Storing Data

Objectives

1. Understand how to define storage in .bss and .data
2. Understand how to use data from storage

Notes

• This is from Jorgensen chapter 4.
• And some digging on the NASM reference page.
• For the following discussion let #### be a valid string of digits in a given base.
• Labels:
  • valid : letters, numbers, _, $, #, @, ~, . and ?
  • Must start with a letter
  • Starting with a $ means it is a label
    • $eax is not eax
    • But I would not do this.
  • Maximum length of 4095 characters!
  • Please note, I use label:, but the : is not necessary.
  • I will never remember this, but a label starting with a . is a local label.
    • It is associated with the previous non-local label.
    • Probably good for .top, .else, .exit
    • This example is from the documentation

    • loop1: 
      .top:   ; this is loop1.top
              ; some code
              jne .top
      loop2:
      .top   
              ; some code
              jne .top
              jmp loop1.top 

• Numeric constants
  • A number is assumed to be decimal
  • ####d is also decimal
  • 0d#### is also decimal
  • Hex include 0x####, 0h####, and ####x
  • Octal 0q or 0o at the beginning and q or o at the end
  • Binary 0b or 0y at the beginning and b or y at the end
  • Any of these can include a _ as a separator
• Strings seem less consistent
  • "", '' or ``
  • backquotes `` support c style escapes (\n,\t)
  • But #, at least, seems to cause a problem in ``
  • I would not use strings in instructions as literals
    • The byte order will be reversed
  • For any c level routines, strings must be null terminated.
• Declaring initialized (.data section) memory
  • db, dw, dd, dq, and others are pseudo instructions.
  • They are not real intel instructions
  • But tell the assembler to allocate memory.
  • Integer
    • db : byte
    • dw : word (16 bits, 2 bytes)
    • dd : double (32 bits, 4 bytes)
    • dq : quad (64 bits, 8 bytes)
  • Floating point
    • dd: float
    • dq: double precision
    • dt: quad precision
    • do: but the manual does not address this
  • We may consider floating point later.
  • label: dd data [, data]
  • A ? means no value, or uninitialized
  • There is more, but this will do for now.
• Declaring uninitialized memory
  • res*
    • resb : reserve a byte array
    • resw : reserve a word array
    • resd : reserve a double array
    • resq : reserve a quad array
  • There is also t,o,y,z for floats.
  • label: res* size
• equ is another pseudo-instruction
  • It defines a symbolic constant.
  • label: equ constant
  • label: equ expression
  • This is a CONSTANT, assigned once.