8086

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Intel 8086 An Introduction Hindu College, Amritsar.

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Intel 8086 CPU: An Introduction 8086 Features 16-bit Arithmetic Logic Unit • 16-bit data bus • 20-bit address bus - 2 20 = 1,048,576 = 1 meg The address refers to a byte in memory. 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 least significant byte of a word on an 8086 family microprocessor is at the lower address. Hindu College, Amritsar.

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8086 Architecture • The 8086 has two parts, the Bus Interface Unit (BIU) and the Execution Unit (EU). • The BIU fetches instructions, reads and writes data, and computes the 20-bit address. • The EU decodes and executes the instructions using the 16-bit ALU. • The BIU contains the following registers: IP - the Instruction Pointer CS - the Code Segment Register DS - the Data Segment Register SS - the Stack Segment Register ES - the Extra Segment Register The BIU fetches instructions using the CS and IP, written CS:IP, to construct the 20-bit address. Data is fetched using a segment register (usually the DS) and an effective address (EA) computed by the EU depending on the addressing mode. Hindu College, Amritsar.

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8086 Block Diagram Hindu College, Amritsar.

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The EU contains the following 16-bit registers: AX - the Accumulator BX - the Base Register CX - the Count Register DX - the Data Register SP - the Stack Pointer BP - the Base Pointer SI - the Source Index Register DI - the Destination Register These are referred to as general-purpose registers, although, as seen by their names, they often have a special-purpose use for some instructions. The AX, BX, CX, and DX registers can be considered as two 8-bit registers, a High byte and a Low byte . This allows byte operations and compatibility with the previous generation of 8-bit processors, the 8080 and 8085. The 8-bit registers are: AX --> AH,AL BX --> BH,BL CX --> CH,CL DX --> DH,DL 8086 Architecture ] Default to stack segment Hindu College, Amritsar.

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8086 Architecture The EU also contains the Flag Register which is a collection of condition bits and control bits. The condition bits are set or cleared by the execution of an instruction. The control bits are set by instructions to control some operation of the CPU. Bit 0 - CF Carry Flag - Set by carry out of msb Bit 2 - PF Parity Flag - Set if result has even parity Bit 4 - AF Auxiliary Flag - for BCD arithmetic Bit 6 - ZF Zero Flag - Set if result is zero Bit 7 - SF Sign Flag = msb of result Bit 8 - TF Single Step Trap Flag Bit 9 - IF Interrupt Enable Flag Bit 10 - DF String Instruction Direction Flag Bit 11 - OF Overflow Flag Bits 1, 3, 5, 12-15 are undefined. Hindu College, Amritsar.

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8086 Programmer’s Model 16-bit Registers ES CS SS DS IP AH BH CH DH AL BL CL DL SP BP SI DI FLAGS AX BX CX DX Extra Segment Code Segment Stack Segment Data Segment Instruction Pointer Accumulator Base Register Count Register Data Register Stack Pointer Base Pointer Source Index Register Destination Index Register BIU registers (20 bit adder) EU registers 16 bit arithmetic Hindu College, Amritsar.

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Segments Segment Registers EXTRA 64K Data Segment 64K Code Segment CODE STACK DATA MEMORY Address 000000H 0FFFFFH Segments are < or = 64K, can overlap, start at an address that ends in 0H.  C S:0 Segment Starting address is segment register value shifted 4 place to the left. Hindu College, Amritsar.

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8086 Memory Terminology CODE DATA STACK EXTRA 0100H 0B200H 0CF00H 0FF00H DS: SS: ES: CS: 001000H 0B2000H 0CF000H 0FF000H 10FFFH 0C1FFFH 0DEFFFH 0FFFFFH 000000H Segment Registers Memory Segments Segments are < or = 64K and can overlap. Note that the Code segment is < 64K since 0FFFFFH is the highest address. Hindu College, Amritsar.

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The Code Segment The offset is the distance in bytes from the start of the segment. The offset is given by the IP for the Code Segment. Instructions are always fetched with using the CS register. The physical address is also called the absolute address 000000H Memory Segment Register Offset Physical or Absolute Address 0 + CS: IP 0400H 0056H 4000H 4056H 0400 0056 04056H CS:IP = 400:56 Logical Address 0FFFFFH Left-shift 4 bits Hindu College, Amritsar.

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The Data Segment Data is usually fetched with respect to the DS register. The effective address (EA) is the offset. The EA depends on the addressing mode. Memory Segment Register Offset Physical Address + DS: EA 05C0 0050 05C00H 05C50H 05C0 0 0050 05C50H DS:EA 000000H 0FFFFFH Hindu College, Amritsar.

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Addressing Modes DATA1 DW 25H DATA1 is defined as a word (16-bit) variable, i.e., a memory location that contains 25H. DATA2 EQU 20H DATA2 is not a memory location but a constant. Direct Addressing MOV AX,DATA1 [DATA1]  AX, the contents of DATA1 is put into AX. The CPU goes to memory to get data. 25H is put in AX. Immediate Addressing MOV AX,DATA2 DATA2 = 20H  AX, 20H is put in AX. Does not go to memory to get data. Data is in the instruction. MOV AX, OFFSET DATA1 The offset of SAM is just a number. The assembler knows which mode to encode by the way the operands SAM and FRED are defined. Assembler directive, DW = Define Word Hindu College, Amritsar.

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Addressing Modes Register Addressing MOV AX,BX AX BX Register Indirect Addressing MOV AX,[BX] AX DS:BX Can use BX or BP -- Based Addressing (BP defaults to SS) or DI or SI -- Indexed Addressing The offset or effective address (EA) is in the base or index register. Register Indirect with Displacement MOV AX,SAM[BX] AX DS:BX + Offset SAM Indexed with displacement Based with displacement Based-Indexed Addressing MOV AX,[BX][SI] EA = BX + SI Based-Indexed w/Displacement MOV AX,SAM[BX][DI] EA = BX + DI + offset SAM AX DS:EA where EA = BX + offset SAM Hindu College, Amritsar.

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Addressing Modes Branch Related Instructions Intrasegment (CS does not change) Direct -- IP relative displacement new IP = old IP + displacement Allows program relocation with no change in code. Indirect -- new IP is in memory or a register. All addressing modes apply. Intersegment Direct -- new CS and IP are encoded in (CS changes ) the instruction. Indirect -- new CS and IP are in memory. All addressing modes apply except immediate and register. NEAR JUMPS and CALLS FAR Hindu College, Amritsar.

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Assembly Language The Assembler is a program that reads the source program as data and translates the instructions into binary machine code. The assembler outputs a listing of the addresses and machine code along with the source code and a binary file (object file) with the machine code. Most assemblers scan the source code twice -- called a two-pass assembler . The first pass determines the locations of the labels or identifiers. The second pass generates the code. Hindu College, Amritsar.

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Assembly Language To locate the labels, the assembler has a location counter . This counts the number of bytes required by each instruction. When the program starts a segment, the location counter is zero. If a previous segment is re-entered, the counter resumes the count. The location counter can be set to any offset by the ORG directive. In the first pass, the assembler uses the location counter to construct a symbol table which contains the offsets or values of the various labels. The offsets are used in the second pass to generate operand addresses . Hindu College, Amritsar.

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adc Add with carry flag add Add two numbers and Bitwise logical AND call Call procedure or function cbw Convert byte to word (signed) cli Clear interrupt flag (disable interrupts) cwd Convert word to doubleword (signed) cmp Compare two operands dec Decrement by 1 div Unsigned divide idiv Signed divide imul Signed multiply in Input (read) from port inc Increment by 1 int Call to interrupt procedure Instruction Set Hindu College, Amritsar.

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iret Interrupt return j?? Jump if ?? condition met jmp Unconditional jump lea Load effective address offset mov Move data mul Unsigned multiply neg Two's complement negate nop No operation not One's complement negate or Bitwise logical OR out Output (write) to port pop Pop word from stack popf Pop flags from stack push Push word onto stack Instruction Set (Contd.) Hindu College, Amritsar.

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pushf Push flags onto stack ret Return from procedure or function sal Bitwise arithmetic left shift (same as shl) sar Bitwise arithmetic right shift (signed) sbb Subtract with borrow shl Bitwise left shift (same as sal) shr Bitwise right shift (unsigned) sti Set interrupt flag (enable interrupts) sub Subtract two numbers test Bitwise logical compare xor Bitwise logical XOR Instruction Set (Contd.) Hindu College, Amritsar.

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Conditional Jumps Name/Alt Meaning Flag setting JE/JZ Jump equal/zero ZF = 1 JNE/JNZ Jump not equal/zero ZF = 0 JL/JNGE Jump less than/not greater than or = (SF xor OF) = 1 JNL/JGE Jump not less than/greater than or = (SF xor OF) = 0 JG/JNLE Jump greater than/not less than or = ((SF xor OF) or ZF) = 0 JNG/JLE Jump not greater than/ less than or = ((SF xor OF) or ZF) = 1 JB/JNAE Jump below/not above or equal CF = 1 JNB/JAE Jump not below/above or equal CF = 0 JA/JNBE Jump above/not below or equal (CF or ZF) = 0 JNA/JBE Jump not above/ below or equal (CF or ZF) = 1 JS Jump on sign (jump negative) SF = 1 JNS Jump on not sign (jump positive) SF = 0 JO Jump on overflow OF = 1 JNO Jump on no overflow OF = 0 JP/JPE Jump parity/parity even PF = 1 JNP/JPO Jump no parity/parity odd PF = 0 JCXZ Jump on CX = 0 --- Hindu College, Amritsar.

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More Assembler Directives ASSUME Tells the assembler what segments to use. SEGMENT Defines the segment name and specifies that the code that follows is in that segment. ENDS End of segment ORG Originate or Origin: sets the location counter. END End of source code. NAME Give source module a name. DW Define word DB Define byte. EQU Equate or equivalence LABEL Assign current location count to a symbol. $ Current location count Hindu College, Amritsar.

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? Hindu College, Amritsar.