This project is a soft processor core compatible with 586 instruction set.
The processor core has AXI4 interface and uses XILINX AXI4 IPs such as AXI4 CROSSBAR and MIG with AXI 4 interface.
The platform boots linux kernel with a ramdisk contained in the SPI flash.
The package contains code for NEXYS4 DDR and ESA-11-7a100t-v1.1 boards , with artix7 device inside external DDR2/3 size 128MB DDR and 128Mbit SPI Flash.
** CORE DETAILS:
- 586 instruction set implementation, conditional mov added.
- MMU with protected and paged mode supported with 4KB page size.
- TLB 16 entries.
- CPUID (0x0617)and CMPXCHG8B are implemented, with conditional mov.
- pre-fetch queue is 32 bytes long.
- 16kB Instruction cache, 16kB Data cache, 128bit loaded at once into prefetch queue.
- Hardware multiplication and Division.
- 32 bit AXI4 interface with bursts.
- ~17DMIPS or ~30500 dhrystones v2.1 @ 100MHz with -O2 option under gcc 4.8, no register.
* Known limitations:
- no FPU, but emulation mechanism with "device not available" fault #7, whetsone gives 0.125 MIPS score with emulation.
- some instruction are missing, but not used in Linux/GCC : like the decimal adjust.
- Protected and Paged mode are supported, there are some limitation in the protected mode implementation like segment limit.
- bits and features for MSR/TSC/PAE/NX bit not implemented.
- MMU page size is 4KB , no PSE / PSE36 bit supported.
- Segment prefix in protected mode are the default or GS descriptor, FS prefix in protected mode is not used in Linux 32b.
- RESET is the CPU Push buttton on NEXYS4.
- Minimal set of peripherals : timer and interrupt controller.
- one 16750 interface with 64byte fifo , re-used from uart16750 opencore project.
- IT87xx SuperIO chip implemented only for GPIOs parts. 16x GPIOs Connected to leds of nexys4.
( GPIO Linux driver fot IT87 compatible with userspace interface. )
- SPI interface serial-in , parallel out
- Bus Interface Unit to arbitrate between instruction fetch and data operations and
also MMU specific operation like updating the bit for page directory/table entries.
- The Platform mapping in the Artix7-100 fills 42% and clock frequency used in the project is 100MHz
- clock divider for timer and uart are hard coded for 100MHz base clock.
- if an SD car is present on NEXYS4 DDR SD slot it can be mounted with "mount /dev/mmcblk /mnt"
- internal ROM to boot and copy SPI flash into RAM, provide minimal description
and configuration for Linux with command line to configure TTY console on uart
at 115200 baud.
- SPI image with Linux 3.19 ( 3.17/3.14/3.12/2.6.x were tested as well) and initramfs
with busybox built from buildroot 2016.02 with dhrystone/whetstone/e2fs/micropython.
- Kernel has been built on regular PC UBUNTU 14.04LTS with regular GCC 4.8 as well.
- patch for linux and .config file for the kernel included, settings optimized for size and FPU emulation on plus device declaration.
videos on youtube can be found here : https://www.youtube.com/channel/UCNbm8Bah54cwhedmCRWyXMAvideos