Dhrystone Min

This is an implementation of the standard Dhrystone benchmark, which measures the performance of the ZPU in its smallest configuration.

There are no noteworthy differences in the makefile between this and the Hello World project. I have commented out a large portion of the report at the end of the test, just to keep the ROM size down, otherwise making it fit in the rather small amount of block RAM available on the DE1 board is tricky. Also to keep down the code size, I’ve used a very simple and compact printf implementation which only supports very basic format strings. Its size can be reduced even further (and its reliance upon division and modulo can be eliminated) by specifying “-DPRINTF_HEX_ONLY” in the CFLAGS within the Makefile.

As with Hello World, the Makefile builds an elf binary, converts that to raw binary format, then uses the “zpuromgen” command and some sed trickery to create a VHDL ROM file that can be included directly in the Quartus project. The RTL is essentially the same as the Hello World hardware, with the addition of a “millisecond counter” register which the firmware uses to time the benchmark.

The entire project is 808 logic elements when built for the DE1. The size of the Dhrystone_min.bin file is 4671 bytes, but this is deceptive; the project requires quite a lot of working space which is in the uninitialized BSS section. To make it possible to gauge the required RAM size, the Makefile spits out a report, showing the location of the end of code, the start of BSS and the end of BSS:

End of code:	00001240 g       *ABS*	00000000 _romend
Start of BSS:	00001240 g       .bss	00000000 __bss_start__
End of BSS:	00003b88 g       *ABS*	00000000 __bss_end__

In this case the BSS section ends at 0x3b88. Any space above this will be available for the stack – so we round up to the nearest power of 2 – our Block RAM size will be 0x4000 or 16384 bytes, which leaves 0x478 (1144) bytes available for the stack. This should be plenty. We require 14 address bits (13 downto 0) to address 16384 bytes of RAM, so in the ROM and ZPU instantiations, maxAddrBit and maxAddrBitBRAM, respectively, are set to 13.

When clocked at 125MHz, the Dhrystone_min project turns in 1.7 DMIPS, which is quite respectable for a CPU that’s taking up a mere 599 logic elements!

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