New/Old Assembler Directives

Data Definition Statement syntax –[name] directive initializer [,initializer]… At least one initializer is required in a data definition even if it is ? (does not assign a specific value) –The name is a label that marks the offset of a variable from the beginning of its enclosing segment.

Examples Var1BYTE35h;use h for hex Var2byte255;case not checked Var3db‘A’;old syntax Var4sbyte-128;signed byte Var5byte?;uninitialized

Defining Lists.data List byte 10, 20, 30, 40 List2 byte 10, 32, 41h, b List3byte 10, 20, 30, 40 byte50, 60, 70, 80 byte90, 100, 120

Defining Strings String1byte“This is a string”, 0 String2byte‘T’,’h’,’i’,’s’,‘i’,…. String3byte “This is a long string that” byte“that extends across many” byte“lines”, 0Dh, 0Ah,0 String4\ byte “Continuation character \ may be” byte“ used to concatenate two lines” byte“ into one.”,0

Using the DUP Operator Generates a repeated storage allocation, using a constant expression as a counter BYTE20 DUP(0) –Results in 20 bytes, all equal to 0 BYTE20 DUP(?) –Results in 20 bytes, uninitialized BYTE4DUP(“STACK”) –Results in 20 bytes “STACKSTACKSTACKSTACK”

Symbolic Constants Created by associating an identifier with either an integer value or some text. Does not reserve any storage. They are only used during assembly of a program, so they cannot change value during runtime.

Equal-Sign Directive Associates a symbol name with an integer value.code –COUNT = 500 –Mov al, COUNT.code –ESC_KEY = 27 –Moval, ESC_KEY.data –COUNT = 50 –Array COUNT DUP(0) Useful for constants. Easy to later change one line of code.

Calculating the Sizes of Strings and Arrays Use the current location counter ($). –List byte 10, 20,30, 40 –ListSize = ($ - list) The calculation must be done immediately following the list definiton –mystring byte “This is my string”,0 –String_len = ($ - mystring –1)

Calculating the Sizes of Strings and Arrays of Words and Doublewords ListWORD1000h, 2000h, 3000h Listsize = ($ - list) / 2 List DWORD h, h, DWORD h, h Listsize = ($ - list) / 4 This is important when using loops through arrays

Lists and Strings List1 db 10, ‘A’, 41h, 0Ah, b, 101q Listptr db List1 Cstring db “This is a string”,0 Clength = ($ - Cstring); sets Clength to length of Cstring Array1 db 20 dup(0); 20 bytes, all equal to zero Array2 db 4 dup(“ABC”); 12 bytes, “ABCABCABCABC”