Z-Scale: Tiny 32-Bit RISC-V Systems with Updates to the Rocket Chip Generator

Z-scale: Tiny 32-bit RISC-V Systems With Updates to the Rocket Chip Generator

Yunsup Lee, Albert Ou, Albert Magyar 2nd RISC-V Workshop [email protected] 6/30/2015 1 What is Z-scale? Why?

§ Z-scale is a tiny 32-bit RISC-V core generator suited for microcontrollers and embedded systems § Over the past months, external users have expressed great interest in small RISC-V cores - So we have listened to your feedback! § Z-scale is designed to talk to AHB-Lite buses - plug-compatible with ARM Cortex-M series § Z-scale generator also generates the interconnect between core and devices - Includes buses, slave muxes, and crossbars

2 Berkeley’s RISC-V Core Generators

§ Z-scale: Family of Tiny Cores - Similar in spirit to ARM Cortex M0/M0+/M3/M4 - Integrates with AHB-Lite interconnect § Rocket: Family of In-order Cores - Currently 64-bit single-issue only - Plans to work on dual-issue, 32-bit options - Similar in spirit to ARM Cortex A5/A7/A53 - Will integrate with AXI4 interconnect § BOOM: Family of Out-of-Order Cores - Supports 64-bit single-, dual-, quad-issue - Similar in spirit to ARM Cortex A9/A15/A57 - Will integrate with AXI4 interconnect - BOOM talk right after this one

3 Z-scale Pipeline

WB PC DE IF MEM Gen. EX MUL

I-Bus I-Bus D-Bus D-Bus Request Response Request Response

§ 32-bit 3-stage single-issue in-order pipe § Executes RV32IM ISA, has M/U privilege modes § I-bus and D-bus are AHB-Lite and 32-bits wide § Interrupts are supported § Will publish a “microarchitecture speciﬁcation” 4 ARM Cortex-M0 vs. Z-scale

Category ARM Cortex-M0 RISC-V Zscale ISA 32-bit ARM v6 32-bit RISC-V (RV32IM) Architecture Single-Issue In-Order 3-stage Single-Issue In-Order 3-stage Performance 0.87 DMIPS/MHz 1.35 DMIPS/MHz Process TSMC 40LP TSMC 40GPLUS Area w/o Caches 0.0070 mm2 0.0098 mm2 Area Efficiency 124 DMIPS/MHz/mm2 138 DMIPS/MHz/mm2 Frequency ≤50 MHz ~500 MHz Voltage (RTV) 1.1 V 0.99 V Dynamic Power 5.1 µW/MHz 1.8 µW/MHz

§ Note: numbers are very likely to change in the future as we tune the design and add things to the core.

5 RV32E

§ New base integer instruction set - Reduced version of RV32I designed for embedded systems § Cut number of integer registers to 16 § Remove counters that are mandatory in RV32I - Counter instructions (rdcycle[h], rdtime[h], rdinstret[h]) are not mandatory

6 Building a Z-scale System

AHB-Lite JTAG Z-Scale Debugger Core APB

D Devices J-Bus I-Bus D-Bus

AHB-Lite P Peripherals Crossbar

S-Bus

NOR Boot SRAM Flash ROM AHB DRAM D D D APB § Working on P-Bus “platform speciﬁcation” P P P P P

7 Z-scale Generator is Written in Chisel

§ Chisel is a new HDL embedded in Scala - Rely on good software engineering practices such as abstraction, reuse, object oriented programming, functional programming - Build hardware like software § 604 Unique LOC written in Chisel - Control: 274 lines - Datapath: 267 lines (99 lines could be generalized) - Top-level: 63 lines § 983 LOC in Chisel borrowed from Rocket § Reuse and parameterization in Chisel and Rocket chip actually works!

8 Functional Programming 101

§ (1, 2, 3) map { n => n+1 } - (2, 3, 4) § (1, 2, 3) map { _+1 } § (1, 2, 3) zip (a, b, c) - ((1, a), (2, b), (3, c)) § (1, a)._1 - 1 § ((1, a), (2, b), (3, c)) map { _._1} § ((1, a), (2, b), (3, c)) map { n => n._1} - (1, 2, 3) § (0 until 3) foreach { println(_) } - 0/1/2

9 Functional Programming Example Used in AHB-Lite Crossbar masters(0) masters(1) AHB-Lite Crossbar masters(0,1) master AHB-Lite Bus AHB-Lite Bus AHB-Lite Bus

AHB-Lite Crossbar slaves(0,1,2)

ins(0,1) slaves(0,1,2) AHB-Lite Arbiter Arbiter Arbiter Slave Mux Arbiter Slave Mux Slave Mux Slave Mux

out slaves(0) slaves(1) slaves(2) class AHBXbar(n: Int, amap: Seq[UInt=>Bool]) extends Module { val io = new Bundle { val masters = Vec.fill(n){new AHBMasterIO}.flip val slaves = Vec.fill(amap.size){new AHBSlaveIO}.flip }

val buses = io.masters map { m => Module(new AHBBus(amap)).io } val muxes = io.slaves map { s => Module(new AHBSlaveMux(n)).io }

(buses.map(_.master) zip io.masters) foreach { case (b, m) => b <> m } (0 until n) foreach { m => (0 until amap.size) foreach { s => buses(m).slaves(s) <> muxes(s).ins(m) } } (io.slaves zip muxes.map(_.out)) foreach { case (s, x) => s <> x } } Z-scale in Verilog

§ Talked to many external users, and perhaps the #1 reason why they can’t use our stuﬀ is because it’s written in Chisel - So we have listened to your feedback! § We have implemented the same Z-scale core in Verilog § 1215 LOC § No more excuses for adoption! - If there still is any reason why you can’t use RISC-V, please do let us know

11 Z-scale FPGA DEMO System

0x8000_0800 JTAG Z-Scale CORE RESET Debugger Core (1KB) 0x8000_0400 GPIO LED J-Bus I-Bus D-Bus (1KB) 0x8000_0000 AHB-Lite Empty Crossbar 0x2400_4000 SPI FLASH (16KB) S-Bus 0x2400_0000 Boot DRAM ROM SPI AHB (64MB) DRAM FLASH APB 0x2000_0000 P-Bus Empty 0x0000_4000 Boot ROM AHB-Lite (16KB) GPIO CORE 0x0000_0000 APB LED RESET 12 Z-scale FPGA DEMO System Mapped to Xilinx Spartan6 LX9 § Avnet LX9 Microboard - $89 - Xilinx Spartan6 LX9 - 64MB LPDDR RAM - 16MB SPI FLASH Resource Used Percentage - 10/100 Ethernet Registers 2,329 20% - USB-to-UART LUTs 4,328 75% - USB-to-JTAG RAM16 8 25% - 2x Pmod headers RAM8 0 0% - 4x LEDs Test program is stored in bootrom. - 4x DIP switches It is a memory test program, which - RESET/PROG buttons writes 32-bit words generated from an LFSR to 64MB of DRAM, and § 4 boards for raﬄe! checks it by reading 64MB of data, and toggles LED if it succeeds. 13 Z-scale Use Cases

§ Microcontrollers - Implement your simple control loops - If code density matters § Embedded Systems - Build your system around Z-scale § Validation of Tiny 32-bit RISC-V Systems - You don’t need to use our code, just consider Z- scale as an existence proof and implement your own RV32I core § Both Chisel and Verilog versions of Z-scale is open-sourced under the BSD license - https://github.com/ucb-bar/zscale - https://github.com/ucb-bar/fpga-spartan6

14 What is the Rocket Chip Generator?

Tile Tile HTIF § Rocket Rocket HTIFIO Parameterized SoC Core Core ROCC ROCCIO Accel. generator written in FPU FPU HostIO ROCC Chisel Accel. L1 Inst L1 Inst L1 Data L1 Data § Generates n Tiles sets, sets, sets, ways ways ways - (Rocket) Core client client client client client - RoCC Accelerator - TileLink L1 I$ L1 Network arb - L1 D$ TileLinkIO TileLinkIO TileLinkIO Coherence Manager § Generates Uncore L2Cache L2Cache L2Cache L2Cache L2Cache mngr mngr mngr mngr mngr - L1 Crossbar

sets, sets, sets, sets, sets, - Coherence Manager ways ways ways ways ways - Shared L2$ with

client client client client client TileLink directory bits

mngr TileLinkIO TileLink / MemIO Converter - Exports a simple MemIO memory interface 15 Rocket Chip Generator Updates Since the 1st RISC-V Workshop

Tile Tile HTIF Rocket Rocket HTIFIO § Implemented L2$ with Core Core ROCC ROCCIO Accel. directory bits FPU FPU HostIO ROCC § RoCC coprocessor has Accel. L1 Inst L1 Inst L1 Data L1 Data sets, sets, sets, a memory port ways ways ways

client client client client client directly into the L2$

TileLink § Main development will L1 Network arb

TileLinkIO TileLinkIO happen on the rocket- TileLinkIO Coherence Manager L2Cache L2Cache L2Cache L2Cache L2Cache chip repository mngr mngr mngr mngr mngr

sets, sets, sets, sets, sets, § Moving towards ways ways ways ways ways standardized memory

client client client client client TileLink interfaces

mngr TileLinkIO TileLink / MemIO Converter MemIO

16 Important Memory Interfaces

§ TileLink - Our cache-coherent interconnect - For more details, watch my talk from last workshop § NASTI (pronounced nasty) - Not A STandard Interface - Our implementation of the AXI4 standard § HASTI (pronounced hasty) - Highly Advanced System Transport Interface - Our implementation of the AHB-Lite standard § POCI (pronounced pokey) - Peripheral Oriented Connection Interface - Our implementation of the APB standard

17 Rocket Chip Generator Grand Plan with Z-scale

RocketTile RocketTile

RoCC Rocket L1I$ Rocket L1I$ Accel. JTAG RoCC Debug CSR CSR Accel. File L1D$ File L1D$

L1 Network Z-scale L2$ Bank Cache- L2$ Bank Cache- L2$ Bank Coherent L2$ Bank DeviceCoherent AHB-Lite Bus Device

Low- Scratch SCR Speed L2 Network Pad File IO Device

TileLink/AXI4 TileLink/AXI4 AXI4/AHB AHB/APB Bridge Bridge Bridge Bridge

AXI4 Crossbar APB Bus

High- High- DRAM DRAM Speed Speed IO Peripheral Peripheral Peripheral Controller Controller IO Device Device 18 Conclusion, Future Work, and Raﬄe

§ Z-scale is a RISC-V tiny core generator suited for microcontrollers and embedded systems § Z-scale - Microarchitecture document will be released ﬁrst - Improve performance - Implement “C” extension as an option - Add MMU option to boot Linux - More devices on the LX9 board to come § Rocket Chip Generator - JTAG debug interface (get rid of HTIF) - Move to standardized interfaces (NASTI/HASTI/POCI) - Add Z-scale option § Raﬄe time!