Intel 8086 Microprocessor Architecture Notes

Intel  8086 Microprocessor  Architecture

Introduction

The 8086 was the first 16-bit Microprocessor to be introduced by Intel Corporation. The word 16-bit means that its arithmetic logical unit, internal registers, and most of its instructions are designed to work with 16-bit binary words. The 8086 has a 16-bit data bus, so it can read data form or write data to memory and ports either 16-bits or 8- bits at a time. The 8086 has a 20-bit address bus, so it can address any one of 220 or 1,048,576 memory locations. Each of the 1,048,576 memory addresses of the 8086 represents a byte-wide location. Words will be stored in two consecutive memory locations. If the first byte of a word is at an even address, the 8086 can read the entire word in one operation. If the first byte of the word is at an odd address, the 8086 will read the first byte of the word in one operation, and the second byte in another operation.

Features:

·              Released by Intel in 1978

·              Produced from 1978 to 1990s

·              A 16-bit microprocessor chip.

·              Max. CPU clock rate:5 MHz to 10 MHz

·              Instruction set:  x86-16

·              Package: 40 pin DIP

·              16-bit Arithmetic Logic Unit

·              16-bit data bus  (8088 has 8-bit data bus)

·              20-bit address bus - 220 = 1,048,576 = 1 meg

·              The address refers to a byte in memory.

·               In the 8088, these bytes come in on the 8-bit data bus.  In the 8086, bytes at even addresses come in on the low half of the data bus (bits 0-7) and bytes at odd addresses come in on the upper  half of the data bus (bits 8-15).

·              The 8086 can read a 16-bit word at an even address in one operation and at an odd address in two operations.  The 8088 needs two operations in either case.

·              The least significant byte of a word on an 8086 family microprocessor is at the  lower address.

The 8086 microprocessor architecture is internally divided into two separate functional units.

·        Bus Interface Unit (BIU)

·        Execution Unit (EU).

(REFER DIAGRAM 1-Figure shows a block diagram of the 8086 internal architecture.)

The BIU fetches instructions, reads data from memory and ports, and writes data to memory and I/O ports. The EU executes instructions that have already been fetched by the BIU. The BIU and EU function independently. The BIU interfaces the 8086 to the outside world. The BIU provides all external bus operations. The BIU contains segment registers, instruction pointer, instruction queue, and address generation bus control circuitry to provide functions such as fetching and queuing of instructions, and bus control.

The BIU’s instruction queue is a First-In First-out (FIFO) group of registers in which up to six bytes of instruction code are perfected from memory ahead of time. This is done in order to speed up program execution by overlapping instruction fetch with execution. This mechanism is known as pipelining. If the queue is full and the EU does not request BIU to access memory, the BIU does not perform any bus cycle. On the other hand, if the BIU’s is not full and if it can store at least two bytes and the EU does not request it to access memory, the BIU may prefect instructions. However, if BIU is interrupted by EU for memory access while the BIU is in the process of fetching an instruction, the BIU first completes fetching and then services the EU : the queue allows the BIU to keep the EU supplied with perfected instructions without typing up the system bus. If an instruction such as Jump or subroutine call is encountered, the BIU will reset the queue and begin refilling after passing the new instruction to the EU.

            The BIU contains a dedicated adder, which is used to produce the 20-bit address. The bus control logic of the BIU generates all the bus control signals such as read and write signals for memory and I/O.

            The BIU has four 16-bit segment registers. These are the Code Segment (CS) register, the Data Segment (DS) register, the Stack Segment (SS) register, and the Extra Segment (ES) register. The 8086’s one-megabyte memory is divided into segments of up to 64K bytes each. The 8086 can directly address four segments (256K byte within the 1 Mbytes memory) at a particular time. Programs obtain access to code and data in the segments by changing the segment register contents to point to the desired segments. All program instructions must be located in main memory pointed to by the 16-bit CS register with a 16-bit offset in the segment contained in the 16-bit instruction pointer (IP). The BIU computes the 20-bit physical address internally using the programmer-provided logical address (16-bit contents of CS and IP) by logically shifting the contents of CS four bits to left and then adding the 16-bit contents of IP. In other words, the CS is multiplied by 16₁₀ by the BIU for computing the 20-bit physical address. This means that all instructions of a program are relative to the contents of the CS register multiplied by 16 and then offset is added provided by the 16-bit contents of IP. For example, if [CS] = 456A₁₆ and [IP] = 1620₁₆, then the 10-bit physical address is generated by the BIU as follows:

                                                                                                    

            Four times logically shifted [CS] to left  =  456A0₁₆

            + [IP] as offset                                                  =  1620₁₆

            20-bit physical address                     =  46CC0₁₆

            The BIU always inserts four Zeros for the lowest 4-bits of the 20-bit starting address (physical) of a segment. In the other words, the CS contains the base or start of the current code segment, and IP contains the distance or offset from this address to the next instruction byte to be fetched. Note that immediate data are considered as part of the code segment.

            The SS register points to the current stack. The 20-bit physical stack address is calculated from SS and SP for stack instruction such as PUSH and POP. The programmer

Can use the BP register instead of SP for accessing the stack using the based addressing mode. In this case, the 20-bit physical stack address is calculated from BP and SS.

            The DS register points to the current data segment; operands for most instructions are fetched from this segment. The 16-bit contents of Source Index (SI) or Destination Index (DI) are used as offset for computing the 20-bit physical address.

            The ES register points to the extra segment in which data (in excess of 64k pointed to by DS) is stored. String instructions always use ES and DI to determine the 20-bit physical address for the destination.

            The segment can be continuous, partially overlapped, fully overlapped, or disjoint. An example of how five (segment 1 through segment 5) may be stored in physical memory are shown.
a)Segments are contiguous (adjacent)

 

SEGMENTS 1 an d 2 are partially overlapped, SEGMENTS 2 and 3 are fully overlapped, and SEGMENTS 2 and 4 are disjoint.. Every segment must start on 16-byte memory boundaries.

            Typical examples of values of segments should then be selected based on physical addresses starting at 00000₁₆ , 00010₁₆ , 00020_{16 ,} 00030_16,  … FFFF0₁₆  . A physical memory location may be mapped into (contained in) one or more logical segments. Many applications can be written to simply initialize the segment and then forget them.

           

The 8086 uses segmented memory. A memory address on the 8086 consists of two numbers, usually written in hexadecimal and separated by a colon, representing the segment and the offset. This combination of segment and offset is referred to as a logical address. The segment number refers to a 64 KB block of memory and the offset is an index into the memory segment.

When the processor obtains a logical address (segment and offset), it performs a simple calculation to determine the 20-bit physical address in memory to which the logical address refers:

 physical address = (segment << 4) + offset

This is equivalent to multiplying the segment by 16 and adding the offset (i.e., physical address = segment * 16 + offset). This means that the 64 KB segments overlap, with a new segment starting every 16 bytes. This also means that there can be more than one address for the same memory location. For example, 0000:0100, 0001:00F0, and 0010:0000 all refer to physical address 0x100

The EU decodes and executes instructions. A decoder in the EU control system translates instructions. The EU has a 16-bit ALU for performing arithmetic and logic operations.

            The EU has eight 16-bit general registers. These are AX, BX, CX, DX, SP, BP, SI, and DI. The 16-bit registers AX, BX, CS, and DX can be used as two 8-bit registers (AH, AL, BH, BL, CH, CL, DH, DL). For example, the 16-bit register DX can be considered as two 8-bit registers DH (high byte of DX) and DL (low byte of DX). The general-purpose registers AX, BX, CX, and DX are named after special functions carried out by each one of them. For example, the AX is called the 16-bit accumulator while the AL is the 8-bit accumulator. The use of accumulator registers is assumed by some instructions. The Input/Output (IN or OUT) instructions always use AX or AL for inputting/outputting 16- or 8-bit data to or from and I/O port.

            Multiplication and division instructions also use AX or AL. The AL register is the same as the 8085 A register.

BX  register is called the base register. This is the only general-purpose register, the contents of which can be used for addressing 8086 memory. All memory references utilizing these register contents for addressing use Ds as the default segment register. The BX register is similar to 8085 HL register. In other words, 8086 BH and BL are equivalent to 8085 H and L registers, respectively.

The CX register is known as the counter register. This is because some instructions such as shift rotate, and loop instructions use the contents of CX as a counter. For example, the instruction LOOP START will automatically decrement CX by 1 without affecting flags and will check if [CX]=0. If is zero, the 8086 executes the next instruction; otherwise the 8086 branches to the label START.

The data register DX is used to hold high 16-bit result (data) in 16* 16 multiplication or high 16-bit dividend (data) before a 32 ¸16 division and the 16-bit remainder after the division.

The two pointer registers, SP (stack pointer) and BP (base pointer), are used to access data in stack segment. The SP is used as an offset from the current SS during execution of instructions that involve stack segment in external memory. The SP contents are automatically updated (incremented or decremented) due to execution of POP or PUSH instruction.

The base pointer contains an offset address in the current SS. This offset is used by the instructions utilizing the based addressing mode.

The FLAG register in the EU holds the status flags typically after an ALU operation.

The 8086 have six one-bit flags. AF (Auxiliary carry flag) is used by BCD bit) into the high nibble or a borrow from the high nibble into the low nibble of the low-order 8-bit of a 16-bit number. CF (Carry Flag) is set if there is a carry from addition or borrow from subtraction. OF (Overflow Flag) is set if there is an arithmetic overflow, that is, if the size of the result exceeds the capacity of the destination location. An interrupt on overflow instructions is available which will generate an interrupt in this situation. SF (Sign Flag) is set if the most significant bit of the result is one (Negative) and is cleared to zero for non-negative result. PF (Parity Flag) is set if the result has even parity; PF is zero for odd parity of the result. ZF (Zero Flag) is set if the result is zero; ZF is zero for non-zero result.

The 8086 has three control bits in the flag register which can be set or reset by the programmer: setting DF (Direction Flag) to one causes string instructions to auto decrement and clearing DF to zero causes string instructions to auto increment. Setting IF (Interrupt Flag) to one causes the 8086 to recognize external mask able interrupts; clearing IF to zero disables these interrupts. Setting TF (Trace Flag) to one places the 8086 in the single-step mode. In this mode, the 8086 generate an internal interrupt after execution of each instruction. The user can write a service routine at the interrupt address vector to display the desired registers and memory locations. The user can thus debug a program