1. The Processor and Machine Language
Computer Science 101
The Processor and Machine Language

2. The Processor The control unit Sequences and executes the instructions
The ALU Performs primitive arithmetic and logic
Registers High-speed memory cells that serve
special functions for the two units

3. The ALU in Action a b a + b a - b a * b a / b
One circuit for each function, all performing simultaneously

4. The ALU in Action a b a + b a - b Output Multiplexor circuit a * b
line 0
a + b
line 1
a - b
Output
Multiplexor
circuit
a * b
line 2
a / b
line 3
Selector lines
00, 01, 10, 11 are codes that determine which input is sent out

5. Executing Instructions
Program instructions, written in machine language, are stored in memory
The control unit
Fetches an instruction from memory
Decodes the instruction
Executes the instruction
Repeats this until a halt instruction is reached

6. Format of a Machine Language Instruction
Let’s assume a memory cell size of 16 bits and an address space of 12 bits.
Then, each instruction consists of
a 4-bit opcode
a 12-bit operand code, called the address field 1 1 1 1 1 1 1 1 1 1
opcode
address field

7. Interpreting an Instruction
The opcode 1101 means input a number and store it in the memory cell specified by the instruction’s address field.
So, this particular instruction says, input a number and store it in the cell whose location is 127. 1 1 1 1 1 1 1 1 1 1
opcode
address field

8. Registers
We’ve already seen the memory data register (MDR) and the memory address register (MAR), which are used for fetching and storing data.
MDR 1 1 1 1 1 1 1 1 1 1
MAR 1 1 1 1 1 1

9. Other Registers Program counter (PC) 12 bits Contains the address of
the next instruction
Instruction register (IR) 16 bits Contains the currently
executing instruction
Data register (R) 16 bits Used for temporary
storage by some
instructions
Condition code register (CCR) 3 bits Used to test data and
make decisions

10. Other Registers PC 1 1 1 1 1 1 IR 1 1 1 1 1 1 1 1 1 1 R 1 1 1 1 1 1 1 1 1 1
CCR 1
The state of the program at any given time is determined by the data contained in
memory (RAM)
the registers

11. The Instruction Set We will want instructions for Input and output
Moving data from RAM to the data register and back again (like assignment statements)
Arithmetic (at least addition and subtraction)
Making decisions based on comparisons of two data values (if statements and loops)

12. The Instruction Set
A machine’s instruction set depends on the number of possible opcodes
4 bits allows 24 or 16 possible opcodes
In general, N opcodes requires log2N bits

13. The 16 Opcodes and Their Operations
Binary opcode Operation
LOAD X
STORE X
CLEAR X
ADD X
INCREMENT X
SUBTRACT X
DECREMENT X
COMPARE X
JUMP X
JUMPGT X
JUMPEQ X
JUMPLT X
JUMPNEQ X
IN X
OUT X
HALT
X = a memory address

14. Data Movement – 0000 (LOAD)
LOAD X Copy the contents of the cell at address X into
the data register R
LOAD X = IR 1 1 1 R 1 1 1 1 1 1 1 1 1 1
RAM 1 1 1 1 1 1 1 1 1 1

15. Data Movement – 0001 (STORE)
STORE X Copy the contents of the data register R into
the cell at address X
STORE X = IR 1 1 1 1 R 1 1 1 1 1 1 1 1 1 1
RAM 1 1 1 1 1 1 1 1 1 1

16. Arithmetic – 0011 (ADD)
ADD X Add the contents of the data register R to
the contents of the cell at address X and leave
the sum in the data register R
ADD X = IR 1 1 1 1 1
2 + 3 R 1
RAM 1 1

17. Arithmetic – 0011 (ADD) Add the contents of the data register R to
the contents of the cell at address X and leave
the sum in the data register R
ADD X = IR 1 1 1 1 1
2 + 3 = 5 R 1 1
RAM 1 1

18. General Method for Addition (Z = X + Y)
LOAD the value at address X
ADD the value at address Y
STORE into address Z

19. Example: Set Z to X + Y Fetch instruction at address stored in PC (0)IR R
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

20. Example: Set Z to X + Y Increment PC PC IR 1 1 R RAM 000000000000 1 1IR 1 1 R
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

21. Example : Set Z to X + Y Execute the instruction in IR PC 1 IR 1 1 R1 IR 1 1 R
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

22. Example : Set Z to X + Y Fetch the next instruction PC 1 IR 1 1 R 11 IR 1 1 R 1
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

23. Example : Set Z to X + Y Increment the PC PC 1 IR 1 1 1 R 1 RAM1 IR 1 1 1 R 1
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

24. Example : Set Z to X + Y Execute the instruction in IR PC 1 IR 1 1 1 R1 IR 1 1 1 R 1
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

25. Example : Set Z to X + Y Fetch the next instruction PC 1 IR 1 1 1 R 11 IR 1 1 1 R 1 1
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

26. Example : Set Z to X + Y Increment the PC PC 1 IR 1 1 1 R 1 1 RAM1 IR 1 1 1 R 1 1
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

27. Example : Set Z to X + Y Execute the instruction in IR PC 1 1 IR 1 1 11 1 IR 1 1 1 R 1 1
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z

28. Example : Set Z to X + Y PC 1 1 IR 1 1 1 R 1 1 RAM 000000000000 1 11 1 IR 1 1 1 R 1 1
RAM 1 1 1 1 1
instructions 1 1 1 x 1 y 1 1
data z 1 1