General Purpose registers are used for temporary storage of data and memory access. Since the processor accesses register more quickly than memory. 8086 has four 16-bit general-purpose registers AX, BX, CX and DX. These are available to the programmer, for storing values during programs. Each of these can be divided into two 8-bit registers such as AH, AL; BH, BL; CL, CH and DL, DH. Beside their general use, these registers also have some specific functions.

• AX Register (16-Bits): It holds operands and results during multiplication and division operations. All IO data transfers using IN and OUT instructions use A register (AL/AH or AX). It functions as accumulator during string operations.

Example:

MUL BL ; AX = (AL × BL)

MUL BX ; DX-AX = (AX × BX)

MUL BYTE PTR [BX] ; AX = (AL x DS:[BX])

• BX Register (16-Bits): It holds the memory address (offset address), in Indirect Addressing modes.

Example:

MOV CL, [BX] ; Moves a byte from the address pointed by BX in Data Segment into CL. Physical Address calculated as DS * 10H + BX

MOV CH, [BX+6] ; Moves a byte from the address pointed by BX+6 in Data Segment to CH; Physical Address: DS * 10H + BX + 6H

MOV CL, [BX+SI] ; Moves a byte from the address pointed by BX+SI in Data Segment to CL. Physical Address: DS * 10H + BX + SI

• CX Register (16-Bits): It holds count for instructions like: Loop, Rotate, Shift and String Operations.

Example:

MOV CX, 40H

BACK: MOV AL, BL

ADD AL, BL

. . MOV BL, AL

LOOP BACK

MOV CX, 40H BACK: MOV AL, BL ADD AL, BL . . MOV BL, AL LOOPZ BACK

• DX Register (16-Bits): It is used with AX to hold 32 bit values during Multiplication and Division. It is used to hold the address of the IO Port in indirect IO addressing mode.

Example:

MUL BX ; DX-AX = (AX × BX)

MOV DX, 2000H

IN AL, DX

Gateway operates at all 7 layers of the OSI model. It is a device, which connects two different dissimilar n/w which have the same function of communication. The situation where gateways are necessary for different n/w like Ethernet, Token Ring, FDDI, etc. It can communicate if they are using the same protocol for communication like TCP/IP 08 Apple talk if they are using different protocols from gateway cash forward packets across different n/w s that may also use different protocols. Eg: if n/w A is a Token Ring network using TCP/IP & network B is a Novell Network, a gateway can relay frames between the two. This means that a gateway has not only had but also between different protocols. In certain situations, the only changes required are to the frame header. In other cases, the gateway must take the case of different frame sizes, data rates, formats, acknowledgment schemes, priority schemes, etc.

Features of Gateways

• Gateway is located at the boundary of a network and manages all data that inflows or outflows from that network.
• It forms a passage between two different networks operating with different transmission protocols.
• A gateway operates as a protocol converter, providing compatibility between the different protocols used in the two different networks.
• The feature that differentiates a gateway from other network devices is that it can operate at any layer of the OSI model.
• It also stores information about the routing paths of the communicating networks.
• When used in enterprise scenario, a gateway node may be supplemented as proxy server or firewall.