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Assembly Instructions Assembly language instructions may involve mnemonics, labels, variables, constants, and directives. Examples are as follows. here1 mov5 ax,23 msg2 db6 "help$" bytes assigned to msg var3 db6 10 of 10 num4 equ6 20 ;set ax value ;ASCII ;var assigned a value ;num has the value 20 _____________________________________________________________ 1 label 4 constant 2 variable of ASCII bytes 5 mnemonic 3 one-byte numeric variable 6 directive 1 Define Byte, Word Define Byte Variables (memory locations) Define Word Variables (memory locations) 2 Define Byte, Word • list db 10,20,30,40 will store the four values in consecutive locations. If the offset of list is 0000, they will be stored in 0000, 0001, 0002, 0003, respectively. list refers to 0000, list+1 refers to 0001, etc. • Value1 dw 2AB6h will place two bytes in consecutive locations with the low order byte in the lower memory location as: B6 2A 3 Character or String Constants • ‘ABC’ • “This is a test.” • ‘This is my file.’ • Define a variable called message stored at offset 0000 for any of the above. For example: message db ‘ABC’. • Then “B” of “ABC” is at offset 0001 and “i” of “This …” is at offset 2 4 Pointer Given: message db ‘ABC’ ;define 3 bytes P db message ;P points to message The variable P contains the offset of message 5 ASCII Table 6 MOV Instruction • Allowable MOVs with scratch registers MOV MOV MOV MOV MOV reg,reg mem,reg reg,mem mem,immed reg,immed • Allowable MOVs with segment registers (except CS) MOV MOV MOV MOV segreg,reg16 segreg,mem16 reg16,segreg mem16,segreg 7 Direct Operands 8 Illegal Moves 9 Label Directive .data countB label byte ;byte attribute, ;no storage allocated countw dw 0020h ;word attribute .code mov al,countB mov cx,countW ;retrieve low byte of count ;retrieve all of count countB countW 20 00 Addressing Modes Addressing Mode Direct Example mov ax,bx Description Effective address (EA) are the registers ax,[bx] EA is the contents of a register ax,[bp + 1] EA is the sum of a base register and a displacement [di + 2],ax EA is the sum of an index register and a displacement ax,[bx + si] EA is the sum of a base register and an index register ax,[bx + si + 2] EA is the sum of a base register, an index register and a displacement Register Indirect Based mov mov Indexed mov Based Indexed mov Based Indexed with Displacement mov 11 Based Index Addressing Example Adding Bytes In the following example of based indexed addressing, the contents of 16d bytes beginning at ds:1000h and ending with ds:100Fh are accumulated in the al register. cs:100 mov al,0 cs:102 mov cx,10h cs:105 mov si,0 cs:108 mov bx,1000h cs:10b add al,[bx + si] cs:10d inc si cs:11e1 loop 10bh ;initialize AL register ;set loop counter to 10h=16d ;set si=0 ;set bx=1000h as offset address ;accum. contents of mem. loc. [bx+si] ;increment si ;return to add 12 Based Index Addressing Example Adding Words In the following example of based indexed addressing, the contents of 16d words beginning at ds:1000h and ending with ds:101Dh are accumulated in the ax register. cs:100 mov ax,0 cs:103 mov cx,10h cs:106 mov si,0 cs:109 mov bx,1000h cs:10c add ax,[bx + si] cs:10e add si,2 cs:111 loop 10ch ;initialize AX register ;set loop counter to 10h=16d ;set si=0 ;set bx=1000h as offset address ;accum. contents of mem. loc. [bx+si] ;increment si by 2 ;return to add 13 Stack Operation To save register contents before calling a subroutine: ;save register contents before calling subroutine push ax push bx push cx push dx ;restore register contents after returning from subroutine pop dx pop cx pop bx pop ax 14 Assemble-Link-Execute Cycle .asm MASM .exe .obj .lst .map 15 Hello World .lst File 0000 0000 0000 0000 0000 0003 0005 0007 000A 000C 000F 0011 title Hello World Program (hello.asm) ;This program displays "Hello, world!" directive ->.model small <-code and data each < 64K directive -> .stack 100h <- 256 bytes reserved directive -> .data 48 65 6C 6C 6F 2C message db "Hello, world!",0dh,0ah,'$' 20 77 6F 72 6C 64 21 0D 0A 24 directive ->.code main proc <-begin procedure B8 ---- R mov ax,@data <-assigns seg. Addr. 8E D8 mov ds,ax to DS B4 09 mov ah,9 BA 0000 R mov dx,offset message CD 21 int 21h B8 4C00 mov ax,4C00h CD 21 int 21h main endp <-end procdure end main <-end assembly 16 Hello World .map File Start Stop Length Name 00000H 00010H 00011H _TEXT 00012H 00021H 00010H _DATA 00030H 0012FH 00100H STACK STACK Origin 0001:0 Class CODE DATA Group DGROUP Program entry point at 0000:0000 17 XCHG Instruction Problem: move bx to ax and ax to bx mov cx,ax ;ax stored temporarily in cx mov ax,bx ;move bx to ax mov bx,cx ;move cx (really ax) to bx or use: xchg ax,bx Allowed: xchg reg,reg xchg reg,mem xchg mem,reg 18 XCHGing Two Variables Cannot do: xchg mem1,mem2, but 19 Arithmetic Instructions INC and DEC Instructions inc destination dec destination ;add 1 to destination ;subtract 1 from destination where destination is reg or mem Examples: inc al dec ax dec byte ptr membyte dec memword inc word ptr memword ;dec 8-bit memory operand ;dec memory operand ;inc 16-bit memory operand 20 Arithmetic Instructions ADD Instruction add destination, source Example: add ax,bx Contents of Registers Before add ax,bx AX |0FFF BX |0001 add ax,bx AX |0002 BX |FFFF After |1000 |0001 | | |0001 |FFFF | plus a | carry 21 Arithmetic Instructions ADD Instruction Consider the way in which add and adc, add with a carry, deal differently with the carry flag. Both the AX and BX registers contain 0000 and the carry flag is set, CY. add ax,bx carry) yields AX=0, BX=0 and NC (no adc ax,bx yields AX=1, BX=0 and NC 22 Arithmetic Instructions SUB Instruction sub destination, source Example: sub ax,bx Contents of Registers Before After sub ax,bx AX |00A0 BX |0001 |009F |0001 | | sub ax,bx AX |0005 BX |0006 |FFFF |0006 | | 23 Arithmetic Instructions MUL Instruction mul multiplier ;multiplicand in ax ;product in dx,ax Example: mul bx Contents of Registers Before After mul bx AX |FFFF BX |0010 DX |0000 |FFF0 |0010 |000F | | | 24 Arithmetic Instructions DIV Instruction div divisor ;dividend in dx,ax: quotient in ax ;remainder in dx Example: div bx Contents of Registers Before After div bx AX |FFF1 BX |0010 DX |000F |FFFF |0010 |0001 | | | 25 Memory Models Produces .com files What we will use Linear addressing 26 Overlapping Segments ) 27