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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 Configs9, 10, 11, 12
Pipeline
TypeSuperscalar, Pipelined
OoOEYes
SpeculativeYes
Reg RenamingYes
Stages17
Decode6-wa8
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[edit]

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[edit]

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

Architecture[edit]

Key changes from z14[edit]

  • 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)
    • 0-12 PCIe+ I/O drawers (up from 0-5 drawers)
    • Rack
      • standard 19" racks (from 24-inch racks)
      • 1-4 frame configs (up from only 2-frame configs)
      • rear only I/O and power connections (from front & rear)


  • 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[edit]

CP Chip[edit]

z15 chip block diagram.svg

Individual core[edit]

z15 block diagram.svg

Memory Hierarchy[edit]

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[edit]

Introduced in early September 2019, the z15 mainframe is the successor to the z14. The z15 is designed to improve both single-thread and multi-core performance as well as introduce better data security features for pervasive encryption through new hardware acceleration.

Under construction icon-blue.svg This article is a work in progress!


Mainframe[edit]

The z15 mainframe has changed from the z14 in a number of important ways. IBM switched from a 24-inch to standard 19-inch rack chassis, meaning they can now be incorporated into standard data center aisles. For this, IBM provides new options for hot and cold aisle configurations. Unlike the last few generations which came in only 2-frame options, the new z15 is intended to scale from one to four frames depending on the desired configuration. Additionally, the z15 comes with 1 to 5 CPC drawers.

Models[edit]

The z15 comes in a single model - T01. There are 5 feature codes.

z15 T01 Configurations
Feature
Code
Feature
Name
CPC Drawers Core Spare Cores SAPs Memory Frequency Max PCIe
fanout
0655 Max34 1 34 2 4 512 GiB - 7.75 TiB 5.2 GHz 12
0656 Max71 2 71 2 8 512 GiB - 15.75 TiB 5.2 GHz 24
0657 Max108 3 108 2 12 512 GiB - 23.75 TiB 5.2 GHz 36
0658 Max145 4 145 2 16 512 GiB - 31.75 TiB 5.2 GHz 48
0659 Max190 5 190 2 22 512 GiB - 39.75 TiB 5.2 GHz 60

Note that the z15 reserves a fixed amount of 256 GiB (0.25 TiB) for the hardware system area (HSA).

Frames[edit]

The z15 supports anywhere from one to four frames. The frames are labeled Z, A, B, and C.

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System[edit]

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Drawer[edit]

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Hardware system area (HSA)[edit]

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Central Processor[edit]

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Core[edit]

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Die[edit]

Central Processor (CP) Chip[edit]

  • 14HP FinFET on SOI
    • 17 metal layers
  • 9,200,000,000 transistors
    • 15.6 miles of wire
    • 26,200,000,000 wire connections
  • 5.2 GHz
  • 12 cores
  • Die size
    • 25.3 mm x 27.5 mm
    • 695.75 mm²


z15 cp floorplan.png

Core[edit]

z15 core floorplan.png

System Controller (SC) Chip[edit]

  • 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 count9 +, 10 +, 11 + and 12 +
designerIBM +
first launchedSeptember 12, 2019 +
full page nameibm/microarchitectures/z15 +
instance ofmicroarchitecture +
instruction set architecturez/Architecture +
manufacturerGlobalFoundries +
microarchitecture typeCPU +
namez15 +
pipeline stages17 +
process14 nm (0.014 μm, 1.4e-5 mm) +