Skylake Bus Speed Tip

Here is a quick tip:

System Information > Hardware Overview should not list “Bus Speed: 400 MHz” for Skylake processors, and in case you (still) see that, then please ask the developers of the used boot loader to fix this (SMBIOS) issue for you.

Note: sysctl hw should not report:

hw.busfrequency: 400000000
hw.busfrequency_min: 400000000
hw.busfrequency_max: 400000000

but:

hw.busfrequency: 100000000
hw.busfrequency_min: 100000000
hw.busfrequency_max: 100000000

But there is more. Check the value of the IOService:/clock-frequency and IOService:/cpuN@N/bus-frequency properties, with either IORegistryExplorer or ioreg, and note that they are also wrong.

Background information

Here is a bit of background information. We already know that the AppleSMBIOS.kext binary fetches the bus transfer rate from a SMBIOS type 132 structure, which is optional, and when that is missing in the SMBIOS, then it assumes that a quad pumped bus is used and multiplies the frequency by 4, as can be seen in the source code snippet from AppleSMBIOS:

    // Fetch cpu and bus nominal frequencies.

    SMB_ANCHOR_RESET(&anchor);
    if (findSMBIOSStructure(&anchor, kSMBTypeProcessorInformation,
        kSMBProcessorInformationMinSize))
    {
        const SMBProcessorInformation * cpuInfo =
            (const SMBProcessorInformation *) anchor.header;

        cpuSpeedMHz = cpuInfo->maximumClock;
        DEBUG_LOG("SMBIOS: CPU speed (MHz) = %u\n", cpuSpeedMHz);
        if (busSpeedMTs == 0)
        {
            busSpeedMTs = cpuInfo->externalClock;
            busSpeedMTs *= 4;  // Assume quad-pumped FSB
            DEBUG_LOG("SMBIOS: FSB speed (MT/s) = %u\n", busSpeedMTs);
        }
    }

Giving you the 400MHz instead of the expected 100MHz. Old stuff by now.

Simple Workaround

There are different ways of checking what frequency we should use, and one of the things that you can do is to check register ecx of CPUID leaf 0x16 which in this case will return a value of 100(MHz).

A simple workaround is to check whether or not a SMBIOS type 132 structure is injected, and if not then you divide the value of the External Clock frequency in your SMBIOS type 4 structure by 4.

Anyway. This information should help Slice and the Chameleon developers to fix this issue in their code 😉

Edit: Sorry. I asked Jens – a friend of mine – to update this blog article for me, but as usual… I didn’t make myself clear so he ended up mixing up stuff – the ARTFrequency was meant to become a next blog article. This was clearly my mistake, for which I apologise, because all Jens tried to do was help me. And he surely did.

Thank you Jens, for helping me during the holidays. We’re almost home – having a coffee in Madrid and I cannot wait to begin hacking the science out some of the unsolved problems 😉