The basic elements of an assembler program Statements – one per line. The format of a line: [identifier] instruction/directive [operands] [; comment]

Identifier • name of a variable • label (including the name of a procedure) • name of a constant • name of a segment Make your labels meaningful! Find out if register AL contains a code for a lower case letter. If yes, convert it to the corresponding upper case letter. cmp al,’a’ jb L1 cmp al,’z’ ja L1 sub al,20h ; convert to upper case L1: …

Assembly Language 4/ 1

Better: cmp al,’a’ jb NotALowerCaseLetter cmp al,’z’ ja NotALowerCaseLetter sub al,20h NotALowerCaseLetter: …

Directives They are not compiled to a machine code, but they may:
define constants, variables, labels, segments, procedures and macros
enable compilation of instructions of the 80186, 80286, 80386, 80486, Pentium processors and the 80287 and 80387 coprocessors
control the listing contents and format
control the conditional compilation (e.g. directives IF, ENDIF, ELSE)

Assembly Language 4/ 2

Operands Possible operands:
registers
addresses
constant values An instruction may have:
none operand
1 operand
2 operands o the right one: a source o the left one:
a source cmp al,0
a destination mov al,0
a source and a destination add al,bl; al := al+bl The left operand cannot be a constant value. If the operands are registers or memory locations, they must be of the same types. Two memory operands are not allowed in the instruction!
3 operands

imul ax,bx,5; ax := bx*5

o the leftmost one: a destination o the middle and rightmost ones: sources Assembly Language 4/ 3

Variables - symbolic addresses of data items (offsets in the data segment) - defined by directives DB, DW, DD, DF, DP, DQ, DT. Syntax: [name of the variable] Dx expression [,expression] ... Directive Dx:
determines the variable type (according to the letter x)
allocates the space in memory (one or more bytes)
initialises the contents of the memory locations (does not initialise, if the expression is ?)

Assembly Language 4/ 4

Directive Allocated memory size in bytes DB

1

Variab Variable may contain le type

byte

Signed integer in the range Unsigned integer in the range Character

DW

2

word

Signed integer in the range Unsigned integer in the range 16-bit offset

DD

4

dword Signed integer Unsigned integer Single precision floating point number in the range about ±1038 Far pointer in 16-bit mode, i.e. address in the segment:offset form 32-bit offset Assembly Language 4/ 5

DP

6

pword Signed integer fword Unsigned integer

DF

Far pointer in 32-bit mode, i.e. address in the segment:offset form DQ

8

qword Signed integer Unsigned integer Double precision floating point number in the range about ±10308

DT

10

tbyte

Signed integer Unsigned integer Packed BCD number Extended precision floating point number in the range about ±104932

Assembly Language 4/ 6

.DATA ORG 100h ; contents of memory locations beginning at offset 100h: One DB –1,12+1; FF 0D DB ’abcd’

; 61 62 63 64

DB 3 dup (?)

; ?? ?? ??

DW -32768

; 00 80

DW One

; 00 01

DD 0.1

; CC CC CC 3D

DD 0ABCDEF23h ; 23 EF CD AB DD One

; 00 01 xy uv

Multi-byte data are stored in the reverse order of bytes. 0100 is the offset of variable One uvxy is the base address of the data segment

DT 1234567890 ; 90 78 56 34 12 00 00 00 00 00 DT -1234567890; 90 78 56 34 12 00 00 00 00 80

Assembly Language 4/ 7

Labels - symbolic addresses of instructions - type: near a far Type near The label is the offset of an instruction in the code segment. A jump to the label is in the same code segment. Loop: mov ax,bx Type far The label is the complete address of an instruction (segment:offset). A jump to the label is in another code segment. DoIt LABEL far mov ax,bx

Attributes of variables and labels
segment – the base address of the segment where the variable/label is defined
offset
type

Assembly Language 4/ 8

Value-returning operators Operator

Use

seg

Return the value of attribute segment.

offset

Return the value of attribute offset.

type

Return the value

according to the variable/label type

1

byte

2

word

4

dword

6

fword/pword

8

qword

10

tbyte

-1

near

-2

far

length

Return the number of data items allocated to the variable.

size

Return the number of bytes in the variable, i.e. the value length*type.

Assembly Language 4/ 9

Value DW 1234h Test_data DB 5,6,7 Array DW 100 dup(0) mov al,type Value; al := 2 mov bl,type Test_data; bl := 1 mov cl,type Array; cl := 2 mov ah,length Value; ah := 1 mov bh,length Test_data; bh := 1 mov ch,length Array; ch := 100 mov dx,size Array; dx := 200 length returns value > 1 only if the variable has been defined using operator dup

Assembly Language 4/ 10

Standard and alternative segment registers Memory reference

Standard segment register

Alternative segment register

Label

CS

-

Direct address

DS

CS, ES, FS, GS, SS

SS

CS, DS, ES, FS, GS

Source string

DS

CS, ES, FS, GS, SS

Destination string

ES

-

Indirect address with base BX and/or index SI, DI Indirect address with base EAX, EBX, ECX, EDX, ESI, EDI Indirect address with base BP Indirect address with base EBP, ESP

Assembly Language 4/ 11

Operators, which override attribute values Operator

Override attribute

:

segment

ptr

typ

mov al,ds:[bp] Value DW 1234h Number DB 5,6 mov ax,Value; al := 34h, ah := 12h

Value:

34h

AX 12h

34h

12h Number:

5 6

Assembly Language 4/ 12

mov ah,byte ptr Value; ah := 34h mov bx,word ptr Number; bx := 605h The same as mov bl,Cislo mov bh,Cislo+1 AH Value:

34h

34h

12h Number:

BX

5 6

06h

05h

byte? word? mov [bx],1 byte! mov byte ptr [bx],1 mov word ptr [bx],1 word! Similarly: inc [bx],1 Assembly Language 4/ 13

Constants - make the orientation in the program and its modification more easier - defined by directives EQU The $ predefined symbol is equal to the current offset in the segment. Example Display the string “Ahoj!” stored in variable String.

String:

‘A’

String + 1:

‚h‘

String + 2:

‚o’

String + 3:

‚j‘

String + 4:

‚!‘

StringLength

$:

Assembly Language 4/ 14

.MODEL small .STACK 100h .DATA String DB ’Ahoj!’ StringLength EQU $-String .CODE Start:mov ax,@data mov ds,ax; point ds to the data segment mov di,0; the 1st character has index 0 mov bx,offset String; point bx to the storage location for the first character mov cx,StringLength DisplayChar: mov dl,[bx+di]; copy a character at address ds:bx+di to dl mov ah,2 int 21h; display the character whose ASCII code is in dl inc di; increment index by 1 – go to the next character loop DisplayChar; cx := cx – 1, if cx > 0, jump to DisplayChar Stop: mov ax,4C00h int 21h END Start

Assembly Language 4/ 15