PCI-E x12?

This is probably a stupid question, or it's so obvious it's gone right over my head - why are there never any PCI-E x12 slots on motherboards? We get x16, x8, x4, x1.

Is there a technical reason why x12 isn't used? Thinking of a situation such as with the Z790 boards that can only use x8 GPU lanes if the Gen 5 NVMe slot is used - could it not run at x12 instead in this case? (x12 GPU + x4 NVMe for a total of 16 lanes supported by the CPU)

Just being curious but someone must have the answer!

According to Professor Wikipedia x12 and x32 are defined in the technical specification but haven't ever been used.

Mentioned in the Architecture section 3rd paragraph.

BrianInEngland said:

why are there never any PCI-E x12 slots on motherboards

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Like everything else in computers... manufacturers like powers of 2.
You'll see powers of two, over and over in hardware and software.

For example... 2^16 = 65536

65536 (216) is the number of different values representable in a number of 16 binary digits (or bits), also known as an unsigned short integer in many computer programming systems.

• A 65,536-bit integer can represent up to 265536 (2.00352993...×1019,728) values.
• 65,536 is the number of characters in the original Unicode, and currently in a Unicode plane.

This number is a limit in many common hardware and software implementations, some examples of which are:

• The Motorola 68000 family, x86 architecture, and other computing platforms have a word size of 16 bits, representing 65536 possible values. (32- or 64-bit operations are supported equally or better in modern microprocessors.)
• Many modern CPUs allow a memory page size of 64KiB (65536 bytes) to be configured in their memory-management hardware.[5]
• The default buffer size of a pipeline (Unix) is 64KiB (65536 bytes).
• 65536 is the maximum number of spreadsheet rows supported by Excel 97, Excel 2000, Excel 2002 and Excel 2003. Text files that are larger than 65536 rows cannot be imported to these versions of Excel.[6] (Excel 2007, 2010 and 2013 support 1,048,576 rows (220)).
• A 16-bit microprocessor chip can directly access 65536 memory addresses, and the 16-bit highcolor graphics standard supports a color palette of 65536 different colors.
• The maximum number of methods allowed in a single dex file Android application is 65536.[7]
• The limit for the amount of code in bytes for a non-native, non-abstract method in Java.
• The number of available ports to combine with a network address to create a network socket.
• The maximum character limit for one message in WhatsApp is 65536.

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Ghot said:

Like everything else in computers... manufacturers like powers of 2.

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That would make the meetings to set computer standards a lot simpler.

BrianInEngland said:

This is probably a stupid question, or it's so obvious it's gone right over my head - why are there never any PCI-E x12 slots on motherboards? We get x16, x8, x4, x1.

Is there a technical reason why x12 isn't used? Thinking of a situation such as with the Z790 boards that can only use x8 GPU lanes if the Gen 5 NVMe slot is used - could it not run at x12 instead in this case? (x12 GPU + x4 NVMe for a total of 16 lanes supported by the CPU)

Just being curious but someone must have the answer!

Click to expand...

Devices are made to use x16 x8 x4 and x1 lanes. You probably could put a x12 device in your main graphics slot - if you use the CPU graphics. Also these are shared and use an electronic PCI-E switch that will, for example, split the 16 lane CPU provided graphics into 8 x 8 if something is plugged into the second 16 lane form-factor slot. Thing would get quite complicated (more costly) if the combinations of shared lanes were to be expanded. At least that is my guess - that it is a cost optimization.

IMO, I don't think it has much to do with the power of two.