Intro to Computer Systems

Chapter 6a: System Expansion

Form-Factor Considerations

The form factor (external design) of a computer can have serious consequences on the expansion interfaces available to a system. Furthermore, some expansion interfaces have special, miniaturised versions designed for compact spaces.

Processor Sockets

As discussed earlier, processor sockets are specific to a particular family of CPUs, based on the signalling requirements and the level of integration between the CPU internals and other core components (e.g. an integrated memory or graphics controller).

It's quite common for there to be low-power variants of CPUs, designed for compact computers, which not only reduce the thermal requirements of the case, but are in a physically smaller package (both in terms of socket, and the space around the socket designated for a heatsink and fan) to allow for smaller circuit boards.

Socket G1   Socket LGA 1366
The Socket G1 (mobile, left) and LGA 1366 (desktop, right) support similar era processors, but for different power variants.

The pictures above are of a Socket G1 processor socket for mobile CPUs, and LGA 1366, for desktop CPUs. Although the two sockets are used for processors of a similar era in terms of processing power, the Socket G1 is much more compact. Furthermore, there is clear space reserved on the desktop processor socket (the white outlined area on the circuit board) for mounting a cooling solution, which is not present in the Socket G1 standard.

In this particular example, there is one compromise in the portable CPU socket: it has fewer pins, and as such can only support dual-channel memory modes, and not triple-channel like the desktop socket.

(The designers removed the pins for the third memory channel, in order to save space.)

In more extreme cases of miniaturisation, such as subnotebook PCs, tablets, and phones, the processors are soldered onto the motherboard and un-upgradable.

Memory Modules

Modern memory modules are arranged on circuit board packages known as a Dual In-Line Memory Module (DIMM) - a module with memory chips, with edge-card connectors on both sides.

A comparison photo of a DDR3 DIMM and SO-DIMM.
A comparison photo of a DDR3 DIMM and SO-DIMM. Photo:

Compact desktops and notebook computers use a smaller memory packaging standard, known as a Small Outline DIMM (SO-DIMM). The SO-DIMM packaging of a particular memory standard is about half the size of a standard DIMM. They are electrically comparable in potential performance, but not physically compatible.

Different keys for DDR, DDR2 and DDR3 modules.
Different keys for DDR, DDR2 and DDR3 modules. Diagram:

The memory modules across memory standards can look very similar - however they are not electrically compatible (e.g. a DDR2 DIMM will not work in a DDR3 socket, and would lead to either the module or motherboard being damaged). To ensure that these sorts of mismatches are not possible, the modules are 'keyed' so that they will only fit in the socket corresponding to that specific memory technology.

Expansion Cards

There are also miniaturised versions of expansion card interfaces for portable computing. Given the more flexible requirements of portables, expansion slots may be internally or externally accessible.

Internally Accessible Portable Expansion

The expansion card interfaces for compact and portable computers designed for internal expansion (i.e. inside the case and/or behind an access panel, and not designed for regular changing) are simply miniaturised versions of their desktop counterparts, similar to the relationship between a memory DIMM and SO-DIMM.

These internal expansion options for compact computers are very commonly used for wireless networking, wireless broadband adapters, and solid state disks. It's not unusual for systems to be designed with expansion slots that will only allow a certain type, or a specific range of cards to actually work (even if they physically fit into the slot).

A Mini PCI and Mini PCIe card.
A Mini PCI and Mini PCIe card.


Note that the multiple interface options of a mini-PCIe card can cause confusion, as not all mini-PCIe slots support all modes.

For example, a mini-PCIe slot in a netbook may only support Serial ATA connectivity (and not USB or a PCIe 1x), as the system designers intended it to be used only for a solid state disk.

Externally Accessible Portable Expansion

For more general-purpose expansion of compact computers, an externally facing slot is used. This is to allow for the expansion card to be removed when not needed (saving both weight and power consumption), and for additional functions to be swapped out as the user sees fit - there is often only one or two of these external expansion ports available.

A variety of ExpressCards.
A variety of ExpressCards. Photo: PCMCIA Association

Another advantage of these externally-accessible expansion cards is that one edge is exposed from the computer case: this allows for external ports to be put onto the edge. Furthermore, the cards are designed to be hot-swappable.

Relative sizes of CardBus, ExpressCard/54, and ExpressCard/34 cards
Relative sizes of CardBus, ExpressCard/54, and ExpressCard/34 cards. Diagram: Amtron Technology, Inc.

Like the internal expansion slots, they are miniaturised versions of their desktop counterparts: