Intel 8086 and 8088

An index register has the function of receiving, storing, and outputting instruction-changing codes in a computer. The index register is in a computer's CPU.

The 8088 is slower than the 8086 because the 8088 is running an 8-bit bus, while the 8086 runs a 16-bit bus. The two processors are the same, 16-bit processors, but the 8088 requires twice as many memory accesses to do the same amount of work as the 8086.

Each segment in the 8086/8088 is 64KB because that is how Intel designed the microprocessor. The offset address is 16 bits, making the allowable range 64KB. See the related questions link for a further discussion of segmented architecture.

15 bits

A data bus is bidirectional because the processor needs it to both read and write. Also, in the case of the 4004, the data bus was tri-multiplexed with the 12 bit address bus and the 8 bit opcode bus.

Because that's how Intel designed it.

The 8085 is an 8-bit computer operating on a 16-bit address space.

The interrupt vector table in the 8085 is a region of low memory that contains the target addresses for the RST instructions. RST can be invoked by the program, by an INTR request which provides an RST x instruction in response to INTA, or by one of the four direct interrupt pins, TRAP, RST5.5, RST6.5, and RST7.5 Each of these interrupt sequences place the PC on the stack, and then execution goes to one of the vectors. The vectors are as follows... RST 0: 0000H

RST 1: 0008H

RST 2: 0010H

RST 3: 0018H

RST 4: 0020H

TRAP: 0024H

RST 5: 0028H

RST5.5: 002CH

RST 6: 0030H

RST6.5: 0034H

RST7: 0038H

RST7.5: 003CH

At this point you may be wondering why Intel designed the 8086 family devices to access memory using the segment: offset approach rather than accessing memory directly with 20-bit addresses. The segment: offset, scheme requires only a 16-bit number to represent the base address for a segment, and only a 16-bit offset to access any location in a segment. This means that the 8086 has to manipulate and store only 16-bit quantities instead of 20-bit quantities. This makes for an easier interface with 8- and 16-bit-wide memory boards and with the 16-bit registers in the 8086.

The second reason for segmentation has to do with the type of microcomputer in which an 8086-family CPU is likely to be used. In a timesharing system, several users share a CPU. The CPU works on one user's program for perhaps 20 ms, then works on the next user's program for 20 ms. After working 20 ms for each of the other users, the CPU comes back to the first user's program again. Each time the CPU switches from one user's program to the next, it must access a new section of code and new sections of data. Segmentation makes this switching quite easy. Each user's program can be assigned a separate set of logical segments for its code and data. The user's program will contain offsets or displacements from these segment bases. To change from one user's program to a second user's program, all that the CPU has to do is to reload the four segment registers with the segment base addresses assigned to the second user's program. In other words, segmentation makes it easy to keep users' programs and data separate from one another, and segmentation makes it easy to switch from one user's program to another user's program.

In the 8086/8088 microprocessor, there are eight 8 bit general registers, AH, AL, BH, BL, CH, CL, and DH, DL. They can be used as four 16 bit registers AX, BX, CX, and DX. There are four 16 bit pointer/index registers, SP (stack pointer), BP (base pointer), SI (source index), and DI (destination index). There are four 16 bit segment registers, CS (code segment), DS (data segment), SS (stack segment), and ES (extra segment). There is the 16 bit instruction pointer, IP, and there is a 16 bit flags register.

Basically , Demultiplexing is breaking of multiplexed signal .Recall that A/D0 -A/D15 and A16/S3-A19/S6 are the multiplexed signals in 8086.To do so, three demultiplexing latches are used .ALE (Address Enable Latch) is used for strobe Demultiplexing.8086 is 16bit data lines and 20 bit address line microprocessor.BY the Demultiplexing ,we Get A0-A19 separate Address lines and D0-D15 Data lines .

Ajmal Shahbaz

Each instruction of a computer specifies an operation on certain data. The are various ways of specifying address of the data to be operated on. These different ways of specifying data are called the addressing modes. The most common addressing modes are: * Immediate addressing mode * Direct addressing mode * Indirect addressing mode * Register addressing mode * Register indirect addressing mode * Displacement addressing mode * Stack addressing mode To specify the addressing mode of an instruction several methods are used. Most often used are : a) Different operands will use different addressing modes.

b) One or more bits in the instruction format can be used as mode field. The value of the mode field determines which addressing mode is to be used. The effective address will be either main memory address of a register. Immediate Addressing: This is the simplest form of addressing. Here, the operand is given in the instruction itself. This mode is used to define a constant or set initial values of variables. The advantage of this mode is that no memory reference other than instruction fetch is required to obtain operand. The disadvantage is that the size of the number is limited to the size of the address field, which most instruction sets is small compared to word length. INSTRUCTION

OPERAND Direct Addressing: In direct addressing mode, effective address of the operand is given in the address field of the instruction. It requires one memory reference to read the operand from the given location and provides only a limited address space. Length of the address field is usually less than the word length. Ex : Move P, Ro, Add Q, Ro P and Q are the address of operand. Indirect Addressing: Indirect addressing mode, the address field of the instruction refers to the address of a word in memory, which in turn contains the full length address of the operand. The advantage of this mode is that for the word length of N, an address space of 2N can be addressed. He disadvantage is that instruction execution requires two memory reference to fetch the operand Multilevel or cascaded indirect addressing can also be used. Register Addressing: Register addressing mode is similar to direct addressing. The only difference is that the address field of the instruction refers to a register rather than a memory location 3 or 4 bits are used as address field to reference 8 to 16 generate purpose registers. The advantages of register addressing are Small address field is needed in the instruction. Register Indirect Addressing: This mode is similar to indirect addressing. The address field of the instruction refers to a register. The register contains the effective address of the operand. This mode uses one memory reference to obtain the operand. The address space is limited to the width of the registers available to store the effective address. Displacement Addressing: In displacement addressing mode there are 3 types of addressing mode. They are : 1) Relative addressing

2) Base register addressing

3) Indexing addressing. This is a combination of direct addressing and register indirect addressing. The value contained in one address field. A is used directly and the other address refers to a register whose contents are added to A to produce the effective address. Stack Addressing: Stack is a linear array of locations referred to as last-in first out queue. The stack is a reserved block of location, appended or deleted only at the top of the stack. Stack pointer is a register which stores the address of top of stack location. This mode of addressing is also known as implicit addressing.

In the 8086/8088, the logical address (or effective address) is mapped to a physical address by adding it to the left shifted by 4 value of a segment register.

For example, is the logical address is 1234H, and the segment register's contents is 5678H, the physical address is 1234H + 56780H, or 579B4H.

Logical address is the address generated by the CPU (from the perspective of a program that is running) whereas physical address (or the real address) is the address seen by the memory unit and it allows the data bus to access a particular memory cell in the main memory. All the logical addresses need to be mapped in to physical addresses before they can be used by the MMU. Physical and logical addresses are same when using compile time and load time address binding but they differ when using execution time address binding.

Loosely coupled microprocessors are more like independent processors joined via a small communication link (a high speed bus or cable). This means that the connected microprocessors have their own local memory sets. They are low in performance. Also only microprocessors that can work independently may be used.

Tightly coupled microprocessors share a common memory for the purpose of communication. One processor (slave) is dependent on the other (master). They are good at performance. The connected processors also have local memory sets for general purposes.

The 8086 is not a co-processor. The 8087 is. The 8087 is intended to be coupled to an 8086/8088 to do math co-processing.

You need 12 address lines to access 4K of memory. 212 = 4096. log2 (4096) = 12.

The 8086 Microprocessor operate to require frequency that is provided by clock generator to 8086 Microprocessor and also Synchronization various component of 8086.

16KB, or 16384 bytes, can be addressed with 14 address lines. (214 = 16384)

A word in a computer is the native integer for that computer. In a 16 bit computer, a word is a 16 bit integer.

Inertia.

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The address bus is unidirectional becos address information is always given by microprocessor to i/o devices. The data bus is bidirectional bcos it takes the data from other devices & also give the data to other i/o devices