Slide 5.13: Calculating data sizes

Calculating Data Sizes and EQU Directive

The ‘$’ operator (current location counter) returns the offset associated with the current program statement.

String Size	Array Size	Instruction Size
.data s BYTE "Hello, world!" len = .code mov eax, len call WriteInt	.data a WORD 10, 20, 30, 40 len = .code mov eax, len call WriteInt	.code add eax, eax len = $ - a1 mov eax, len call WriteInt
Output	Output	Output
13	4	2

The EQU directive associates a symbolic name with an integer expression or some text.


 name  EQU  expression
 name  EQU  symbol
 name  EQU  <text>

In the first format, expression must be a valid integer expression.
In the second format, symbol is an existing symbol name, already defined with ‘=’ or EQU.
In the third format, any text may appear within the brackets <...>.

When the assembler encounters name later in the program, it substitutes the integer value or text for the symbol. Code 3 and 4 are equal. myWord1 stores the integer value 200, and myWord2 stores the string “20 * 10” .

^†Unlike the ‘=’ directive, a symbol defined with EQU cannot be redefined in the same source code.

Code 3	myInt EQU 20 * 10 myString EQU <20 * 10> .data myWord1 WORD myInt myWord2 WORD myString
Code 4	.data myWord1 WORD 200 myWord2 WORD 20 * 10