8086 Microprocessor architecture

INTRODUCTION

·        It is a semiconductor device consisting of electronic logic circuits manufactured by using either a Large scale (LSI) or Very Large Scale (VLSI) Integration Technique.

·        It includes the ALU, register arrays and control circuits on a single chip. The microprocessor has a set of instructions, designed internally, to manipulate data and communicate with peripherals.

·        The era microprocessors in the year 1971, the Intel introduced the first 4-bit microprocessor is 4004. Using this the first portable calculator is designed.

·        The 16-bit Microprocessor families are designed primarily to complete with microcomputers and are oriented towards high-level languages. They have powerful instruction sets and capable of addressing mega bytes of memory.

·        The era of 16-bit Microprocessors began in 1974 with the introduction of PACE chip by National Semiconductor. The Texas Instruments TMS9900 was introduced in the year 1976. The Intel 8086 commercially available in the year 1978, Zilog Z800 in the year 1979, The Motorola MC68000 in the year 1980.

·        The 16-bit Microprocessors are available in different pin packages. Ex: Intel 8086/8088 40 pin package Zilog Z8001 40 pin package, Digital equipment LSI-II 40 pin package, Motorola MC68000 64 pin package National Semiconductor NS16000 48 pin package.

·        The primary objectives of this 16-bit Microprocessor can be summarized as follows.

1. Increase memory addressing capability

2. Increase execution speed

3. Provide a powerful instruction set

4. Facilitate programming in high-level languages.

Microprocessor Architecture:

·        The 8086 CPU is divided into two independent functional parts, the Bus interface unit (BIU) and execution unit (EU).

The Bus Interface Unit contains Bus Interface Logic, Segment registers, Memory addressing logic and a Six byte instruction object code queue. The BIU sends out address, fetches the instructions from memory, read data from ports and memory, and writes the data to ports and memory.

·        The execution unit: contains the Data and Address registers, the Arithmetic and Logic Unit, the Control Unit and flags. tells the BIU where to fetch instructions or data from, decodes instructions and executes instruction. The EU contains control circuitry which directs internal operations. A decoder in the EU translates instructions fetched from memory into a series of actions which the EU carries out. The EU is has a 16-bit ALU which can add, subtract, AND, OR, XOR, increment, decrement, complement or shift binary numbers. The EU is decoding an instruction or executing an instruction which does not require use of the buses.

In other words the BIU handles all transfers of data and addresses on the buses for the execution unit.

The Queue: The BIU fetches up to 6 instruction bytes for the following instructions. The BIU stores these prefetched bytes in first-in-first-out register set called a queue. When the EU is ready for its next instruction it simply reads the instruction byte(s) for the instruction from the queue in the BIU. This is much faster than sending out an address to the system memory and waiting for memory to send back the next instruction byte or bytes.

Except in the case of JMP and CALL instructions, where the queue must be dumped and then reloaded starting from a new address, this prefetch-and-queue scheme greatly speeds up processing. Fetching the next instruction while the current instruction executes is called pipelining.

·        Word Read: Each of 1 MB memory address of 8086 represents a byte wide location.16-bit

words will be stored in two consecutive memory locations. If first byte of the data is stored at an even address, 8086 can read the entire word in one operation.

For example if the 16 bit data is stored at even address 00520H is 9634H MOV BX, [00520H]

8086 reads the first byte and stores the data in BL and reads the 2nd byte and stores the data in BH

BL= (00520H) i.e. BL=34H BH= (00521H) BH=96H

If the first byte of the data is stored at an odd address, 8086 needs two operations to read the 16 bit data.

For example if the 16 bit data is stored at even address 00521H is 3897H MOV BX, [00521H]

In first operation, 8086 reads the 16 bit data from the 00520H location and stores the data of 00521H location in register BL and discards the data of 00520H location In 2^nd operation, 8086 reads the 16 bit data from the 00522H location and stores the data of 00522H location in register BH and discards the data of 00523H location.

BL= (00521H) i.e. BL=97H BH= (00522H) BH=38H

·        Byte Read: MOV BH, [Addr]

For Even Address:

Ex: MOV BH, [00520H]

8086 reads the first byte from 00520 location and stores the data in BH and reads the 2^nd byte from the 00521H location and ignores it

BH =[ 00520H]

For Odd Address

MOV BH, [Addr]

Ex: MOV BH, [00521H]

8086 reads the first byte from 00520H location and ignores it and reads the 2nd byte from the 00521 location and stores the data in BH

BH = [00521H]

·        Physical address formation:

The 8086 addresses a segmented memory. The complete physical address which is 20-bits long is generated using segment and offset registers each of the size 16-bit.The content of a segment register also called as segment address, and content of an offset register also called as offset address. To get total physical address, put the lower nibble 0H to segment address and add offset address. The fig 1.3 shows formation of 20-bit physical address.

·        Register organization of 8086:

All the registers of 8086 are 16-bit registers. The general purpose registers, can be used either 8-bit registers or 16-bit registers used for holding the data, variables and intermediate results temporarily or for other purpose like counter or for storing offset address for some particular addressing modes etc. The special purpose registers are used as segment registers, pointers, index registers or as offset storage registers for particular addressing modes. Fig 1.3

ü AX Register: Accumulator register consists of two 8-bit registers AL and AH, which can be combined together and used as a 16- bit register AX. AL in this case contains the low-order byte of the word, and AH contains the high-order byte. Accumulator can be used for I/O operations, rotate and string manipulation.

ü BX Register: This register is mainly used as a base register. It holds the starting base location of a memory region within a data segment. It is used as offset storage for forming physical address in case of certain addressing mode.

ü CX Register: It is used as default counter - count register in case of string and loop instructions.

ü DX Register: Data register can be used as a port number in I/O operations and implicit operand or destination in case of few instructions. In integer 32-bit multiply and divide instruction the DX register contains high-order word of the initial or resulting number.

Segment registers:

1Mbyte memory is divided into 16 logical segments. The complete 1Mbyte memory segmentation is as shown in fig 1.4. Each segment contains 64Kbyte of memory. There are four segment registers.

Code segment (CS) is a 16-bit register containing address of 64 KB segment with processor instructions. The processor uses CS segment for all accesses to instructions referenced by instruction pointer (IP) register. CS register cannot be changed directly.

The CS register is automatically updated during far jump, far call and far return instructions. It is used for addressing a memory location in the code segment of the memory, where the executable program is stored.

ü Stack segment (SS) is a 16-bit register containing address of 64KB segment with program stack. By default, the processor assumes that all data referenced by the stack pointer (SP) and base pointer (BP) registers is located in the stack segment. SS register can be changed directly using POP instruction. It is used for addressing stack segment of memory. The stack segment is that segment of memory, which is used to store stack data.

ü Data segment (DS) is a 16-bit register containing address of 64KB segment with program data. By default, the processor assumes that all data referenced by general registers (AX, BX, CX, DX) and index register (SI, DI) is located in the data segment. DS register can be changed directly using POP and LDS instructions. It points to the data segment memory where the data is resided.

ü Extra segment (ES) is a 16-bit register containing address of 64KB segment, usually with program data. By default, the processor assumes that the DI register references the ES segment in string manipulation instructions. ES register can be changed directly using POP and LES instructions. It also refers to segment which essentially is another data segment of the memory.

ü It also contains data.

ü Pointers and index registers.

The pointers contain within the particular segments. The pointers IP, BP, SP usually contain offsets within the code, data and stack segments respectively

Stack Pointer (SP) is a 16-bit register pointing to program stack in stack segment.

Base Pointer (BP) is a 16-bit register pointing to data in stack segment. BP register is usually used for based, based indexed or register indirect addressing.

Source Index (SI) is a 16-bit register. SI is used for indexed, based indexed and register indirect addressing, as well as a source data addresses in string manipulation instructions.

Destination Index (DI) is a 16-bit register. DI is used for indexed, based indexed and register indirect addressing, as well as a destination data address in string manipulation instructions.

ü Flag Register:

Flags Register determines the current state of the processor. They are modified automatically by CPU after mathematical operations, this allows to determine the type of the result, and to determine conditions to transfer control to other parts of the program. The 8086 flag register as shown in the fig 1.5. 8086 has 9 active flags and they are divided into two categories:

1. Conditional Flags

2. Control Flags

ü Conditional Flags

Carry Flag (CY): This flag indicates an overflow condition for unsigned integer arithmetic. It is also used in multiple-precision arithmetic.

Auxiliary Flag (AC): If an operation performed in ALU generates a carry/barrow from lower nibble (i.e. D0 – D3) to upper nibble (i.e. D4 – D7), the AC flag is set i.e. carry given by D3 bit to D4 is AC flag. This is not a general-purpose flag, it is used internally by the Processor to perform Binary to BCD conversion.

Parity Flag (PF): This flag is used to indicate the parity of result. If lower order 8-bits of the result contains even number of 1’s, the Parity Flag is set and for odd number of

1’s, the Parity flag is reset.

Zero Flag (ZF): It is set; if the result of arithmetic or logical operation is zero else it is reset.

Sign Flag (SF): In sign magnitude format the sign of number is indicated by MSB bit. If the result of operation is negative, sign flag is set.

ü Control Flags

Control flags are set or reset deliberately to control the operations of the execution unit. Control flags are as follows:

Trap Flag (TF): It is used for single step control. It allows user to execute one instruction of a program at a time for debugging. When trap flag is set, program can be run in single step mode.

Interrupt Flag (IF): It is an interrupt enable/disable flag. If it is set, the maskable interrupt of 8086 is enabled and if it is reset, the interrupt is disabled. It can be set by executing instruction sit and can be cleared by executing CLI instruction.

Direction Flag (DF): It is used in string operation. If it is set, string bytes are accessed from higher memory address to lower memory address. When it is reset, the string bytes are accessed from lower memory address to higher memory address.