Pep/9

• Virtual machine
• 40 machine instructions
• Has 16-bit (65,536 bytes) of computer memory
• [[CPU] has seven registers
  • PC (See arithmetic logic unit)
  • IR (See arithmetic logic unit)
  • Accumulator (A)
    • Holds data and results of operations
• Uses ASCII
• Has memory-mapped input-output

Instruction Format

• Either 1 byte or 3 bytes in length
  • 1 byte --> operand specifier not included
  • 3 bytes --> operand specifier included
• Two parts
  • 8-bit instruction specifier
  • 16-bit operand specifier

Instruction Specifier

• Op codes vary from 4 bits to 8 bits long
• 5th bit is the register specifier
• 3-bit addressing mode specifier
  • Indicates hot to interpret contents of operand specifier
  • Address mode $000$ --> operand specifier contains data value to be used as operand
    • Immediate
  • Address mode $001$ --> operand specifier contains memory address
    • Direct

Computer Science Computing Systems