[Oberon] Resend: Re: Risc-5 Instructions statistics

Skulski, Wojciech skulski at pas.rochester.edu
Sun Feb 17 05:17:06 CET 2019


Walter Gallegos [waltergallegos at vera.com.uy] said:

> Implementing a general propose microprocessor system alone
> in FPGA is impractical beyond the research or ASIC prototypes.

I agree, but... The motivation for RISC5 was that Oberon System software was available. The traditional rules of computer design have been turned upside down. Traditionally, the CPU is designed first, and compilers and other software are then developed to support the CPU. Here it was just the opposite. The operating system existed. The CPU was then designed to run the software, not the other way around. This course of actions seems pretty unique to me.

> For industrial applications the microprocessor must coexist
> with some special hardware that justifies the use of FPGA. 

A very good point. But it can be turned upside down too. The "special hardware" can be termed a "coprocessor" for the microprocessor. It depends what is more important to you. The Oberon programmers will probably put the RISC5 and its software in the foreground. They will probably say that the "special hardware" is bunch of registers plus some obscure logic delivering the content to these registers. Your point of view may be the opposite. To you, the "special hardware" is the focus, while the CPU is a necessary devil to set it up and to monitor its operation.

> In this environment, wasting DSP blocks
> could be a not recommended technique.

If you say "wasting", it means that you need them for something else. Then you are presumably building a coprocessor ("special hardware") where you need these blocks. This in turn implies that you are designing in a resource starved environment and you need to compromise either the CPU or the "special hardware" because there are not enough DSP blocks for both. If so, then I would ask: why not choose a slightly larger FPGA with more DSP blocks? 

Let me give an example of a particular Artix FPGA and the next higher Artix in the same FT256 footprint. I took the price from DigiKey. 

XC7A15T-1FTG256C $27.93  45 DSP blocks
XC7A35T-1FTG256C $34.37  90 DSP blocks

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