Review of Assembly language

Recalling main concepts

Segment: special areas defined to contain CODE, DATA and STACK Paragraph boundary: location evenly divisible by 16 or 10H

Recalling main concepts Stack Segment Data Segment Code Segment SS DS CS Segment Registers

Example program ; Add two numbers and store the results into the third variable page 60,132 TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' FLDDDW 215 FLDEDW 125 FLDFDW ? DATASEGENDS ; CODESEGSEGMENT PARA 'Code' MAINPROC FAR ASSUME SS:STACK,DS:DATASEG,CS:CODESEG MOV AX,DATASEG;Set address of data MOV DS,AX; segment in DS MOV AX,FLDD;Move 0215 to AX ADD AX,FLDE;Add 0125 to AX MOV FLDF,AX;Store sum in FLDF MOV AX,4C00H;End processing INT 21H MAINENDP;End of procedure CODESEGENDS;End of segment END MAIN;End of program

; Add two numbers and store the results into the third variable page 60,132 TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' FLDDDW 215 FLDEDW 125 FLDFDW ? DATASEGENDS ; CODESEGSEGMENT PARA 'Code' MAINPROC FAR ASSUME SS:STACK,DS:DATASEG,CS:CODESEG MOV AX,DATASEG;Set address of data MOV DS,AX;Segment in DS MOV AX,FLDD;Move 0215 to AX ADD AX,FLDE;Add 0125 to AX MOV FLDF,AX;Store sum in FLDF MOV AX,4C00H;End processing INT 21H MAINENDP;End of procedure CODESEGENDS;End of segment END MAIN;End of program Comments ; COMMENTS

; Add two numbers and store the results into the third variable page 60,132 TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' FLDDDW 215 FLDEDW 125 FLDFDW ? DATASEGENDS ; CODESEGSEGMENT PARA 'Code' MAINPROC FAR ASSUME SS:STACK,DS:DATASEG,CS:CODESEG MOV AX,DATASEG;Set address of data MOV DS,AX;Segment in DS MOV AX,FLDD;Move 0215 to AX ADD AX,FLDE;Add 0125 to AX MOV FLDF,AX;Store sum in FLDF MOV AX,4C00H;End processing INT 21H MAINENDP;End of procedure CODESEGENDS;End of segment END MAIN;End of program IDENTIFIERS

Identifiers Identifier is a name applied to an item in a program to reference Name (e.g: FLDDDW 215) Label (e.g: MAINPROC FAR) Identifiers must not a reserved word and only contain: Alphabetic letters (A-Z,a-z) Digits (0-9) (.) (but not for the first character) Maximum length is 247 IDENTIFIERS

Instructions: ADD, MOV Directives:.TITLE,.MODEL Operators: FAR, SIZE Register: AX,BX RESERVED WORDS

STATEMENT Instructions: are translated to object code MOV, ADD, LEA.. Directives: tell the assembler to perform a specific action. [identifier] operation [operand(s)] [;comments]

; Add two numbers and store the results into the third variable page 60,132 TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' FLDDDW 215 FLDEDW 125 FLDFDW ? DATASEGENDS ; CODESEGSEGMENT PARA 'Code' MAINPROC FAR ASSUME SS:STACK,DS:DATASEG,CS:CODESEG MOV AX,DATASEG;Set address of data MOV DS,AX;Segment in DS MOV AX,FLDD;Move 0215 to AX ADD AX,FLDE;Add 0125 to AX MOV FLDF,AX;Store sum in FLDF MOV AX,4C00H;End processing INT 21H MAINENDP;End of procedure CODESEGENDS;End of segment END MAIN;End of program STATEMENTS

Directives Control the way a source program assembles and lists Generate no machine code (unlike instructions which generate object code)

; Add two numbers and store the results into the third variable page 60,132 TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' FLDDDW 215 FLDEDW 125 FLDFDW ? DATASEGENDS ; CODESEGSEGMENT PARA 'Code' MAINPROC FAR ASSUME SS:STACK,DS:DATASEG,CS:CODESEG MOV AX,DATASEG;Set address of data MOV FLDF,AX;Store sum in FLDF MOV AX,4C00H;End processing INT 21H MAINENDP;End of procedure CODESEGENDS;End of segment END MAIN;End of program Segment directive

NameOperationOperand Segment-nameSEGMENT[align][combine] [`class’] Segment-nameENDS Example: STACKSEGMENT PARA STACK 'Stack‘ STACK ENDS

; Add two numbers and store the results into the third variable page 60,132 TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' FLDDDW 215 FLDEDW 125 FLDFDW ? DATASEGENDS ; CODESEGSEGMENT PARA 'Code' MAINPROC FAR ASSUME SS:STACK,DS:DATASEG,CS:CODESEG MOV AX,DATASEG;Set address of data MOV DS,AX;Segment in DS MOV AX,FLDD;Move 0215 to AX MOV FLDF,AX;Store sum in FLDF MOV AX,4C00H;End processing INT 21H MAINENDP; End of procedure CODESEGENDS;End of segment END MAIN;End of program PROC directive

Format: Procedure-namePROC Operand Comment Procedure-nameENDP Operand: relates to program execution (FAR)

; Add two numbers and store the results into the third variable page 60,132 TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' FLDDDW 215 FLDEDW 125 FLDFDW ? DATASEGENDS ; CODESEGSEGMENT PARA 'Code' MAINPROC FAR ASSUME SS:STACK,DS:DATASEG,CS:CODESEG MOV AX,DATASEG;Set address of data MOV DS,AX;Segment in DS MOV AX,FLDD;Move 0215 to AX MOV FLDF,AX;Store sum in FLDF MOV AX,4C00H;End processing INT 21H MAINENDP;End of procedure CODESEGENDS;End of segment END MAIN;End of program ASSUME directive

Tells the assembler the purpose of each segment in the program Example: ASSUME SS:STACK,DS:DATASEG,CS:CODESEG

Simplified Segment Directives Model memory-model # Code segment#Data segment Small: 1, <=64K1,<=64K Medium: any number,size 1, <=64K Compact: 1, <=64K any number,size Large: any number,size any number,size Huge: any number,size any number,size

Simplified Segment Directives STACK [size] (default: 1K) DATA (default size is 1K) CODE (default size is 1K).EXIT directive

EQUATE directives Equal-Sign directive COEFFICIENT= 100 EQU directive COEFFICIENT EQU 100

Data type Format for data definition [name] Dn expression Name: identifier Dn: Directives and can be: DB: byteDF:farword DW: wordDQ:quadword DD: doublewordDT:tenbytes Expression: can be unnitialized: ? can be assigned a constant: such as 25, 21. Example: DATAZ DB 21,22.. DW 10 DUP(?)

Data type Constant: String: is defined within ‘ ‘ or “ “ MESSAGE DB “I am learning assembly language” Numeric: Is stored in reverse sequence Binary: 01B Decimal: 9D( D is optional) Hexadecimal: 1FH Real: 12R

Directives for defining Data Byte: DB Word: DW Doubleword: DD Farword: DF Quadword: DQ Tenbytes: DT

Some instructions on arithmetic calculation ADD: ADD registerregister/memory/immediate Example:ADD AX,FLDE Subtract SBB register register/memory/immediate Example:SUB AX, 100 Multiplication IMUL register Example:IMULCX Division DIV register ExampleDIVCX

Data transfer instructions MOV instruction Transfers data referenced by the address of the second operand to the address of the first operand Destination has to have the same length as source [label:] MOV register/memory register/memory/immediate Example: MOV F, AX; // Move content of AX to the variable F MOV CX, D;// Move value of D to CX MOV ES, AX MOV AX, 215

Note MOV instruction can’t: set the value of the CS and IP registers. copy value of one segment register to another segment register (should copy to general register first). MOV ES, DS copy immediate value to segment register (should copy to general register first). MOV DS, 100 MOV instruction can’t: set the value of the CS and IP registers. copy value of one segment register to another segment register (should copy to general register first). MOV ES, DS copy immediate value to segment register (should copy to general register first). MOV DS, 100

MOVSB: Copy byte at DS:[SI] to ES:[DI]. Update SI and DI. Algorithm: ES:[DI] = DS:[SI] if DF = 0 then SI = SI + 1 DI = DI + 1 else SI = SI - 1 DI = DI - 1 DF: direction flag from the flag register MOVSB and MOVSW

MOVSW: Copy word at DS:[SI] to ES:[DI]. Update SI and DI. ES:[DI] = DS:[SI] if DF = 0 then SI = SI + 2 DI = DI + 2 else SI = SI - 2 DI = DI - 2 DF: direction flag from the flag register MOVSB and MOVSW

XCHG swap the two data items [label:] XCHG register/memory, register/memory Example: MOV AL, 5 MOV AH, 2 XCHG AL, AH ; AL = 2, AH = 5 XCHG AL, AH ; AL = 5, AH = 2 XCHG instruction

Load Effective Address. REG = address of memory (offset) [label:] LEA register/memory Example: LEA AX, m ;load offset address of m to AX LEA instruction

Arithmetic instructions INC and DEC instruction Increasing or decreasing the contents of register or memory location by 1 [label:] INC/DEC register/memory Flag: OF, SF and ZF OF:is set when an instruction resulted in a carry into the sign bit of the result. SF: is set if the sign bit of a result is set ZF: is set if the result is equal to 0.

Arithmetic instructions ADD [label:] ADD/SUB operand1, operand 2 operand1 =operand 1 + operand 2 Operand 1: register/memory Operand 2: register/memory/immediate

Arithmetic instructions SUB [label:] SUB operand1, operand 2 operand1 =operand 1 - operand 2 operand 1: register/memory operand 2: register/memory/immediate

Arithmetic instructions MULoperand Unsigned multiply. Operand: register/memory

Arithmetic instructions IMULoperand Signed multiply. Operand: register/memory Example: MOV AX, -2 MOV CX, -3 IMUL CX; AX = +6 CF = 0

Arithmetic instructions DIVoperand Unsigned multiply. Operand: register/memory when operand is a byte: AL = AX / operand AH = remainder (modulus) when operand is a word: DX = remainder (modulus)

Arithmetic instructions IDIVoperand Signed multiply. Operand: register/memory when operand is a byte: AL = AX / operand AH = remainder (modulus) when operand is a word: DX = remainder (modulus)

Repetitive move instructions TITLEA04ASM1 (EXE) Move and add operations ; STACKSEGMENT PARA STACK 'Stack' DW 32 DUP(0) STACKENDS ; DATASEGSEGMENT PARA 'Data' STRING1 DW " ","$" STRING2 DW ? DATASEGENDS

Repetitive move instructions MAINPROC FAR MOV AX, dataseg MOV DS, AX MOV ES, AX MOV CX, 09 ; Initialize to move 9 characters LEA SI, STRING1 ; Initialize source index register to offset of string 1 LEA DI, STRING2 ; Initialize destination index register to offset of string 2 BEGINLOOP: MOV AL,[SI] ; Get a current character from string 1 to AL MOV [DI], AL ; Move it to the current character in string 2 INC SI ; Move to the next character in string 1 INC DI ; Move to the next character in string 2 DEC CX ; Decrease the count for loop JNZ BEGINLOOP ; Continue to loop if count is not 0 MOV AH, 09H LEA DX, STRING2 int 21H ; Display String 2 MAINENDP;End of procedure END MAIN;End of program CODESEG ENDS

Result

Repetitive move instructions DEC CX ZF = 1 if CX = 0 JNZ LABEL if ZF = 0 then jump to the label

Addressing mode Register addressing: E.g ADD AX, BX fastest type of operations Immediate addressing Immediate contains a constant value or an expression E.g: MOV AX, 0245H Direct memory addressing One of operand references a memory location and the other operand references a register E.G MOV FLDF, AX

Addressing mode Direct-Offset addressing use arithmetic instruction to modify an address e.gMOV CX, DATAZ+2 Indirect memory addressing UseBX and BP, DI and SI within [ ] e.g. MOV [BX], CL

Addressing mode Base Displacement Addressing Uses BX, BP and DI, SI and combine with a displacement to form an effective address E.g MOV AL,[SI+2] Base-Index Addressing Combine BX,BP with DI,SI to form effective address E.G MOV AL,[BX+SI]

Addressing mode Base-Index Displacement Addressing Combine BX, BP and DI, SI and a displacement to form an effective address E.g MOV AL,[BX+SI+2]

NEAR and FAR address NEAR address consists of 16 bit offset portion of an address used in real mode FAR address consists of both the segment and offset portions in the form of 32 bit segment:offset

CMP Instruction [label:] CMP register/memory, register/memory/immediate Compares the first to the second operand Affects: AF, CF, OF, PF, SF and ZF flag CMP AX, DX JE Startloop

Conditional Jump instructions Jump based on unsigned data [label:] JE/JZshort-address Jump if equal or Jump if zero [label:] JNE/JNZshort-address Jump if not equal or Jump if not zero Flag: ZF

Example MOV AL, 5 CMP AL, 5 JE label1 JMP exit label1: MOV CX, BX exit: …..

Conditional Jump instructions JG: Jump if first operand is Greater then second operand (as set by CMP instruction). Signed. if (ZF = 0) and (SF = OF) then jump Syntax: [label:] JGshort-address

Example MOV AL, 5 CMP AL, -5 JG label1 JMP exit label1: MOV CX, -5 ; in this case AL > -5 exit:

Conditional Jump Instruction JL: Jump if first operand is Less then second operand (as set by CMP instruction). Signed. if SF <> OF then jump Syntax: [label:] JLshort-address

Example MOV AL, -2 CMP AL, 5 JL label1 JMP exit label1: MOV CX, 5; in this case AL < 5 exit: …