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z15 - Microarchitectures - IBM
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z15 µarch
General Info
Arch TypeCPU
DesignerIBM
ManufacturerGlobalFoundries
IntroductionSeptember 12, 2019
Process14 nm
Core Configs12
Pipeline
TypeSuperscalar, Pipelined
OoOEYes
SpeculativeYes
Reg RenamingYes
Instructions
ISAz/Architecture
Cache
L1I Cache128 KiB/core
8-way set associative
L1D Cache128 KiB/core
8-way set associative
L3 Cache256 MiB/chip
32-way set associative
L4 Cache960 MiB/drawer
60-way set associative
Succession

z15 is the successor to the z14, a 14 nm z/Architecture mainframe microarchitecture designed by IBM and introduced in 2019.


Process Technology

IBM fabricates its z15 microprocessors and system controllers on GlobalFoundries's 14 nm (14HP) FinFET Silicon-On-Insulator (SOI) process featuring highly-dense deep trench structures used for high-density eDRAM.

Release Dates

The z15 was launched by IBM on September 12, 2019. General availability of the z15 mainframe started September 23.

Architecture

Key changes from z14

  • Mainframe
    • Up to 190-way multiprocessing (from 170-way)
    • Less CPs per drawer (4, down from 6)
    • Less CPs per logical cluster (2, down from 3)
    • 1 more drawer (5, up from 4)
    • 1-4 racks (up from 2)


  • Central Processor (CP)
    • 2 more cores (12, up from 10)
    • Core
      • 10-13% higher IPC (IBM claim)
      • Front-end
        • Improved branch predictor
          • New TAGE predictor
          • BTB pre-buffer (BTBp) replaced by a simpler write buffer
            • single double-bandwidth port (two independentread ports)
          • 2x larger L1 BTB (8 sets of 2K rows, up from 4 sets of 2K rows)
      • Back-end
        • Larger GCT (60 groups, up from 48 groups)
          • Wider retire (12 instructions/cycle, up from 10)
        • Larger Issue Queues (2 x 36-entry, up from 2 x 30-entry)
        • 2x larger mapper (128-entry, up from 64-entry)
        • Larger integer physical register files (???, up from 120 entries)
        • Larger vector physical register files (???, up from 127 entries)
      • Execution engine
        • Wider execute (12 instructions/cycle, up from 10)
        • New Modulo Arithmetic (MA) unit
      • Memory subsystem
        • 2x larger L2 instruction cache (4 MiB, up from 2 MiB)
        • 2x larger 2 GiB pages STLB (256-entry, up from 64 entries)
    • Shared L3
      • 2x larger L3 (256 MiB, up from 128 MiB)
    • I/O
      • GX Bus removed
      • X Bus removed (2 interface, down from 3)
      • New PCIe Gen interface (3 interfaces, up from 2)
    • Memory
      • Larger memory support (40 TiB, up from 32 TiB)
    • New integration
      • Nest Acceleration Unit (NXU)


  • System Controller (SC)
    • 1.4x Larger L4 cache (960 MiB, up from 672 MiB)
      • Non-exclusive (from inclusive)

Block Diagram

CP Chip

z15 chip block diagram.svg

Individual core

z15 block diagram.svg

Memory Hierarchy

The z15 features a memory structure very similar to the z14.

  • Cache:
    • L1 instruction cache
      • 128 KiB, 8-way set associative
      • 64 sets, 256 B line size
    • L1 data cache
      • 128 KiB, 8-way set associative
      • 64 sets, 256 B line size
    • L2 instruction cache
      • 4 MiB, 8-way set associative
      • 2K sets, 256 B line size
    • L2 data cache
      • 4 MiB, 8-way set associative
      • 2K sets, 256 B line size
    • L3 cache
      • 256 MiB, 32-way set associative
      • 32K sets, 256 B line size
    • L4 cache (off-chip, on the SC chip)
      • 960 MiB, 40-way set associative
      • 64K sets, 256 B line size
      • Non-exclusive (almost always inclusive, but not strictly)
    • System Memory
  • TLBs:
    • ITLB
      • 4 sets x 128 entries
    • DTLB
      • 4 KiB pages
        • 4 sets x 256 entries
      • 1 MiB pages
        • 4 sets x 128 entries
      • 2 GiB pages
        • 2 sets x 32 entries
    • STLB
      • 4 KiB pages
        • 6K entries
      • 2 GiB pages
        • 256 entries
      • Segment Table
        • 8 sets x 256 entries
      • Page Table
        • 8 sets x 256 entries

Overview

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Mainframe

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System

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Drawer

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Central Processor

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Core

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Die

Central Processor (CP) Chip

  • 14HP FinFET on SOI
    • 17 metal layers
  • 9,200,000,000 transistors
  • 5.2 GHz
  • 12 cores
  • Die size
    • 25.3 mm x 27.5 mm
    • 695.75 mm²


z15 cp floorplan.png

Core

z15 core floorplan.png

System Controller (SC) Chip

  • 14HP FinFET on SOI
    • 17 metal layers
  • 9,700,000,000 billion transistors (note that this number, from the technical document, is likely incorrect as it's the same number as the z14)
  • 960 MiB shared eDRAM L4 cache.
  • Die size
    • 25.3 mm x 27.5 mm
    • 695.75 mm²


z15 sc floorplan.png
codenamez15 +
core count12 +
designerIBM +
first launchedSeptember 12, 2019 +
full page nameibm/microarchitectures/z15 +
instance ofmicroarchitecture +
instruction set architecturez/Architecture +
manufacturerGlobalFoundries +
microarchitecture typeCPU +
namez15 +
process14 nm (0.014 μm, 1.4e-5 mm) +