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Comment by abortz | Hacker News Reader

Perhaps in terms of gates, but LUTs represent a much larger fraction of resources on an FPGA than the block RAMs. In other words, if you wanted to pack a lot of these onto an FPGA you'd run out of LUTs before block RAM.

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I'm too lazy to look up numbers for the Kintex-7 part in question, but I'm almost certain you're wrong on this. A block RAM is a big chunk of die, and there are comparatively few of them to go around. A LUT is a tiny object (comparable in computation power to 10-50 dedicated transistors) and there are hundreds of thousands of them on the FPGA.

I'm willing to bet lots that 4/n_block_ram > 308/n_lut.

Yeah, I went and looked up. The details are hairy, becuase Xilinx. But a KC7K410T part, which is roughly their mid-range offering has 63550 slices, where IIRC a slice has two LUTs (pay no attention to their "logic cells" number -- that's a normalized thing scaled so as to be linear with the old 4-input LUT design from long ago) and 28620kbit of block RAMs, where each block is 36kbit.

So that design uses 308/(2*63550) =~ 0.2% of the logic resources on the FPGA, but 4/(28620/36) = 0.5% of the RAM.

Not nearly as imbalanced as it sounded to me originally, but still: the LUT numbers are spun by more than a factor of two. The design is more closely equivalent to "640 LUTs". Which interestingly is very comparable to the equivalent transistor count on the original part from Intel.

Yes, you were correct. I was working from the bulk stats on the chip and mixed up Kb and KB ;-)

Block RAMs are 2-port. You can up it to 4 ports by doubling their clock rate, if the rest is merely crawling on 100mhz. Yet, each core would eat up one port for reading its microcode every clock cycle, so you can only have up to 4 cores for each group of 4 brams with stored microcode.

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