Assembly Theory and Addressing Modes

• What is one reason to use a low-level language?

  It reflects the design of the processor
• What is a characteristic of machine code?

  It uses binary notation
• How is machine code specific to a processor?

  It depends on the hardware design of the processor
• What do instructions in machine code operate on?

  Bytes of data
• What are the differences between machine code and assembly language?

  • Machine code: written in binary and hexadecimal
  • Assembly language: includes mnemonics
  • Machine code: no translation needed
  • Assembly language: translated by an assembler
  • Machine code: very difficult to write
  • Assembly language: easier than machine code, harder than high-level languages
• What steps do assemblers perform to produce machine code?

  1. Reserves storage for instructions & data
  2. Replaces mnemonic opcodes with machine codes
  3. Replaces symbolic addresses with numeric addresses
  4. Creates a symbol table for labels & addresses
  5. Checks syntax and provides error diagnostics
• What is a mnemonic in assembly language?

  A three-letter code for an instruction
• What does an opcode indicate?

  What operation the instruction is to perform
• What is the purpose of an operand in an instruction?

  It holds data for the operation specified
• What is flow control in programming?

  The order in which instructions are executed
• What is symbolic addressing?

  Using characters to represent memory addresses
• How is a jump command executed?

  By changing the contents of the PC
• What happens when an instruction arrives in the processor?

  • The processor decodes the instruction
  • Determines the addressing mode to access data
  • Multiple versions of each instruction may exist
• What does #ADD signify in addressing modes?

  Addition using direct addressing
• What does %ADD signify in addressing modes?

  Addition using immediate addressing
• What does $ADD signify in addressing modes?

  Addition using indirect addressing
• If the Accumulator equals 5 and we %ADD 6, what is the result?

  11
• If the Accumulator equals 5 and we #ADD 2, what is the result?

  6
• If the Accumulator equals 5 and we $ADD 4, what is the result?

  6
• What is immediate addressing?

  • Operand is the value to be used
  • Used in assembly language
• What is direct addressing?

  • Operand is the memory location of data
  • Simplest and most common method of addressing
• What is indirect addressing?

  • Operand is a memory location holding a value
  • Increases the memory locations that can be addressed
• What is indexed addressing?

  • Operand is added to contents of Index Register
  • Allows efficient access to a range of memory locations
• Why can't we only use direct addressing?

  Addresses are limited by the address field size
• What is an indexed register?

  • Used in indexed addressing
  • Stores a number to modify an address
  • Allows access to a range of memory locations
• Compare direct addressing and indirect addressing.

  • Direct: simplest method, uses address field directly
  • Indirect: uses address field as a pointer, increases addressable memory
• What is relative addressing?

  • Calculates a real address from a base address
  • Uses an offset as a relative address
  • Useful for arrays and branching
• What does indexed addressing modify?

  • Modifies the address by adding a number
  • Uses the index register to determine the final address