flops ALL CYCLES GO THROUGH A REG! Combinational Logic Blocks (CL) –Acyclic –no internal state (no feedback) –output only a function of inputs Registers (reg) –collections of flip-flops