Niski

This system combines several university assignments into a functional whole project. It integrates FPGA design, low-level programming, and system programming. The ultimate objective of this project is to develop a programmable microcontroller capable of running any multithreaded program while also facilitating communication with the outside world.

Hardware

For this project Cyclone IV EP4CE6E22C8 FPGA development board was used:

It has the following features:

• 6,272 Logic Elements
• 276,480 Total RAM Bits
• 30 Embedded 9 x 9 Multipliers
• 2 PLLs
• 92 I/O Pins
• 50 MHz Clock

The development board has the following peripherals:

• 4 LEDs
• 4 seven-segment displays
• 4 buttons
• buzzer
• 16x2 LCD display
• VGA port
• PS/2 port
• UART port

CPU

Initially, I had a desire to create my own processor with custom instructions. However, I soon recognized the advantages of utilizing a standardized architecture for which compilers for higher-level programming languages already exist. This realization led me to opt for RISC-V, a versatile architecture that permits the addition of extensions.

I specifically selected the 32-bit version due to limited available memory, which will never exceed 4GB. Currently, the processor partially supports the M extension, providing hardware multiplication capabilities, while division operations rely on the GCC library. To ensure seamless compatibility with operating systems, the Zicsr extension was also essential, and I successfully implemented it."

Because of the above, the current name of the architecture is RV32EMZicsr.

Memory

The processor features a 32-bit address bus, allowing it to potentially address up to 4GB of memory. However, due to the limited resources of the FPGA development board, only 64KB of memory is currently available, which is constructed using the onboard Block RAM resources. In the future, I plan to explore adding support for SDRAM memory, which is available on the development board. This upgrade would extend the accessible memory to 8MB.

Peripherals

On the FPGA development board, there are several peripherals that can be used by the processor. For every peripheral, a controller and memory-mapped bus interface was designed.

DMA

In order to facilitate efficient data transfer, I have implemented a DMA (Direct Memory Access) controller. This controller offers flexibility in handling both source and destination addresses, allowing for incrementing, decrementing, or fixed addressing modes. Additionally, data transfers can occur in either burst mode (single bus request for the entire transfer) or normal mode (a bus request for every byte).

Currently, the DMA operation isn't as efficient as it could be, as it reads/writes only one byte per bus request. In my future plans, I intend to enhance the efficiency by enabling the controller to read/write 4 bytes at a time.

PLIC

The PLIC (Platform-Level Interrupt Controller) is a peripheral that manages interrupts from external devices (such as the UART, PS/2, buttons, etc.) and forwards them to the processor. Every interrupt can be enabled or disabled individually. In future, I plan to implement priority levels for interrupts.

Watchdog

To prevent the processor from becoming stuck in an infinite loop when encountering an invalid address (one not associated with any module or memory-mapped register), a watchdog timer has been implemented. This timer generates a signal to halt the ongoing request and trigger an exception on the processor.

Software

A small yet functional operating system kernel is in place, aiming to adhere to the C23 language standard. In addition to its core functionality, the kernel includes functions to facilitate communication with external devices on the FPGA development board.

Dynamic memory allocation for user programs is achieved using a first-fit algorithm, while a more efficient buddy/slab algorithm is employed for kernel objects.

The kernel offers support for multithreading, although multiprocessing is not currently supported. The scheduler operates on a FIFO basis, but I have plans to implement a more efficient scheduling algorithm in the future. The scheduling is preemptive, with a time slice set to 1 second.

C standard library

GCC automatically generates header files containing types and other definitions that are architecture-dependent rather than tied to the specific operating system

Therefore, following header files already exist:

• iso646.h
• stdarg.h
• stdalign.h
• stdbool.h
• stddef.h
• stdint.h
• stdnoreturn.h
• limits.h

The OS developer is responsible for implementing additional header files. Some functions will be designed as system calls, while others will function as library functions. Currently, the following header files and functions have been fully implemented:

• errno.h
• stdlib.h
  • malloc
  • calloc
  • free
  • realloc
  • bsearch
  • rand
  • srand
  • [l/ll]div
  • [l/ll]abs
  • strtol[l]
  • strtoul[l]
  • ato(i|l|ll)
• inttypes.h
  • imaxdiv
  • imaxabs
• stdio.h
  • fputs
  • fputc
  • putc
  • putchar
  • puts
• threads.h
  • thrd_create
  • thrd_equal
  • thrd_current
  • thrd_yield
  • thrd_exit
  • thrd_detach
  • mtx_init
  • mtx_lock
  • mtx_unlock
  • mtx_destroy
  • cnd_init
  • cnd_signal
  • cnd_broadcast
  • cnd_wait
  • cnd_destroy
• time.h
  • difftime
  • time
  • clock
  • timespec_get
  • timespec_getres
• ctype.h
  • isalnum
  • isalpha
  • islower
  • isupper
  • isdigit
  • isxdigit
  • iscntrl
  • isgraph
  • isspace
  • isblank
  • isprint
  • ispunct
  • tolower
  • toupper
• string.h
  • strcpy
  • strncpy
  • strcat
  • strncat
  • strdup
  • strndup
  • strlen
  • strcmp
  • strncmp
  • strchr
  • strrchr
  • strstr
  • strtok
  • memchr
  • memcmp
  • memset
  • memset_explicit
  • memcpy
  • memccpy
  • memmove
  • strerror
• setjmp.h
  • setjmp
  • longjmp
• signal.h
  • signal
  • raise

I/O devices

In order to facilitate communication with external devices on the FPGA development board (such as the buzzer, LEDs, 7-segment displays, LCD, etc.), drivers are essential. At present, the following header files and functions have been fully implemented:

• devices/buzzer.h
  • buzzer_on
  • buzzer_off
  • buzzer_set
  • buzzer_toggle
• devices/leds.h
  • leds_on
  • leds_off
  • leds_set_single
  • leds_toggle_single
  • leds_set
• devices/ssds.h
  • ssds_on
  • ssds_off
  • ssds_set
  • ssds_set_dots
  • ssds_set_digit
  • ssds_set_dec_num
  • ssds_set_hex_num
• devices/lcd.h
  • lcd_init
  • lcd_clear
  • lcd_move_cursor
  • lcd_send_cmd
  • lcd_write_ch
• devices/dma.h
  • dma_transfer
  • dma_fill
  • dma_copy
  • dma_rcopy
• devices/btns.h
  • btns_enable_single
  • btns_disable_single
  • btns_enable_all
  • btns_disable_all
  • btns_get_statuses
  • btns_is_pressed