AltOr32 - Alternative Lightweight OpenRisc CPU

AltOR32 is an OpenRISC 1000 architecture derived RISC CPU targeted at small FPGAs and contains only the most basic ISA features from the OpenRisc project. Instructions & registers relating to Vector, floating-point, 64-bit extensions, MMU & Cache have been omitted. The aim of AltOR32 is to provide a simple 32-bit soft CPU architecture aimed at control applications that can fit in low-end FPGA technology.

This design implements all instructions that cannot be disabled. Anything else is viewed as beyond the scope of this cut-down soft-CPU implementation.

AltOR32 does not make use of delay slots, unlike the original OpenRisc implementation. Due to this, the or1knd toolchain is required.

Toolchain

The 'or1knd' toolchain is required for this target.

To build from source:

git clone git://github.com/openrisc/or1k-src.git git clone git://github.com/openrisc/or1k-gcc.git

Build the first set of tools, binutils etc:

../or1k-src/configure --target=or1knd-elf --prefix=/opt/or1k-toolchain --enable-shared --disable-itcl --disable-tk --disable-tcl --disable-winsup --disable-libgui --disable-rda --disable-sid --disable-sim --disable-gdb --with-sysroot --disable-newlib --disable-libgloss --disable-werror make make install

Build gcc:

../or1k-gcc/configure --target=or1knd-elf --prefix=/opt/or1k-toolchain --enable-languages=c --disable-shared --disable-libssp --disable-werror make make install

Verilator

The project contains a Verilator cycle accurate model of the CPU which can execute the same code as the simulator. Waveforms can be outputted and viewed in GTKWave.

Current Status

• Pipelined Verilog core with optional instruction & data cache.
• Synthesizes to ~100MHz on a Xilinx Spartan 6 LX9 -3
• Harvard architecture (separate instruction & data cache / memory interfaces).
• Support for interrupts & tick timer.
• Also contains a smaller non-pipelined 'lite' version.

Instruction Set Simulator

• A simple simulator for OpenRisc instructions.
• Able to execute OpenRisc 1000 (ORBIS32) code compiled with the following options: -msoft-div -msoft-float -msoft-mul -mno-ror -mno-cmov -mno-sext
• Extensible