Open8 - 8-bit RISC Processor Core Based on the Automation uRISC

Open8 - 8-bit RISC Processor Core Based on the Automation uRISC


Category: Processor

Created: September 11, 2006

Updated: January 27, 2020

Language: VHDL

Other project properties

Development Status: Stable

Additional info: Design done, FPGA proven, Specification done

WishBone compliant: No

WishBone version: n/a

License: BSD


8-bit RISC processor core based on the Vautomation uRISC

This is a "clean" reimplementation of the Vautomation uRISC processor core (aka the "V8", also named the Arclite core) based on ISA documentation only.

It implements the full v8 architecture with a few additions, most of which are optional:

* Thirty-six basic instructions (and four new instructions)
* 8-bit PSR(Program Status Register) with Zero, Carry, Negative, and Interrupt status bits, and 4 general purpose status bits.
* Eight 8-bit registers, R0 though R7.
* Accumulator register (R0)
* A 16-bit program counter
* Any two adjacent registers may be paired to create a 16-bit index register.
* Three basic addressing modes; addressed, indexed, and indexed with offset

The design adds a few new features, which can be enabled through generics:
* An optional auto-increment for indexed addressing modes ("LDX R4++" is equivalent to "LDX R4 ; UPP R4" )
* A new branching instruction, DBNZ (Decrement, and Branch if Not Zero)
* A new math instruction, MUL, uses on-board multipliers.
* The interrupt mask can now be set with the new instructions SMSK and GMSK

* The RSP instruction may now optionally be converted into a "relocate SP" instruction with the ability to copy between R1:R0 <> SP based on the state of a user-specified CPU flag.

The Open8 is being designed to work optimally in newer FPGA architectures. It assumes 2 clocks for memory and register file latency.

This design has now fielded as a test stimulus controller hosted in an Altera 3C40, not once - but twice. It's primarily serving as a data acquisition controller / packet generator in those designs, and has performed trouble-free for well over a year. Additionally, as part of the test stimulus system, the Open8 is responsible for synchronizing output frequencies with the device under test. Due to the nature of these calculations, a 16-bit ALU/co-processor was written to "hardware accelerate" common math functions, rather than have to write emulations in assembly. This ALU has been included in the SVN repository. The Open8, and its ALU coprocessor, use about 2400 LE's in the FPGA.

It has also been fielded in several non-shipping test instruments and small emulators hosted in Altera 3C16's, where it performs a variety of tasks. I am presently looking to use it as a packet processor to bridge between a PC and a custom digital waveform generator design.


- Model is written in VHDL ('93)
- Simple RISC architecture and instruction set. All instructions fit in a single byte, with either 1 or 2 operands.
- 16-bit PC / address allows for 64kB of directly accessible memory (can be expanded with paging) - Flat memory model allows code or data to be placed anywhere in the memory map, as well as easily supporting self-modifying code.
- Moderate number of general purpose registers
+ Eight byte-wide registers.
+ Any two registers may be paired as (Rn+1:Rn) to create an index register
+ R0 acts as the accumulator
- 8 interrupts, 1 NMI, 7 maskable. Interrupt controller is built into the core.
+ Interrupt controller keeps track of interrupt order and priority
+ Interrupt mask is controllable through two new instructions, SMSK and GMSK.
- Reasonably small gate-count, with strong fMax in "low-end" devices.


- The CPU is being used in a system that requires real task switching, so the ability to optionally alter the RSP (reset stack point) so that it behaves as a Retrieve/Relocate Stack Pointer was added. This has been tested in a development system version of a real design and works well - though writing the task switching code is really an eye-opener on context switching costs.

- Several generally useful entities have been added to the repository such as a simple serial port, HD44780 character LCD controller, system timer, real-time clock, and a memory-mapped SDLC interface. These may be broken out as separate projects at some point, but they all are written around the Open8 CPU / bus interface.