co slideset 5

li

puts numbers into registers

what’s system memory

• where does the cpu run programs?

• temporary storage hardware

• it’s smaller, faster, and expensive than the persistent storage

• where the programs and data resides

• cpu can only run programs from system memory

what’s a memory?

• 1d array of bytes

• bytes have addresses

words

• comfy integer size for us

• it’s usually 32 bits which is equal to 4 bytes

accumulator archirectures

• used on register

• add more registers that served specific purposes

what’s a word-addressable memory vs. byte addressable memory

• each address usually points to an entire word

• byte addressable store a 32-bit word

  • each byte is has it’s own address

why does the address increase by 4?

• they increase by 4 because we’re taking in 32 bit words and we continue to increase

• 32 bits = 4 bytes

address vs. value

• addy tells you where the word is (4, 8, etc)

• value tells you what’s stored in there (DECE0EBE)

what does the equation 2^nB mean?

• 2^n is the address representaion

• B is the number of bytes

• it’s a memory capacity

now explain how we could store words into an address that only stores one byte?

• another example, one word= 4 bytes but our memory holds bytes

• so we just take every 4 addresses and make that our word!

• if we have a word and it starts at address 4 and goes up to address 8, then the word is in address 4

  • we always take the address where the word starts the beginning which is address 4 and goes up to 8

what’s an accumulator

• olden days, ppl only added into one register only

• acc is the place and result was also there

general purpose registers

• very fast storage location inside cpu

• used for many things

load

• you take smth from memory and move it into a register

• variables tend to be stored in memory u gotta physically take them out

stores

• copy data from CPU registers into memory

how does memory start, so what do we do?

• it starts off as an empty slate

• we exectute in memory address 0×00400000

  • so the cpu knows when to start

what happens after we execute the memory address?

• we start fetching instructions from memory

t or f does everything in memory has an address?

• true it does memory stores ALL the information

what does every piece of data have

• it has two parts

• an address and a value

how do we access words?

• we use lw

  • this loads word from MEMORY to CPU REGISTERS

• we use sq

  • this stores word from CPU REGISTERS to MEMORY

li

• load immediate means directly put values into the registers

li, sw, lw

• li — I know the number or addy. Put it in the register.

• lw — Go to this memory address and bring me what's stored there.

• sw — Take the value in this register and save it at this memory address.

la vs. li

• la basically we give it a variable and it finds the address and value

• li basically we directly assign it to a register it’s never stored in memory

lw

• This tells the computer:

  1. Look at register to find the address, which is 1000.

  2. Go to memory address 1000.

  3. Read the value stored there, which is 7.

  4. Put that value into $t1.

indirect

memory address is in the value in a register

offset

• you add a constant to that value

what does byte do?

• stores 1 byte of data

• tiny: .byte 4

half

• stores 2 bytes

• small: .half 4

word

• x: .word 4

• stores 4 bytes

lb

• load byte

• memory→ register

lbu

• load byte unsigned

• memory→ register

sb

• stores byte

• memory→ byte

lh

• load half-word

• memory→ register

lhu

• load half word unsigned

• memory→ register

sh

• store half-word

• register→ mem

sign extension vs. zero extension

so sign extension checks if the msb is 1 or 0 and based on that copies the bit and zero extension j keeps copying the 0's

big endian vs. little endian

• big

  • it’s when the number is the smallest to biggest

• little

  • it’s when the number is big to all

we base them off of their place value

comps use little endian

truncating concetp

• sometimes we truncate extras if we want to store a specific amt of bytes, so we always take the stuff from the right and truncate the left so we

• we j get rid of extras that dont fit

• we keep the lowest-order buts and et rid of the upper order buts

• if we’re keep 8 bits then the lsb value is taken

• if we’re keeping half words then the two lsbs are taken

• if we keep a whole word nothing’s rly truncated usually