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Alternatively known as a bus slot or expansion port, an expansion slot is a connection or port inside a computer on the motherboard or riser card. It provides an installation point for a hardware expansion card to be connected. For example, if you wanted to install a new video card in the computer, you'd purchase a video expansion card and install that card into the compatible expansion slot.

A slotted screwdriver is a tool used to apply torque to screws using a flat tip. Torque is the pressure applied when a device is turned clockwise or counterclockwise into place. This type of tool is designed to fit into slotted screw heads, which feature a single diveted flat impression on top of. Looking for online definition of ISA or what ISA stands for? ISA is listed in the World's largest and most authoritative dictionary database of abbreviations and acronyms The Free Dictionary.

Microsoft's ISA Server (Internet Security and Acceleration Server) is the successor to Microsoft's Proxy Server 2.0 and is part of Microsoft's.NET support. Different Kinds of Expansion Slots. There have been several types of expansion slots over the years, including PCI, AGP, AMR, CNR, ISA, EISA, and VESA, but the most popular one used today is PCIe. While some newer computers still have PCI and AGP slots, PCIe has basically replaced all of the older technologies.

Computer expansion slots

Below is a listing of expansion slots commonly found in a computer and the devices associated with those slots. Clicking on any of the links below provide you with additional details.

• AGP - Video card.
• AMR - Modem, sound card.
• CNR - Modem, network card, sound card.
• EISA - SCSI, network card, video card.
• ISA - Network card, sound card, video card.
• PCI - Network card, SCSI, sound card, video card.
• PCI Express - Video card, modem, sound card, network card.
• VESA - Video card.

Many of the above expansion card slots are obsolete. You're most likely only going to encounter AGP, PCI, and PCI Express when working with computers today. In the picture below is an example of what expansion slots may look like on a motherboard. In this picture, there are three different types of expansion slots: PCI Express, PCI, and AGP.

How many expansion slots does my computer have?

Every computer motherboard is different, to determine how many expansion slots are on your computer motherboard identify the manufacturer and model of the motherboard. Once you've identified the model of motherboard, you can find complete information about the motherboard in its manual.

Adding additional expansion slots for older motherboards could be accomplished by using a riser board, which would add several ISA or PCI slots. Today, riser boards are rarely used with motherboards, as there is limited need for additional expansion slots with modern motherboards.

What type of expansion slots are on my motherboard?

As mentioned above, every motherboard model is unique, so to determine the type of expansion slots on the motherboard, consult the board's specifications and owner's manual. You can also open the computer case and visually examine the motherboard.

Why do computers have expansion slots?

Computers have expansion slots to give the user the ability to add new devices to their computer. For example, a computer gamer may upgrade their video card to get better performance in their games. An expansion slot allows them to remove the old video card and add a new video card without replacing the motherboard.

What is the most common expansion slot today?

Today, the most commonly used expansion slot used and found on computer motherboards is the PCI Express expansion slot.

Does a laptop have an expansion slot?

Laptops do not have expansion slots like a desktop computer. However, some laptops do have PC Cards that can be inserted into the side of the laptop. They may also have a Cardbus slot for an ExpressCard to be added.

Related expansion slot pages

Expansion, Expansion card, Motherboard terms, Seated

The Instruction Set Architecture (ISA) is the part of the processorthat is visible to the programmer or compiler writer. The ISA serves asthe boundary between software and hardware. We will briefly describe theinstruction sets found in many of the microprocessors used today. The ISAof a processor can be described using 5 catagories:

Operand Storage in the CPU

Where are the operands kept other than in memory?

Number of explicit named operands

How many operands are named in a typical instruction.

Operand location

Can any ALU instruction operand be located in memory? Or must all operandsbe kept internaly in the CPU?

Operations

What operations are provided in the ISA.

Type and size of operands

What is the type and size of each operand and how is it specified?

Of all the above the most distinguishing factor is the first.

The 3 most common types of ISAs are:

1. Stack - The operands are implicitly on top of the stack.
2. Accumulator - One operand is implicitly the accumulator.
3. General Purpose Register (GPR) - All operands are explicitelymentioned, they are either registers or memory locations.

Lets look at the assembly code of

in all 3 architectures:

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Not all processors can be neatly tagged into one of the above catagories.The i8086 has many instructions that use implicit operands although ithas a general register set. The i8051 is another example, it has 4 banksof GPRs but most instructions must have the A register as one of its operands.
What are the advantages and disadvantages of each of these approachs?

Stack

Advantages: Simple Model of expression evaluation (reverse polish).Short instructions.
Disadvantages: A stack can't be randomly accessed This makes ithard to generate eficient code. The stack itself is accessed every operationand becomes a bottleneck.

Accumulator

Advantages: Short instructions.
Disadvantages: The accumulator is only temporary storage so memorytraffic is the highest for this approach.

GPR

Advantages: Makes code generation easy. Data can be stored forlong periods in registers.
Disadvantages: All operands must be named leading to longer instructions.

Earlier CPUs were of the first 2 types but in the last 15 years allCPUs made are GPR processors. The 2 major reasons are that registers arefaster than memory, the more data that can be kept internaly in the CPUthe faster the program wil run. The other reason is that registers areeasier for a compiler to use.

Reduced Instruction Set Computer (RISC)

As we mentioned before most modern CPUs are of the GPR (General PurposeRegister) type. A few examples of such CPUs are the IBM 360, DEC VAX, Intel80x86 and Motorola 68xxx. But while these CPUS were clearly better thanprevious stack and accumulator based CPUs they were still lacking in severalareas:

1. Instructions were of varying length from 1 byte to 6-8 bytes. Thiscauses problems with the pre-fetching and pipelining of instructions.
2. ALU (Arithmetic Logical Unit) instructions could have operands thatwere memory locations. Because the number of cycles it takes to accessmemory varies so does the whole instruction. This isn't good for compilerwriters, pipelining and multiple issue.
3. Most ALU instructions had only 2 operands where one of the operandsis also the destination. This means this operand is destroyed during theoperation or it must be saved before somewhere.

Thus in the early 80's the idea of RISC was introduced. The SPARC projectwas started at Berkeley and the MIPS project at Stanford. RISC stands forReduced Instruction Set Computer. The ISA is composed of instructions thatall have exactly the same size, usualy 32 bits. Thus they can be pre-fetchedand pipelined succesfuly. All ALU instructions have 3 operands which areonly registers. The only memory access is through explicit LOAD/STORE instructions.
Thus C = A + B will be assembled as:

Although it takes 4 instructions we can reuse the values in the registers.

Why is this architecture called RISC? What is Reduced about it?
The answer is that to make all instructions the same length the numberof bits that are used for the opcode is reduced. Thus less instructionsare provided. The instructions that were thrown out are the less importantstring and BCD (binary-coded decimal) operations. In fact, now that memoryaccess is restricted there aren't several kinds of MOV or ADD instructions.Thus the older architecture is called CISC (Complete Instruction Set Computer).RISC architectures are also called LOAD/STORE architectures.

The number of registers in RISC is usualy 32 or more. The first RISCCPU the MIPS 2000 has 32 GPRs as opposed to 16 in the 68xxx architectureand 8 in the 80x86 architecture. The only disadvantage of RISC is its codesize. Usualy more instructions are needed and there is a waste in shortinstructions (POP, PUSH).

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So why are there still CISC CPUs being developed? Why is Intel spendingtime and money to manufacture the Pentium II and the Pentium III?
The answer is simple, backward compatibility. The IBM compatible PC isthe most common computer in the world. Intel wanted a CPU that would runall the applications that are in the hands of more than 100 million users.On the other hand Motorola which builds the 68xxx series which was usedin the Macintosh made the transition and together with IBM and Apple builtthe Power PC (PPC) a RISC CPU which is installed in the new Power Macs.As of now Intel and the PC manufacturers are making more money but withMicrosoft playing in the RISC field as well (Windows NT runs on Compaq'sAlpha) and with the promise of Java the future of CISC isn't clear at all.

An important lesson that can be learnt here is that superior technologyis a factor in the computer industry, but so are marketing and price aswell (if not more).

References For Further Reading

Isa Slots Definicion En

Philip Koopman Stack Computer Web pages