Program Format

Typical sections of an assembler program are:

• Data section: declaration and definition of initialized data
• BSS section: declaration of uninitialized data
• Text section: where the code is placed

Comments

Code comments are written using the semicolon (;). It can be placed anywhere. Everything to the end of the line is ignored.

Numeric Literals

Constants

<name>  equ   <value>

Constants are substituted with their value during the assembly process. They are not assigned a memory location

Example:

SIZE  equ   127

Initialized Variables (data)

• Declared in section .data
• Name, data type and initial value
• Supported data types:
  • db: 8-bit(byte)
  • dw: 16-bit (word)
  • dd: 32-bit (double word)
  • dq: 64-bit (quad word)
  • ddq: 128-bit integer
  • dt: 128-bit float
• Arrays are Initialized with comma separated values

Format:

<varName>  <dataType>   <initialValue>

Examples:

bVal db 2            ; byte
cVal db "H"          ; char
str db "Hello World" ; string
wVal dw 5000         ; 16-bit word
dVal dd 50000        ; 32-bit (double word)
arr dd 100, 200, 300 ; 3 element array
flt1 dd 3.14159      ; 32-bit float
qVar dq 1000000000   ; 64-bit (quad word)

Uninitialized Variables (BSS)

• Declared in section .bss
• Name, data type and count
• Supported data types:
  • resb 8-bit (byte)
  • resw 16-bit (word)
  • resd 32-bit (double-word)
  • resq 64-bit (quad-word)
  • resdq 128-bit (double quad-word)

Format:

<varName>  <resType>   <count>

Examples:

bArr resb 5   ; 5 element byte array
wArr resw 20  ; 20 element word array
dArr resd 30  ; 30 element double array
qArr resq 25  ; 25 element quad array

Code (text)

• Placed in `section .text
• initial program entry point needs to be defined

For the standard linker (Linux) the entry point is defined:

global _start
_start:

Assembler Directives

Assembler directives are instrutions for the assembler that are not directly translated to CPU instrutions.

Labels

• Labels are used as targets for jumps
• Can contain letters, numbers and _
• Terminated with colon (:)
• Case sensitive (at least in yasm)
• May be definied only once

Examples:

loopBegin:
end: