AltOr32 - Alternative Lightweight OpenRisc CPU

AltOr32 - Alternative Lightweight OpenRisc CPU


Category: Processor

Created: June 09, 2012

Updated: January 27, 2020

Language: Verilog

Other project properties

Development Status: Stable

Additional info: Design done, FPGA proven

WishBone compliant: Yes

WishBone version: n/a

License: LGPL


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.


The 'or1knd' toolchain is required for this target.

To build from source:

git clone git://
git clone 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 install

Build gcc

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


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 non-pipelined 'lite' version available.

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