Directives

Pre-processor

• equ  (Equivalent to #define  in C.)

  MAX_COINS    equ 20
  KEY_W        equ 87
  

  • It tells the assembler "wherever you see this name, substitute this value at assemble-time." It's essentially a find-and-replace that happens before any code is generated.

  • No memory is reserved, no address exists, it doesn't belong to any section at all.

  | Assembler | Syntax           | Example        |
  |-----------|------------------|----------------|
  | NASM      | equ             | MAX equ 20    |
  | MASM      | equ             | MAX equ 20    |
  | GAS (GNU) | .equ  or .set  | .equ MAX, 20  |
  | FASM      | =  or equ      | MAX = 20      |
  | ARM (as)  | .equ            | .equ MAX, 20  |

• struct

  • NASM

    • struc  / endstruc  as shown

    struc Player
        .x: resd 1
        .y: resd 1
    endstruc
    

  • MASM

    • STRUCT  / ENDS , and actually lets you declare instances inline

    Player STRUCT
        x REAL4 ?
        y REAL4 ?
    Player ENDS
    myPlayer Player <> ; declares an actual instance in .data
    

  • FASM

    • Virtual at idiom, more explicit

    virtual at 0
        player.x rd 1
        player.y rd 1
    end virtual
    

  • GAS :

    • No built-in struct support at all.

    • You just use .equ  manually for every offset, which is exactly Pattern 1 from before.

    • This is very common in Linux kernel source and GAS-based codebases.

Linker

• global

  • Makes a symbol visible outside the current object file.

  • Without it, labels are local to the file.

  • It's not required to be inside a section.

  section .text
  
  _start:        ; local symbol (not visible to linker)
      ret
  

  global _start  ; export symbol
  
  section .text
  _start:
      ret
  

• extern

  • Declares a symbol defined elsewhere.

  • It's not required to be inside a section.

  extern printf
  

Assembler

• bits <value>

  • Tells NASM to emit <value> -bit instructions — it controls how the assembler encodes opcodes.

  • Without it, NASM defaults to 16-bit mode ( bits 16 ), so instructions like mov eax, 1  might encode differently or not at all.

  • rax , r8 - r15 , xmm  registers etc. are only valid in 64-bit mode.

  • Note :

    • When you use -f win64  as your output format, NASM already implies 64-bit mode.

    • Where bits 64 would matter is if you were writing a file that mixed modes, or targeting a raw binary format like -f bin (e.g. writing a bootloader), where NASM has no format to infer the mode from and defaults to bits 16.

    • Ex:

      ; bootloader — starts in 16-bit real mode
      bits 16
      ; ... real mode code ...
      
      bits 32
      ; ... protected mode code ...
      
      bits 64
      ; ... long mode code ...
      

  • Ex :

    • bits 64

      • NASM emits 64-bit instructions

Data Types (Data Definition Directives)

• They are not  CPU instructions. The CPU never sees them. Instead, they tell the assembler: “Emit these bytes into the output at this location”.

• There's no type safety.

  • Signed and Unsigned comes down to what instructions you use.

• They are not CPU instructions. They are handled by the assembler to lay out bytes in memory.

• x86-64 is little-endian, so values are stored least-significant byte first.

• 

• Creel - Data Types

• About pointers :

  • In x64 all memory is flat, so all pointers are 64 bits wide.

Integer Data Types

• db  / byte

  • define byte

  • 1 byte

• dw  / word

  • define word

  • 2 bytes

• dd  / dword

  • define double word (dword)

  • 4 bytes

• dq  / qword

  • define quad word (qword)

  • 8 bytes

• dt  / tbyte

  • 80 bits.

  • 10 bytes.

• Using dq  doesn’t guarantee alignment unless you explicitly request it:

  align 8
  value dq 1
  

• Example:

  msg db "Hello", 0
  

  • Equivalent to:

  48 65 6C 6C 6F 00
  

Floating Point Data Types

• dd  / float  (FASM alias) / .float  (GAS alias) / real4  (MASM alias)

  • f32.

• dq  / double  (FASM alias) / .double  (GAS alias) / real8  (MASM alias).

  • f64.

• dt  / .tfloat  (GAS alias) / real10  (MASM alias)

  • f80.

  • Only used with the x87 FPU (Floating-Point Unit); largely ignored nowadays.

  • You can't use real10  with SSE.

• ? / real16

  • f128

  • Not consistently supported

pi  dd 3.1415927      ; float (32-bit)
piq dq 3.141592653589793 ; double (64-bit)

SIMD Data Types

• Xmmword

  • 128 bits.

• Ymmword

  • 256 bits.

• Zmmword

  • 512 bits.

Reserve

• resb

  • reserve bytes

• resw

  • reserve words

• resd

  • reserve dwords

• resq

  • reserve qwords