1. Working With Main Memory

2. Why Main Memory Register space limited Used for communication

3. Sections of Code.data section Things to place into memory at start.text section Code Any order, can have multiple.text/.data segments

4. Defining Memory Memory described as words/bytes/asciiz, etc… Hex: Ascii

5. Endianess Endianess : bytes order of a word in main memory

6. Little vs Big Endian Big is "Normal": Little weird – Words in order – Bytes in a word backwards

7. Integers Normal… Both endians store words (integers) same way – Disagree about how bytes are numbered +0+1+2+3 +2+1+0

8. Single Byte Structures Agree on address of single byte structures – Just look different +0+1+2+3 +2+1+0

9. Endian MARS is little endian: – Bytes/strings look goofy – Words (integers) unaffected

10. Why? Little Endian – Read different sized value at same address Big Endian – Easier to read hex – Reading start of word gives sign/magnitude

11. Base Register Data starts in defined location – 0x1001000 for MARS – 0x1000000 for reading simulator

12. Finding Memory Memory reference consist of offset(base) – base address : start here – offset : Go forward this many bytes 6(0x1001000) 32107654

13. Arrays C++ Array – Base address – Offset : number of elements int quiz[3]; quiz[1] = 5;

14. Load Word / Store Word Memory access – lw : get word from memory – put in register – sw : put register value into memory lw $targetRegister, offset($baseAddressRegister) lw $9, 12($8) – Start from address in register $8 Should have 0x1001000 – Move over 16 bytes from there – Read a word into $9

15. LUI & Base Register lw $9, 12($8) Need address of.text section (0x10010000) in register Could do: ori $8, $0, 0x1001 sll $8, $8, 16

16. LUI & Base Register lui : load upper immediate – Place pattern in upper half of register lui $8, 0x1001  10010000

17. Offsets Locations in order declared: x : 0(0x10010000) y: 4(0x10010000)

18. Accessing Memory x = x + y where x and y are in main memory:

19. Load Delay Load delay: Can't use register in instruction after it is loaded – Real MIPS hardware – Tutorial's SPIM simulator – Don't worry about for MARS

20. Labels Labels usually used to specify memory locations – Not allowed for now

21. Labels Behind the scenes… – lw/sw with label = 2 instructions

22. Weirdness sizeof(2 chars and an int) != sizeof(2 chars and an int)

23. Memory Alignment Memory alignment : restrictions on where read/write can start – Want to read 4 bytes: Start on multiple of 4 – Want to read 2 bytes: Start on multiple of 2 – Want to read 1 byte: Start on any byte

24. Offsets Locations in order declared – Must be aligned

25. Offsets Locations in order declared – Must be aligned 32107654111098 aa

26. Offsets Locations in order declared – Must be aligned 32107654111098 aaff

27. Offsets Locations in order declared – Must be aligned 32107654111098 aaff bb

28. Offsets Locations in order declared – Must be aligned 32107654111098 aaff cc bb

29. Offsets Alternative order

30. Offsets Alternative order 32107654111098 aa

31. Offsets Alternative order 32107654111098 bbaa

32. Offsets Alternative order 32107654111098 cc bbaa

33. Offsets Alternative order 32107654111098 cc bbaaff

34. Byte Packing Byte packing : ordering class/struct members for proper alignment

35. Loading Bytes lb $dest, offset(base) : load byte sb $source, offset(base) : store byte lh $dest, offset(based) : load half word sh $ source, offset(base) : store half word

36. LB Sign Extension Code: Memory Result f f f f f f b b 1111 1111 1111 1111 1111 1111 1011 1011

37. Loading Bytes Loads sign extend value – Perfect for numeric data lbu/lhu load unsigned – no sign extension – use for chars, etc…

38. LB Sign Extension Code: Memory Result 0 0 0 0 0 0 b b 0000 0000 0000 0000 0000 0000 1011 1011

39. Horner's Method Reduce multiplications to evaluate polynomial Ex: 6x 3 – 3x 2 + 7x + 2 First, put the coefficient of the first term into the accumulator: 6 Next, multiply that value by x: 6x Add the coefficient of the next term: 6x - 3 Next, multiply that sum by x: 6x 2 - 3x Add the coefficient of the next term: 6x 2 - 3x + 7 Next, multiply that sum by x: 6x 3 - 3x 2 + 7x Finally, add the coefficient of the last term: 6x 3 - 3x 2 + 7x + 2