Computer architecture Lecture 6: Processor’s structure Piotr Bilski

Procesor’s tasks: Instruction fetching Instruction interpretation Data fetching Data processing Data saving These justify existence of the registers (temporary memory space)

Internal processor’s structure Registers Control Unit ALU Status flags Shifter Complementer Arithmetic and Boolean Logic

Block Scheme of Pentium 3 Processor

Block Scheme of P6 Core (Pentium Pro) – 1995 r. Front-end of the processor Core Completion unit

Register types Accessible for the user (addressing, data etc.) Inaccessible for the user (control, status) This categorization is not formal!

Registers accessible by the user General Purpose Registers (GPR) Data Addressing (segment pointer, stack, indexing) Conditional codes (state pointer, flags) – read-only!

Control and state registers Basic: –Program Counter (PC) –Instruction Decoding Register (IR) –Memory Address Register (MAR) –Memory Buffer Register (MBR) Program Status Word (PSW) Interrupt Vector Register Page Table Pointer

Program Status Word S – sign bit Z – bit set, if operation result is zero P – carry bit R – logical comparison result bit O – overflow bit I – Enable/disable interrupt execution N – supervisor mode SZ PR OIN OTHER

Registers in the Motorola MC68000 processor Data and address registers (32-bit) Specialization: 8 data registers (D0-D7) and 9 address registers (two used interchangeably in the user and supervisor modes) Control bus 24-bit, data bus 16-bit A7 register used as a Stack Pointer (SP) State register (SR)16-bit (another name: CCR) Program counter (PC) 32-bit Instructions are stored under even addresses

Registers in the Intel 8086 Processor 16-bit address and data registers Data/General Purpose Registers (AX, BX, CX, DX) Pointer and index registers (SP, BP, SI, DI) Segment registers (CS, DS, SS, ES) Instruction pointer State register

Intel 8086 Registers (cont.) AX BX CX DX Accumulator Base Counting Data SP BP SI DI Stack pointer Base pointer Source index Displ. ndex

Intel Pentium Processors Registers Organization 32-bit data and address registers Eight General Purpose Registers (EAX, EBX, ECX, EDX, ESP, EBP, ESI, EDI) For the backward compatibility, the lower part of the registers are 16-bit registers 32-bit status register 32-bit instruction pointer

Floating-point registers of the Pentium processor Eight 80-bit numerical registers 16-bit control register 16-bit state register 16-bit floating point register content type word 48-bit instruction pointer 48-bit data pointer

EFLAGS register TF – trap flag IF – interrupt enable flag DF – direction flag IOPL – privileged input/output flag RF – resume flag AC – alignment control ID – identification flag CFCF PFPF AFAF ZFZF SFSF TFTF IFIF DFDF OFOF IO F NTNT 0 15 RFRF VMVM ACAC VI F VI P IDID 21 31

Registers in the Athlon 64 processor Compatibility with x86-64 architecture (40-bit physical address space, 48-bit virtual address space) Data and address registers 64-bit 8 general purpose registers (RAX, RBX, RCX, RDX, RBP, RSI, RDI, RSP), work in the 32-bit compatibility mode Opteron contains additional 8 general purpose registers (R8-R15) 16 SSE registers (XMM0-XMM15) 8 floating-point registers x87, 80-bit

Registers in the PowerPC processor 32 general purpose registers (64-bit) + exception register (XER) 32 registers for the floating point unit (64- bit) + state and control register (FPSCR) Branch processing unit registers: 32-bit condition register, 64-bit counting and binding registers

Instruction mode Instruction fetch Instruction address calc. Instruction decoding Argument address calc. Argument fetching Data operation Interrupts checking Interrupt handling Argument address calc. Writing argument Instruction executed, fetch the next one Multiple arguments Multiple results No interrupts Return to data Indirect addressing

Instruction fetching cycle Processor MAR MBR CU Memory Address bus Control bus Data bus PC IR

Indirect mode Processor MAR MBR CU Memory Address bus Control bus Data bus

Interrupt mode Processor MAR MBR CU Memory Address bus Control bus Data bus PC

Pipeline Problem: during the instruction cycle only one instruction is processed Solution: divide the cycle into smaller fragments Condition: time instants, when no main memory access is required! Cycle 1 Cycle 2 Cycle 3

Pipeline example - laundry LADR PA LA DR PA LA DR PA CYCLE 1 CYCLE 2 CYCLE 3 LADR PA 3 hours / cycle – 9 hours for all 3 hours / cycle – 5 hours for all !!

Prefetch NOTE: acceleration is smaller than double, as the memory access lasts longer than the instruction execution Instruction fetch Execution Instruction Result Instruction fetching Execution Instruction Result Waiting New address Denial

Basic phases of the instruction cycle: Instruction fetching (FI) Instruction decoding (DI) Operands calculation (CO) Operands fetching (FO) Instruction execution (EI) Writing outcome (WO) FI DI CO FO EI WO I1 I2 I3 I4

Branches and pipelining FI DI CO FO EI WO FI DI CO FO FI DI CO I1 I2 I3 I4 I5 I6 I21 I22 FI DI CO FO EI WO FI DI FI

Pipeline implementation algorithm

Problems of the pipelining Subsequent pipe phases don’t last the same amount of time Transferring data between the buffers may significantly increase pipeline execution time Dependency between the registers and memory in the pipeline optimization may be minimized with high stakes

Efficiency of the pipelining Cycle execution time: Time required to execute all the instructions: Instruction pipeline acceleration ratio:

Example of the pipeline efficiency

Modern Processors Pipelines Pentium 3 – 10 stages Athlon – 10 stages for ALU, 15 stages for FPU Pentium M – 12 stages Athlon 64/ 64 X2 – 12 stages for ALU, 17 stages for FPU Pentium 4 Northwood – 20 stages (hyperpipeline!!) Pentium 4 Prescott – 31 stages Core2Duo – 14 stages

Hazards They are pipelining disturbances There are data, resources and control hazards

Branch handling Pipeline multiplication Prefetch of the instruction Loop buffer Branch prediction Delayed branch

Multiplied pipelining Both instructions for simultaneous processing as a result of branch are loaded into two pipelines The main problem is to gain memory access for both instructions

Prefetch and loop buffer When branch instruction is decoded, the target instruction is fetched. It is stored until the branch is executed A buffer in memory to store the subsequent instructions is created It is useful when there are conditional branch instructions and loops involved Loop buffer Prefetch

Conditional Branch Prediction Static –Never occuring branch (Sun SPARC, MIPS) –Always occuring branch –Operation code prediction Dynamic –Occured/Didn’t occur switch –Branch history table

Static prediction The simplest, used as the fallback method, for instance in the Motorola MPC7450 processor Pentium 4 allowed inserting the code suggesting if the static prediction should point at the branch or not (so-called prediction hint)

Dynamic prediction of the conditional branches A conditional branch instruction history is stored It is represented by the bits stored in the cache memory Every instruction has its own history bits Another solution is the table storing informations about the conditional branch result

History bits prediction

Branch history table Branch instruction address History bits Target instruction

Local Branch Prediction Requires a separate history buffer for each instruction, although the history table can be common for all instructions Pentium MMX, Pentium 2 i 3 processors have local prediction circuits with 4 history bits and 16 positions for every type of instruction Local prediction efficiency is estimated at 97 %

Global Branch Prediction A common history for all branch instructions is stored in memory. It allows to consider dependencies between different branch instructions Rarely a better solution than the local prediction Hybrid solutions: shared unit of the global prediction and the history table (AMD processors, Pentium M, Core, Core 2)

Branch Prediction Unit A processor circuit responsible for prediction of the disturbances in the sequential code execution Often connected with the microoperation cache memory In Pentium 4 processor, the buffer for the branch prediction has 4096, in Pentium 3 – only 512. Therefore the former has a 33 percent better hit ratio than the latter

Location of the Branch Prediction Unit