1/2002JNM1 Positional Notation (Hex Digits). 1/2002JNM2 Problem The 8086 has a 20-bit address bus. Therefore, it can access 1,048,576 bytes of memory.

1/2002 JNM 1

Positional Notation (Hex Digits)

1/2002 JNM 2

Problem

• The 8086 has a 20-bit address bus. Therefore, it can access 1,048,576 bytes of memory. How many bits and how many HEX digits are required to access 1M memory?

• 2N = 1M (where N is in bits)

• N = 20 bits = 20/4 = 5 HEX digits

1/2002 JNM 3

Memory Map for an 80x86 computer running MS-DOS

1/2002 JNM 4

The memory space of the 8086 consists of 1,048,576 bytes or 524,288 16-bit words.

1 MB Memory Space of 8086

1/2002 JNM 5

Within the 1 MB of memory, the 8086 defines 4 64KB memory blocks.

Segmented Memory

7FFFF

The segment registers point to location 0 of each segment. (The base address)

DS: E000 CS: B300

SS: 7000 ES: 5D27

1/2002 JNM 6

Assembly Language Statements

Instructions - Executable Statements

Directives – provide information to assembler about how to generate executable code

Format -

[name] [mnemonic] [operands] [;comment]

1/2002 JNM 7

Names

• Max 247 characters (MASM)

• No distinction between uppercase, lowercase letters.

• First character can be a letter, ‘@’, ‘_’, ‘$’

• Cannot use an assembler reserved word.

• Identify labels, variables, symbols, keywords

1/2002 JNM 8

OP Codes and Operands

Op-code destination operand, source operand

HLT ; zero operandsINC AX ; one operandMOV AX, 100 ; two operandsSHLD DX, AX, 4 ; three operands

1/2002 JNM 9

AL 00

Immediate Addressing Mode

Mov AL, 3CH

AL 3C

1/2002 JNM 10

Register Addressing Mode

Mov AL, BL

AL 00BL 4D

AL 4DBL 4D

1/2002 JNM 11

Direct Addressing Mode

0023 12

0022 34

0021 56

0020 78

Mov CL, [0020H]

Mov CX, [0020H]

Memory (data)

Little Endian Format – The “little” end of the number is stored first.

CL 78

CX 5678

Mov ECX, [0020H]ECX 12345678

1/2002 JNM 12

Register Indirect Addressing Mode

location

0023 12

0022 34

0021 56

0020 78Mov CL, [SI]

In the 8086, only BX, BP, SI and DI may be used as memory pointers. Later processors don’t have this restriction.

Mov SI, 0022H

CL 34

SI 0022

1/2002 JNM 13

BX 0020

Base + Displacementlocation

0023 12

0022 34

0021 56

0020 78Mov AL, [BX +2]

Useful when accessing individual elements in an array. Note that the array begins with element 0, element 2 corresponds to the third location in the array. The displacement corresponds to byte offset from the base, not element number in the array.

Mov BX, 0020H

AL 34

1/2002 JNM 14

BX 0020

Base + Index + Displacement(Useful when accessing individual elements in an record)

Mov BX, 0020H ;BX points to record starting address

SI 000C

Mov SI, 000CH ;SI points to record three (4 elements ;per record x 3 records = 000C)

Mov AL, [BX+SI+1] ;AL now has the data in element 1 of ;record #3 (assumes elements are 1 byte)

1/2002 JNM 15

• Base + Displacement

– Mov AL, [BX + 4]

• Base + Index + Displacement

– Mov AL, [BX+SI+3]

• Base + Index*Scale + Displacement

– Mov AL,[BX+SI*4+3]

• Immediate

– Mov AL, 4CH

• Register

– Mov AL, BL

• Direct

– Mov AL, [20H]

• Register Indirect

– Mov AL, [SI]

Instruction Addressing Modes

1/2002 JNM 16

Segment Register Defaults

1/2002 JNM 17

Standard Assembler Directives

Directive Description

title Title of the listing file

.model Specify the program's memory model

.stack Set the size of the stack segment

.data Mark the beginning of the data segment

.code Mark the beginning of the code segment

proc Begin procedure

endp End of procedure

end End of program assembly

1/2002 JNM 18

Data Allocation DirectivesMnemonic Description Bytes

DB Define byte 1DW Define word 2DD Define Doubleword 4

DF,DP Define far pointer 6DQ Define quadword 8DT Define tenbytes 10

Char1 db ‘A’ hex db 41hSigned1 db -128 dec db 65Signed2 db +127 bin db 01000001bUnsigned db 255 hex2 db 0A3h

1/2002 JNM 19

Lists and Strings

List1 db 10, ‘A’, 41h, 0Ah, 00100010b, 101qListptr db List1

Cstring db “This is a string”,0Clength = ($ - Cstring) ; sets Clength to length of Cstring

db 20 dup(0) ; 20 bytes, all equal to zero db 4 dup(“ABC”) ; 12 bytes, “ABCABCABCABC”

1/2002 JNM 20

Ptr OperatorPtr Operator - For some instructions, the size of the operand is not clear (INC [20H]).

0023 12

0022 34

0021 56

0020 00

0023 12

0022 34

0021 57

0020 00

0023 12

0022 34

0021 56

0020 FF

INC Byte Ptr [0020] INC Word Ptr [0020]

1/2002 JNM 21

0006 560005 780004 120003 340002 790001 630000 35

memory (data)

Direct Addressing Direct-Offset Addressing

Offset operator – returns the 16-bit address of the variable. Good for strings and arrays.

.dataBytelist db 35h, 63h , 79hWordlist dw 1234h, 5678h.code…Mov al, Bytelist ; al = 35Mov al, Bytelist+1 ; al = 63Mov bx, Wordlist ; bx = 1234Mov bx, Wordlist+2 ; bx = 5678

1/2002 JNM 22

0006 470005 4E0004 490003 520002 540001 530000 41

memory (data)

Direct-Offset Addressing with Strings

.dataaString db “ASTRING”

.code…Mov al, aString ; al = 41Mov al, aString+1 ; al = 53Mov al, aString+2 ; al = 54Mov al, aString+3 ; al = 52

1/2002 JNM 23

Bit Name0 CF Carry1 12 PF Parity3 04 AF Auxiliary Carry5 06 ZF Zero7 SF Sign8 TF Trap9 IF Interrupt Enable

10 DF Direction11 OF Overflow12 131415

8086 Flags - Bit Positions and Names

1/2002 JNM 24

Debug Flag Mnemonics F=0

F=1

NV UP DI PL NZ NA PE NCOV DN EI NG ZR AC PO CY

Overflow Direction InterruptEnable

CarryCarry(Negative)

Sign Zero Auxiliary Parity

1/2002 JNM 25

Interrupt Functions(Listed in Appendix G of Irvine Book)

INT 10h – (Video Bios)INT 16h – (Keyboard)INT 33h – (Mouse)

INT 20 – terminate COM program (use INT 21,4C instead)INT 21h – DOS Functions (AH holds function number)

1 – Keyboard input (char; store in AL )2 – Display char (DL holds char)3 – print char (DL holds char)9 – print string (DX points to string) 4Ch – terminate process