Computer Organization & Assembly Language Chapter 3

The 8086 and 8088 Microprocessors: Intel introduced the 8086 in 1978 as its first 16-bit microprocessor. A 16-bit processor can operate on 16 bits of data at a time. Internally the 8086 and 8088 essentially have the same internal structure, the name “8086” applies to both 8086 and 8088. Externally, the 8086 has an 16-bit data bus. While the 8088 has an 8-bit data bus.

Registers: General-Purpose Registers Data registers are available to programmers for general data manipulation. Even though a processor can operate on data stored in memory, the same instruction is faster (requires fewer clock cycles) if the data are stored in registers. The high and low bytes of data registers can be accessed separately.

General Purpose: AX BX CX DX

AX (Accumulator Register): AX is the preferred register to use in arithmetic, logic, and data transfer instructions because its use generates the shortest machine code. In multiplication and division operations, one of the numbers involved must be in AX or AL. Divided into AH / AL Input and output operations also require the use of AL and AX.

Registers (16-bit) General Purpose 16-bit AX AH AL 8-bit 8-bit

BX (Base Register): BX also serves as an address register.

Registers (16-bit) General Purpose 16-bit CX - the count register BX BH BL 8-bit 8-bit

CX (Count Register): Program loop constructions are facilitated by the use of CX, which serves as a loop counter. CL is used as a count in instructions that shift and rotate bits. Iterative code segments using the LOOP instruction

DX (Data Register): DX is used in multiplication and division. DX is also used in I/O operations.

Registers General Purpose AX A Accumulator Register BX B Base Register CX C Counter Register DX D Destination Register

Segmentation: It is the process in which the main memory of computer is divided into different segments and each segment has its own base address. Segmentation is used to increase the execution speed of computer system so that processor can able to fetch and execute the data from memory easily and fastly.

 Segmentation in 8086 The size of address bus of 8086 is 20 and is able to address 1 Mbytes ( ) of physical memory. The compete 1 Mbytes memory can be divided into 16 segments, each of 64 Kbytes size. The addresses of the segment may be assigned as 0000H to F000H respectively. The offset values are from 0000H to FFFFFH.

Segment Registers: Address Registers Segment registers store addresses of instructions and data in memory. These values are used by a processor to access memory location. The idea of memory segments is a direct consequence of using a 20-bit address in a 16-bit processor.

Pointer / Index / Base: SP, BP, SI, and DI normally point to (contain the offset addresses of) memory locations. Unlike segment registers, pointer and index registers can be used in arithmetic and other operations.

88 Registers (16-bit) Pointer / Index / Base SI SI Source Index DI DI Destination Index IP IP Instruction Pointer SP SP Stack Pointer BP BP Base Pointer

SP (Stack Pointer): SP is used in conjunction with SS for accessing the stack segment.

BP (Base Pointer): BP is used primarily to access data on the stack. Unlike SP, we can also use BP to access data in the other segments.

SI (Source Index): SI is used to point to memory locations in the data segment addressed by DS. By incrementing the contents of SI, we can easily access consecutive memory locations.

DI (Destination Index): DI performs the same functions as SI.

IP (Instruction Pointer): To access instructions, 8086 uses CS and IP. CS contains the segment number of the next instruction, and IP contains the offset. IP is updated each time an instruction is executed so that it will point to the next instruction. Unlike other registers, IP cannot be directly manipulated by an instruction; that is, an instruction may not contain IP as its operand.

Operating system: An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs. The operating system is a component of the system software in a computer system. Application programs usually require an operating system to function.

OS Function: Reading and executing commands typed by the user :   Recognizing input from the keyboard, sending output to the display screen. Perform I/O operations: Controlling peripheral devices such as disk drives and printers. Generating Error message: Managing memory and other resources; It makes sure that different programs and users running at the same time do not interfere with each other. 

Example: DOS LINUX SOLARIS MAC OS UNIX WINDOWS

DOS Stands for "Disk Operating System." DOS was the first operating system used by IBM-compatible computers. DOS was designed for 8086/8088. DOS uses a command line, or text-based interface, that allows the user to type commands. By typing simple instructions such as pwd (print working directory) and cd (change directory), the user can browse the files on the hard drive, open files, and run programs. While the commands are simple to type, the user must know the basic commands in order to use DOS effectively (similar to Unix). .

It can manage only IMB of memory so does not support multitasking. it can be use also in 80286/80386/80486 based machines when they are run in real mood. The first versions of Windows (through Windows 95) actually ran on top of the DOS operating system. This is why so many DOS-related files (such as .INI, .DLL, and .COM files) are still used by Windows. DOS is still included with Windows, but is run from the Windows operating system instead of the other way around. 

BIOS: When a Computer is switched on POST (Power on Self Test) operation is performed, which checks integrity of all components (CPU, Memory, I/O devices) of the system. If a component is found faulty , error message will be displayed. A chip called ROM - BIOS is read and executed. BIOS (Basic Input Output Services) information is a pre-written program that is permanently stored on ROM chip.  Other important functions perform by BIOS are circuit checking and loading of BIOS routines.