Working With Main Memory

Why Main Memory Register space limited Used for communication

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

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

Endianess Endianess : bytes order of a word in main memory

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

Integers Normal… Both endians store words (integers) same way – Disagree about how bytes are numbered

Single Byte Structures Agree on address of single byte structures – Just look different

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

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

Base Register Data starts in defined location – 0x for MARS – 0x for reading simulator

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

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

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 0x – Move over 16 bytes from there – Read a word into $9

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

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

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

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

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

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

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

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

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

Offsets Locations in order declared – Must be aligned

Offsets Locations in order declared – Must be aligned aa

Offsets Locations in order declared – Must be aligned aaff

Offsets Locations in order declared – Must be aligned aaff bb

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

Offsets Alternative order

Offsets Alternative order aa

Offsets Alternative order bbaa

Offsets Alternative order cc bbaa

Offsets Alternative order cc bbaaff

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

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

LB Sign Extension Code: Memory Result f f f f f f b b

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

LB Sign Extension Code: Memory Result b b

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