2. Stored-Program Computer

• Recall: Subunits of a CPU
  
• Arithmetic Logic Unit (ALU): circuitry that performs operations such as addition
• Registers: fast memory units built into the CPU
• Control Unit: circuitry that fetches data and instructions from main memory as well as controlling the flow of data between ALU and registers

• So far, we have acted as the Control Unit, setting data buses, controlling the flow of data and setting ALU instructions via knobs
  

• Actual Control Unit -- section of the circuitry on a CPU that carries out the operations we have been doing by hand.
  

• Control Unit carries out its work by reading Machine Language Instructions and carrying out the duties encoded by the instruction.
  

• Machine Language Instruction -- binary number (16 bits in our simulator) that represents some type of control.  Usually, some set of initial bits rerepsents the type of instruction, rest of bits rerpesent data for instruction.

• Example: Adding two numbers stored in registers RA and RB and storing the result in RC can be represented as
  
• 1010000100 RC RA RB
  
• 1010000100 03 01 02       (add R1 to R2, store in R3)
• 1010000100030102  (actual instruction in memory)
  

• Note 1: Since we only have 4 registers (R0, R1, R2, R3) we can represent a register with a 2-bit number, like RA = 01.
  
• Note 2: Since we have 32=2^5 memory locations, we can represent a memory location with a 5-bit number, like M[5] = 00101
  

• Machine Language Instruction Set for our CPU Simulator

• Example: Machine language program for adding two numbers in memory

0:           1000000100000101   // R0 = M[5]; load memory location 5 into R0

1:           1000000100100110   // R1 = M[6]; load memory location 6 into R1

2:           1010000100100001   // R2 = R0 + R1; add R0 and R1, store result in R2

3:           1000001001000111   // M[7] = R2; copy R2 to memory location 7

4:           1111111111111111   // HALT

5:           0000000000001001   // 9; data to be added

6:           0000000000000001   // 1; data to be added

7:           0000000000000000   // location where sum is to be stored

• Fetch-decode-execute cycle of control unit  (PC = program counter - stores address of next instruction to be executed)
  

initialize PC = 0;
fetch instruction stored at memory location PC;
set PC = PC + 1;
while (instuction is not HALT) {
     decode the instruction;
     configure the CPU hardware to match the settings indicated in the instruction;
     execute the CPU datapath cycle using those settings;
     when the cycle is complete, fetch next instruction from memory location PC;
     set PC = PC + 1;
}

• Machine language simulator

• Class Exercises:
• What task does the following machine-language program perform?
• 0:   1010001000000000
• 1:   1000001000000011
• 2:   1111111111111111
• What sequence of machine-language instructions would cause the simulator to subtract the contents of memory location 10 from 11 and store the result in memory location 12?