J1 -- very small & simple CPU for an FPGA

Sébastien Bourdeauducq sebastien.bourdeauducq at lekernel.net
Thu Dec 2 07:02:22 EST 2010

On Thursday 02 December 2010 12:51:16 David Kuehling wrote:
> if you are interested in this kind of extremely compact FPGA CPUs, you
> might like this one, too:
> http://opensource.zylin.com/zpu.htm
> Pretty professional, even a GCC toolchain is available, and
> BSD-licensed.  This also uses a zero-operand architecture (i.e. operands
> on hardware-stack), so one might call it a "forth" cpu.

Tried it for the Milkymist USB controller and quickly dropped it (designing 
using my own AVR clone to replace it). It basically looks like a contest 
winner for the slowest and most inefficient CPU you can promote with a straight 

As it turned out, that piece of crap takes more than 256 clock cycles per byte 
for a rather simple buffer copy code... It failed to feed the data to my USB 
UART operating at 1.5Mbps while the CPU itself was running at 48MHz. That's 

By the way, why is it always poor or stupid designs that make hackaday and all 
the buzz? I've read countless times about this J1, Plasma, another crappy AVR 
clone and ZPU.


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