What is the NULL ISA architecture under src/arch/null in gem5? - gem5

Question

I've noticed that there is a src/arch/null directory in the gem5 211869ea950f3cc3116655f06b1d46d3fa39fb3a sitting next to "real ISAs" like src/arch/x86/.
This suggests that there is a NULL ISA in gem5, but it does not seem to have any registers or other common CPU components.
What is this NULL ISA for?
Inspired by: https://www.mail-archive.com/gem5-users#gem5.org/msg16968.html

Answer 1

I believe that the main application of the NULL ISA is to support tests where you don't need to simulate CPU, notably traffic generators such as Garnet mentioned at: http://www.gem5.org/Garnet_Synthetic_Traffic
Traffic generators are setups that produce memory requests that attempt to be similar to a real system component such as a CPU, but with a higher level approximation and without actually implemented a detailed microarchitecture.
The advantage is that traffic generators run faster than detailed models, and can be easier to implement. The downside is that the the simulation won't be as accurate as the real system.
Also doing a NULL build is faster than doing a build for a regular ISA, as it skips all the ISA specifics. This can be a big cost saving win for the continuous integration system.
As a concrete example, on gem5 6e06d231ecf621d580449127f96fdb20154c4f66 you could run scripts such as:
scons -j`nproc` build/NULL/gem5.opt
./build/NULL/gem5.opt configs/example/ruby_mem_test.py -m 10000000
./build/NULL/gem5.opt configs/example/memcheck.py -m 1000000000
./build/NULL/gem5.opt configs/example/memtest.py -m 10000000000
./build/NULL/gem5.opt configs/example/ruby_random_test.py --maxloads 5000
./build/NULL/gem5.opt configs/example/ruby_direct_test.py --requests 50000
./build/NULL/gem5.opt configs/example/garnet_synth_traffic.py --sim-cycles 5000000
These tests can also be run on a "regular" ISA build, for example:
scons -j`nproc` build/ARM/gem5.opt
./build/ARM/gem5.opt configs/example/ruby_mem_test.py -m 10000000
It is just that in that case, you also have all that extra ARM stuff in the binary that does not get used.
If you try to run a "regular" script with NULL however, it blows up. For example:
./build/NULL/gem5.opt configs/example/se.py -u /tmp/hello.out
fails with:
optparse.OptionValueError: option --cpu-type: invalid choice: 'AtomicSimpleCPU' (choose from )
since there is no valid CPU to run the simulation on (empty choices).
If you look at the source of traffic generator Python scripts, you can see that the traffic generator implements the CPU memory interface itself, and is therefore seen a CPU by gem5. For example, configs/example/ruby_mem_test.py does:
cpus = [ MemTest(...
system = System(cpu = cpus,
so we understand that the MemTest SimObject is the CPU of that system.