Lecture 2 Chapter 4 –Requirements for coding in Assembly Language 1

Chapter Outline  Assembly Language Features  Simplified segment Directive  Defining Types of data  Equate Directive 2

Defining Types of data expression Dn[name] Name: a program that references a data item does so by means of name Dn (Directive): define the data item – see next slide— Expression: is an operand may specify an uninitialized value or constant value an uninitialized value defined by item ? EXAMPLE : DATAX DB ? 3 expression Dn[name]

Defining Types of data (Directive): Pseudo-op Stands for DB Define Byte DW Define Word DD Define Doubleword DQ Define Quadword DT Define Tenbytes 4

Defining Types of data -Examples Syntax: name DB initial_value Example: ALPHA DB 4 a memory byte is associated with the name ALPHA, and initialized to 4. BYT DB ? a memory byte is associated with the name BYT, and uninitialized. WRD DW -2 a memory word is associated with the name WRD, and initialized to -2. The decimal range is: Unsigned representation: 0 to 255 Signed representation: -128 to 127 5

Defining Types of data – Array byte an array is a sequence of memory bytes or words. Example: B_ARRAYDB 10H,20H,30H Symbol Address Contents B_ARRAY 200H 10H B_ARRAY+1 201H 20H B_ARRAY+2 202H 30H 6

Defining Types of data – Array word Example: W_ARRAYDW 1000,40,29887,329 Symbol Address Contents W_ARRAY 0300H 1000D W_ARRAY H 40D W_ARRAY H 29887D W_ARRAY H 329D 7

Defining Types of data :The DUP Operator It is possible to define arrays whose elements share a common initial value by using the DUP (duplicate) operator. Syntax: Example: DELTA DB 212 DUP (?) creates an array of 212 uninitialized bytes. GAMMA DW 100 DUP (0) set up an array of 100 words, with each entry initialized to 0. 8 Repeat-count(exp)Dn[name]

High and Low Bytes of a Word WORD1DW1234H low byte WORD1 high byte WORD1+1 9

Character String ASCII codes can be initialized with a string of characters using single quotes like ‘PC’ or double quotes like “PC”. Example: LETTERSDB'ABC' = LETTERSDB41H,42H,43H Inside a string, the assembler differentiates between upper and lower case. It is possible to combine characters and numbers in one definition: Example: MSGDB'HELLO',0AH,0DH, '$' 10

Numeric Constant In an assembly language program we may express data as: Binary: bit string followed by ‘B’ or ‘b’ Decimal: string of decimal digits followed by an optional ‘D’ or ‘d’ Hex: begins with a decimal digit and ends with ‘H’ or ‘h’ Real : end with ‘R’ and the assembler converts a given a decimal or hex constant to floating point number Any number may have an optional sign. 11

Numeric Constant Number Type B D 1,234 1B4DH 1B4D FFFFH 0FFFFH decimal binary decimal illegal hex illegal hex 12

Named Constants - EQU (Equates) To assign a name to a constant, we can use the EQU pseudo-op. Syntax: name EQU constant Examples: LF EQU 0AH MOV DL,0AH = MOV DL,LF PROMPT EQU 'Any Thing' MSG DB 'Any Thing' = MSG DB PROMPT Note: no memory is allocated for EQU names. 13