Assembly Constants

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Assembly Language - Constants

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Constants are values determined at compile time that cannot be changed during program execution. Using constants in assembly makes code more readable and easier to maintain.

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What are Constants in Assembly

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In NASM, constants are symbols defined through pseudo-instructions. The assembler replaces all constant references with their actual values at compile time.

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Unlike variables, constants do not occupy memory in the data segment; they are replaced at compile time, equivalent to #define in the C language.

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EQU - Equal Constants

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EQU is the most common way to define constants. Once defined, they cannot be redefined:

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Example

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; EQU Constant Definition Example\\n\\n; Numeric Value Constants\\n\\n MAX_SIZE equ 256; Maximum Buffer Size\\n\\nDEFA ULT_PORT equ 8080; Default Port Number\\n\\n TRUE equ 1; TrueValue\\n\\n FALSE equ 0; FalseValue\\n\\n; Character Constants\\n\\n LF equ 0xA; Newline character\\n\\n CR equ 0xD; Carriage return\\n\\n NULL equ 0; Null character\\n\\n; Expression Constants\\n\\n ARRAY_SIZE equ 100*4; 100 Number of Bytes in a Double Word\\n\\n BUFFER_SIZE equ MAX_SIZE *2; Reference Another Constant\\n\\n TOTAL equ 10+20+30; Calculation result\\n\\nsection.data\\n\\n; UseConstants\\n\\n buffer db MAX_SIZE dup(0); Define Buffer of MAX_SIZE Bytes\\n\\nsection.text\\n\\nglobal _start\\n\\n_start:\\n\\nmov eax, MAX_SIZE ; eax = 256\\n\\nmov ebx, BUFFER_SIZE ; ebx = 512\\n\\nmov ecx, ARRAY_SIZE ; ecx = 400\\n\\nmov eax,1\\n\\nmov ebx,0\\n\\nint 0x80

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Constants defined with EQU cannot be modified in the program, nor can they be redefined. If you need redefinable constants (similar to macro variables), use %define.

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%define - Macro Definition Constants

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%define is more flexible than EQU, supporting redefinition and scope control:

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Example

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; %define Constant Definition and Redefinition Example\\n\\n%define VERSION 1; Define Version Number\\n\\n%define APP_NAME 'tutorial'; Define Application Name\\n\\n; Use %define Defined Constants\\n\\nmov eax, VERSION ; eax = 1\\n\\n; Can be redefined (EQU does not allow this)\\n\\n%define VERSION 2; Version Upgrade\\n\\nmov eax, VERSION ; Now eax = 2\\n\\n; %undef Undefine\\n\\n%undef VERSION ; VERSION No Longer Valid\\n\\n; mov eax, VERSION  ; Error: VERSION Not Defined\\n\\n; %define Multi-line Definition (Macros with Parentheses)\\n\\n%define sum(a, b)(a + b)\\n\\nmov eax, sum(10,20); mov eax, (10+20) -> eax = 30

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Feature	EQU	%define
Can be redefined	No	Yes
Can be undefined	No	Yes (%undef)
Supports parameters	No	Yes (macro functions)
Use cases	True constants (unchanging values)	Variable configurations, macro substitution

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%assign - Assignable Constants

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%assign is used to define numeric constants that can be modified multiple times at compile time:

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Example

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; %assign Compile-Time Variable Constants\\n\\n%assign counter 0; Initially 0\\n\\nsection.data\\n\\n; Use with %rep Repeat Block\\n\\n; Generate table_0, table_1, ..., table_9\\n\\n%rep 10\\n\\ndb counter ; UseCurrent counter Value\\n\\n%assign counter counter +1\\n\\n%endrep

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Practical Application of Constants in Programs

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Example

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; File Path: constants_practice.asm\\n\\n; Comprehensive Use of Constants to Improve Code Readability\\n\\n; System Call Number Constants\\n\\n SYS_EXIT equ 1\\n\\n SYS_FORK equ 2\\n\\n SYS_READ equ 3\\n\\n SYS_WRITE equ 4\\n\\n SYS_OPEN equ 5\\n\\n SYS_CLOSE equ 6\\n\\n; File Descriptor Constants\\n\\n STDIN equ 0\\n\\n STDOUT equ 1\\n\\n STDERR equ 2\\n\\n; Program Constants\\n\\n MAX_INPUT equ 256\\n\\n EXIT_SUCCESS equ 0\\n\\n EXIT_FAILURE equ 1\\n\\nsection.data\\n\\n msg db'Hello, TUTORIAL!',0xA\\n\\n msg_len equ$- msg\\n\\nsection.bss\\n\\n buffer resb MAX_INPUT

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