Dhrystone scores are in!

A project log for 16-bit 'modern' homebrew CPU

DME is 16-bit custom CPU complete with a full software stack and a multi tasking operating system.

erwinmErwinM 09/24/2017 at 13:381 Comment

Yesterday and today I have been trying to get the Dhrystone benchmark program running on DME. It has turned out to be a typical homebrew cpu/os experience that makes this project so interesting to me.

After I downloaded the c code i tried to compile it. I got an error about missing malloc (which I have not implemented), but after replacing these with two static defines the code compiled, assembled and linked without errors. I tried to run it directly (without the OS): no go, the program gets stuck. Since I am running it without the OS i cannot write to the screen so I decide that debugging this way is a no go.

Next, I try to run it through my (work in progress) OS. It loads the code, runs it and....gets stuck (even at this point in the project, my first reaction is still 'yeah, well can't expect random c code to work like that). After some googling I tracked down a newer version of Dhrystone (version 2.1, was working with 1.1 before). This version is much nicer for two reasons: it has more verbose output telling the user what SHOULD be happening. And secondly, it's source is annotated to explain even more.

I add some debugging printf statements and run the code again. It is only then I noticed that *nothing* is actually getting printed to the terminal (this is not always immediatley obvious as my OS is still very noisy with lots of debugging info being printed). I poke at the problem a bit but soon give up and call it a day.

When I got back today I decided there should be no reason printf should not be working, it has been working for the last few weeks. I put in a halt() just after the first printf, run it and...nothing. The Dhrystone code looks like an ancient dialect of c to me and has fairly long and complex strings in the printf statements. I decide to comment out most of them. Run the code and now the single printf statement is being printed to the screen. Aha! so now I only need to find the offending string and then figure out how to fix it! As I am commenting in 5 or so statements at a time and testing the code, another realisation dawns on me: each added statement makes the code longer, is all the code getting loaded in by the os? A quick look with the simulator reveals that the answer is no it's not. The last part of the program (where the strings are), does not get loaded.

My OS uses a unix-like inode scheme for a FS (based of xv6) and it uses direct blocks and indirect blocks to load files. Indirect blocks are used for larger files. Turns out the Dhrystone program is the first program that utilised my indirect block code and it didn't work. After some probing at the code, I realised a 'greater than' should be replaced with a 'greater than or equal'. Solved! the Dhrystone program is now being loaded in its entirety and each printf statement is working as it should!

Except that the program still gets stuck. Lucky for me, it gets stuck in a VERY basic routine (Proc_5), which only assigns a character to a pre-declared memory location. And this is not working. I isolate the code and sure enough it does not work: it seems as my asm/linker is not correctly assigning the addresses for the pre-defined char location. So I dive back into the assembler. Now, this thing has been working for the last couple of months, so that code is only a little familiar to me. I put in some breakpoints (yay for debugging scripting languages) and find out that no memory is being reserved at all for statements with a size of 1 byte! The assembler essentially skips them. Few lines of code later and the assembler now handles odd byte sizes (any odd number of bytes would have tanked) correctly in its bss section.

I recompile Dhrystone and presto, it no longer gets stuck. Operation succes. Dhrystone 1.1 runs fine as well with the assembler fix. So in trying to get a piece of c code to run, I had to fix a bug in my OS, fix a bug in my assembler/linker and had to implement a missing instruction (unsigned greater than or equal) in my compiler, that I somehow had not gotten around to and which my entire OS has avoided using so far. The fixes are always fairly straightforward, it's the tracking down that is the interesting part.

An lastly of course, the score: DME clocks a Dhrystone score of 465. Just for comparison that puts DME about on par with a PDP-11:

The performance isn't too great when you take the clock speed into account, but simplicity over performance has been the motto.


Ed S wrote 10/13/2017 at 04:55 point

Pretty good for an 8MHz CPU!

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