What Computers Can and Cannot Do

2. What Computers Can and Cannot Do • Computers deterministically perform instructions • No creativity, intuition, etc.

3. The Fetch/Execute Cycle • “Instruction Execution Engine” • Fetch/Execute Cycle • Fetch next instruction • Decode • Gather needed data • Perform op • Save result • Repeat

4. A Five-Step Cycle • Ops repeat forever

5. Anatomy of a Computer • All computers have • Memory, • Control unit, • Arithmetic/logic unit (ALU), • Input unit, and • Output unit

6. Principal Subsystems of a Computer

7. 1. Memory • Memory • Code and data • Sequence of cells to hold bytes

8. 1. Memory • Addresses • 0 to 232 – 1 (32-bit CPU) • Values • 8-bit • Finite capacity

9. Byte-Size Memory Location • Discrete locations are shown as 1B boxes

10. Byte-Size Memory Location • Location can store 8-bits • ASCII character • Number (how big?) • Block of four bytes is word

11. Random Access Memory • Random access memory (RAM) • Order and time of access • Measured in MB or GB

12. 2. Control Unit • Control unit • Circuitry handles Fetch/Execute Cycle • Typical machine instruction ADD 4000, 2000, 2080

13. 2. Control Unit • ADD 4000, 2000, 2080 • Sums all three #s? • OP Dest, Src1, Src2

14. Illustration of a single instruction

15. 3. Arithmetic/Logic Unit (ALU) • ALU does math • Uses logic gates (AND, OR, NOT) • +, -, *, /, etc.

16. 4. And 5. Input and Output Units • Input • Info => computer • Output • Info => User • Useless without

17. The Peripherals • Peripherals connect to I/O ports • Not considered part of computer • Encode or decode info b/w computer and physical world

18. The Peripherals • Keyboard • Encodes keystrokes • Monitor • Decodes memory • Peripherals handle physical part of operation

19. Portable Memory & Hard Drives • Storage peripherals can do both I/O • USB memory • Hard disks/drives

20. Hard Disk • Hard disk stores programs and data • Large capacity relative to RAM • Slow relative to RAM • Programs/data must reside in RAM for use

21. A Device Driver for Every Peripheral • Most peripheral devices are “dumb” • Need drivers • Video card, monitor • Printer • Mouse, keyboard

22. The Program Counter:The PC's PC • Program counter (PC) • Maintains address of next instr. • Instr Fetch (IF) • Instr. = MEM[PC] • PC = PC + 4

23. Branch and Jump Instructions • Next instr. may not come after previous • Instr. may change PC • Jump • Branch

24. Instruction Interpretation • Process of executing a program is instruction interpretation • Our language => computer language

25. The Fetch/Execute Cycle • A five-step cycle: • Instruction Fetch (IF) • Instruction Decode (ID) • Data Fetch (DF) / Operand Fetch (OF) • Instruction Execution (EX) • Result Return (RR) / Write Result (WR) / Store (ST)

26. ADD 800, 428, 884

27. Instruction Fetch (IF) • Instr = MEM[PC] • PC is 2200 • Control Unit (CU) holds instr. • Bits of instr are decoded • PC updated

28. IF ADD 800, 428, 884

29. Instruction Decode (ID) • Decoder • Determines op to perform • Sets up ALU • Determines operand addresses • Sources • Destination • Passes to data fetch stage

30. ID + ADD 800 428 884

31. Data Fetch (DF) • Read data from memory • SRC1 = MEM[SRC_ADDR1] • SRC2 = MEM[SRC_ADDR2]

32. DF 42 12

33. Instruction Execution (EX) • Performs op indicated by ID • RES = SRC1OP SRC2 • Result held in ALU

34. EX 54

35. WriteResult (WR) • Writes result to memory • MEM[DST_ADDR] = RES • Cycle begins again

36. RR 54