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This document describes the teaching plan, architecture and development history of egos-2000.

Teaching plan

We use egos and egos-2000 to teach our Practicum in Operating Systems (CS4411) at Cornell. There are six projects and one is optional.

Description Concepts to teach Platform
P0 Queue in C memory, pointers, basic RISC-V ISA Linux/Mac
P1 Non-preemptive Multi-threading context switch, multi-threading, synchronization Linux/Mac
P2 Multi-level Feedback Queue timer, interrupt handling, scheduling Linux/Mac
P3 Memory Protection and System Call exception, control and status register, privilege level Arty board
P4 SD Card Driver (optional) I/O device, bus controller, device driver Arty board
P5 File System inode layer, directory layer Arty board

Architecture

Earth layer (hardware specific)

  • earth/dev_disk: ROM and SD card (touched by P4)
  • earth/dev_tty: keyboard input and tty output
  • earth/cpu_intr: interrupt and exception handling (touched by P3)
  • earth/cpu_mmu: memory paging and address translation (touched by P3)

Grass layer (hardware independent)

  • grass/timer: timer control registers
  • grass/syscall: system call interfaces to user applications
  • grass/process: data structures for managing processes (touched by P1)
  • grass/scheduler: preemptive scheduling and inter-process communication (touched by P2)

Application layer

  • app/system/sys_proc: allocation and free of user processes
  • app/system/sys_file: the inode layer over the SD card (touched by P5)
  • app/system/sys_dir: the directory layer over the inode layer (touched by P5)
  • app/system/sys_shell: interactive user interface
  • shell commands: pwd, cd, ls, cat, echo, clock, killall

Every layer has dedicated memory regions as described below (and in library/egos.h). The complete memory layout is described in Chapter 4 of the FE310 manual.

Selected RAM regions

Base Top Attributes Description Notes
0x0800_0000 0x0800_27FF RWX A ITIM, 10KB Earth layer bss, data and heap
0x0800_2800 0x0800_4FFF RWX A ITIM, 10KB Grass layer code, bss, data and heap
0x0800_5000 0x0800_7FFF RWX A ITIM, 12KB App layer code, bss, data and heap
...... ...... ...... ......
0x2000_0000 0x203F_FFFF R XC Flash ROM, 4MB FPGA binary of the FE310 RISC-V processor
0x2040_0000 0x207F_FFFF R XC Flash ROM, 4MB Earth layer binary
0x2080_0000 0x20BF_FFFF R XC Flash ROM, 4MB Disk image (disk.img produced by mkrom)
...... ...... ...... ......
0x8000_0000 0x8000_1FFF RW- A DTIM, 8KB App layer stack
0x8000_2000 0x8000_3FFF RW- A DTIM, 8KB Earth layer and grass layer stack
0x8000_4000 0x8001_FFFF RW- A DTIM, 112KB MMU cache of physical frames for suspended grass processes

On the Arty board, the first 1MB of the microSD card is used as 256 physical frames by the MMU for paging (see earth/cpu_mmu.c).

Selected memory-mapped I/O regions

Base Top Attributes Description Notes
0x1001_4000 0x1001_4FFF RW A QSPI 0 Control the 16MB on-board ROM
0x1002_4000 0x1002_4FFF RW A SPI 1 Control the Pmod1 (microSD card)

Software development history

Iteration #7

  • [2022.10] Add support to the QEMU emulator
  • [2022.10] Enable the supervisor mode in the QEMU emulator
  • [2022.10] With the modified QEMU, add page table translation in earth/cpu_mmu.c.

Iteration #6

  • [2022.04] Add a simple boot loader earth/earth.S; Remove dependency on the Metal library
  • [2022.04] Add _write() and _sbrk() in library/libc; Remove the Metal library entirely
  • [2022.04] Enrich struct grass in order to improve clarity of the architecture
  • [2022.04] Experiment with Physical Memory Protection (PMP) and switching privilege level (machine <-> user)

Iteration #5

  • [2022.03] Implement system calls and 4 shell commands: pwd, ls, cat and echo
  • [2022.03] Add support of background shell commands and killall
  • [2022.03] Add servers in library and cleanup access to the file system
  • [2022.03] Cleanup the code controlling UART and SPI; Remove dependency on the Metal library

Iteration #4

  • [2022.02] Read Chapter 1, 2 and 3 of RISC-V manual
  • [2022.02] Read Chapter 8, 9 and 10 of FE310 manual
  • [2022.02] Implement timer reset, preemptive scheduling and inter-process communication
  • [2022.02] Implement the kernel processes: GPID_PROCESS, GPID_FILE, GPID_DIR, GPID_SHELL

Iteration #3

  • [2022.01] Confrim that the processor clock frequency cannot be further increased
  • [2022.01] Implement the dev_tty, dev_disk, cpu_intr and cpu_mmu interfaces in earth
  • [2022.01] Load the ELF format grass kernel binary file from the SD card to memory
  • [2022.01] Implement the control transfer from earth to grass kernel and then to a user application
  • [2022.01] Implement mkrom so that creating the bootROM image no longer require Vivado or Docker

Iteration #2

  • Yeah, I didn't work on this project in most of 2021...
  • [2021.12] Create a docker image for portable toolchain setup: Docker Hub repo
  • [2021.12] Reconnect the processor SPI bus controller to the Arty Pmod1 pins
  • [2021.12] Implement the SD card initialize, read and write functions
  • [2021.12] Increase the processor clock frequency from 32MHz to 65MHz so that read/write blocks become faster

Iteration #1

  • [2020.12] Increase the processor memory from 24KB to 160KB (128KB + 32KB)
  • [2020.12] Confirm that the memory cannot be further increased due to the limitation of Artix-7 35T FPGA chip

Iteration #0

  • [2020.09] Setup the toolchain provided by SiFive; Compile and run Hello World on Arty
  • [2020.09] Test the basic input and print functionalities using the SiFive Metal library