MB-Lite

Introduction

The MB-Lite microprocessor is a ligth-weight implementation of the Microblaze Instruction Set Architecture. It is instruction and cycle compatible with the Microblaze EDK 10.1i. It is successfully tested on older and newer Xilinx platforms (EDK 9 and 11). The design has been successfully synthesized for an Altera board as well to show platform independence.

Features

MB-Lite is a highly modular design and is therefore very simple to understand and modify. Features of the MicroBlaze architecture and MB-Lite implementation are:

• 32-bit data- and instruction bus
• Harvard architecture
• Five pipeline stages
• Cycle as well as instruction compatible with the MicroBlaze Instruction Set Architecture
• Optional interrupt support
• Optional wishbone bus
• Optional multiplier
• Optional barrel shifter

The following instructions are currently not implemented. All of these instructions are not used by the compiler (mb-gcc) or can be replaced by software libraries.

• MULH, MULHU, MULHSU
• IDIV, IDIVU (Integer Division)
• TN* (Fast Simplex Link instructions)
• F* (Floating point instructions)
• PCM* (Pattern Compare)
• WIC, WDC (Cache instructions)
• MTS, MFS, MSR* (Special purpose register instructions)
• RTBD, RTED (Return from break / exception)

Included design examples

• Integer unit directly connected to instruction- and datamemory
• Integer unit with wishbone bus wrapper
• Integer unit with address decoder
• Integer unit with address decoder and wishbone bus adapter
• SOC for synthesis purposes

Methodology

The core is designed using the two-process design methodology of Jiri Gaisler. All modules use inferred components, the design is not targeted specificly to any platform. However, currently it is only tested on a Xilinx Spartan 3 FPGA. All memory blocks and registers will synthesize to BRAM on xilinx devices.

Organization

The organization of the hardware corresponds closely to the implementation of the classic RISC pipeline of Hennesy & Patterson (Computer Organization and Design: The Hardware/Software Approach).

Performance

MB-Lite is tested on a Virtex 5 development board (XC5VLX110-3FF 1760). It is able to obtain 229 MHz (c.f. the original implementation obtains 227 MHz). Furthermore, MB-Lite has a lower Cycles Per Instruction (CPI) than MicroBlaze since MicroBlaze has a prefetch buffer which reduces the rate of instructions which can be fed into the processor. The execution time of MB-Lite is therefore approximately 10% lower than MicroBlaze.

TODO / Wishlist

• Modify wishbone bus to support single cycle transfers
• Add Fast Simplex Link (FSL)