Pseudo-ops

Pseudo-ops(Assembler Directive)

Pseudo-ops are helpful to the assembler in performing that task of taking a string input of characters representing a computer program written in LC-3 assembly language and translating it into a program in the ISA of the LC-3.

• Know the following pseudo-ops: .orig, .end, .fill, .blkw, .stringz
  • cheat sheet

    

  .ORIG

  .ORIG tells the assembler where in memory to place the LC-3 program.

  .FILL

  Reserve and fill a word with a numeric value, initialization data with memory

  • Example

    

  • Fill array example(inefficient)

    

  • Fill String example(inefficient)

    

    .fill0 —> Identify where the String ends(Null terminator)

  .BLKW

  Block Word, Reserve n memories in a row —> Array

  C. .blkw 20 —> Set C to the current address and reserve 20 words of memory starting at C

  • Example(efficient)

    

  .STRINGZ

  Store an ASCII string memory with a zero terminator(string length + 1 memory locations), optionally naming it

  • Example

    

  memory used is length of string + 1 (accounting for 0 to terminate string)

  Decimal: #

  Hex: x

  Binary: b

  • Times 6

    

  BRP —> Branch if positive

  LC3 has 8 Registers R1 - R8

• What is the purpose of each of these pseudo-ops? How do they affect the resulting memory layout of your program?

• Example types of questions that you may be asked:
  • Compute the total amount of memory used by a sequence of pseudo-ops
  • Determine the address of data/an instruction based on .orig
  • Figure out what data gets placed at an address by a pseudo-op