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ppcpu - Pipelined PCPU

This is a third revision of 16-bit PCPU processor with custom, deisgned from scratch RISC-type architecture.

It is a direct upgrade to pcpu project - it uses almost the same instrucion set, but inside is cleaner, more modern and advanced pipelined procesor (making it 40x faster). Additionaly it features caches and multi-core operation.

Features

  • 16-bit custom RISC type architecture
  • 4-stage pipelined microarchitecture
  • Multi-core operation
  • Memory paging, virtualization and protection
  • 24-bit address space of Compressed Wishbone Bus
  • Internal and external interrupts
  • Bootloader
  • Instruction and data caches

Processor Structure

rtl/ directory contains code of dual-core PPCPU processor, with data and instruction caches.

Core

Architecture of core pipeline:

  [ FETCH             ]      [ DECODE                         ]     [ EXECUTE                 ]      [ MEMORY/REGISTER WRITEBACK   ]
I [ BRANCH PREDICTOR  ]      [ INSTRUCTION TO CONTROL SIGNALS ]     [ [REGISTER FILE]->[ALU]->]      [ REGISTER FILE WRITEBACK     ] D
N [                   ]      [                                ]     [             IMM->[ALU]  ]      [ MEMORY ACCESS STATE MACHINE ] A
S [                   ]  ==> [                                ] ==> [ BRANCH RESOLUTION       ]  ==> [                             ] T
T [                   ]      [                                ]     [ HAZARD DETECION         ]      [                             ] A
R [                   ]      [                                ]     [ SREGS-CONTROL REGISTERS ]      [                             ]
  [                   ]      [                                ]     [ MUL/DIV UNIT            ]      [                             ]

More details about supported instructions and SREG functions are located inside docs/ directory

Inter-Core connections

  [CORE 0]<->[I&D MMU]<->[                      ]     [                    ] <>  [    MAIN OUTSIDE CW BUS            ]
                         [  INNER_INTERCONNECT  ] <-> [ OUTER_INTERCONNECT ] 
  [CORE 1]<->[I&D MMU]<->[                      ]     [ +CLOCKING & CDC    ] <>  [EMBEDDED MODULE - SPI TO WB BRIDGE,]
                          ^v         ^v        ^v                                [INTERNAL RAM, GPIO                 ]
                      [ICACHE 0] [ICACHE 1] [DCACHE]

  | CORE | INTERNAL BUS             |           WISHBONE BUS    |  COMPRESSED WISHBONE BUS                            |

SoC

PPCPU processor is located inside soc/ module, that runs inside FPGA on my devboard.

SoC moudle decompresses CW bus back to Wishbone bus and connects it to following peripherals

  • SDRAM controller (most of address space)
  • VGA graphics card with text mode
  • ROM with bootloader, that listens for commands on serial port
  • UART interface
  • I2C interface
  • SPI interface
  • Interrupt controller
  • Programmable timers
  • PS/2 interface

Interesting links

  • pcsn - PCPU SoC emulator
  • llvm-pcpu - full LLVM toolchain retargeted for PCPU
  • piOS - full Operating System for PCPU!
  • ppcpu_caravel - silicon design of ppcpu core (with CW bus and embedded mode)

(C) 2022-2023 Piotr Węgrzyn