Computer Architecture

The part of the design visible to the user. The architecture of MU0 consists of:

• Memory: locations 0, 1, 2, etc., each of 16 bits, so each can contain 2¹⁶ different values, which we normally take to mean numbers in the range -2¹⁵ to 2¹⁵-1
• Registers: ACC and PC. As the ACC contains values from memory, it is also 16-bit.
• Instruction set:

  LDA

  copy contents of operand-location to ACC

  STO

  copy contents of ACC to operand-location

  ADD

  add contents of operand-location to ACC

  SUB

  subtract contents of operand-location from ACC

  JMP

  copy operand-location (not contents) to PC

  JGE

  JMP if the ACC is $\ge$ 0

  JNE

  JMP if the ACC is $\ne$ 0

  STP

  stop program

  Each memory location can contain an instruction, so they are also 16-bit. Instructions consist of a 4-bit action (allowing 2⁴=16 different actions, of which 8 are currently unused) and a 12-bit operand-location (allowing 2¹² different memory locations i.e. 0 to 4095). As the PC holds locations of instructions in memory, it is also 12-bit.

See figure 1.

  

Figure 1: The basic MU0 architecture, with the visible registers ACC & PC