lecture 12

Question 1

cache memory (real definition)

Answer 1

small fast SRAM-based memory
- managed in automatically in the hardware (NOT SOFTWARE)
- located on CPU
- hold frequently accessed memory blocks in RAM (main memory)

Question 1

cache (online definition)

Answer 1

fast storage buffer that is located in the CPU (central processing unit) of a computer
*also called cache memory

Question 2

where is cache located?

Answer 2

CPU (central processing unit) in your computer

Question 3

what are blocks?

Answer 3

blocks –> equal-sized chunks of main memory that are held in the cache

Question 4

are blocks physical partitions?

Answer 4

no, block is a contiguous range of physical address locations

Question 5

controller
————
0
1
2
…
N - 1
—————–

how many blocks is RAM partitioned into?

Answer 5

RAM = N-1 blocks in total

Question 6

what is the process for a Block Read Operation for a block that is not in the cache?

Answer 6

1. CPU sends Block Read operation
   to RAM controller
   
   • operation –> CPU sends the start
     address of the block in RAM
2. RAM controller goes to that address in main memory
   
   • makes a copy of the block (aka
     data)
   • puts the copy of the block onto
     the BUS
3. CPU controller reads the copy of the block from the BUS
   - puts it in cache memory in the
   CPU

Question 7

what is the process for a Block Write Operation for a block that is in the cache that needs to be in main memory?

Answer 7

1. CPU controller goes into cache
   
   • puts a copy of block onto the BUS
     with a specific address in the
     main memory
2. RAM controller reads from the BUS
   
   • goes to that specific address in
     main memory
   • replaces current block with the
     copy of the block from the BUS
     from the cache in the CPU

Question 8

block size in main memory = ? bytes

Answer 8

4 bytes

Question 9

0000 —- start of block 0
0001
0010
0011 —- end of block 0

0100 —- start of block 1
0101
0110
0111 —- end of block 1

what are these binary numbers an example of?

Answer 9

block address bits

Question 10

0000 —- start of block 0
0001
0010
0011 —- end of block 0

0100 —- start of block 1
0101
0110
0111 —- end of block 1

what is ‘10’, ‘11’, ‘01’, ‘10’ an example of?

Answer 10

block offset bits

Question 11

what are block offset bits?

Answer 11

the last two binary digits in the block address bits –> references specific byte within the block

ex) byte at offset 11
- references one byte within
block with unique block offset
bits

Question 12

block size =

Answer 12

2^b
b = bytes

Question 13

total number of blocks =

Answer 13

2^m/ 2^b

m = physical address bits (4)
b = bytes

Question 14

3 types of cache mapping algorithms

Answer 14

1. Fully Associative
2. Direct Mapping
3. Set Associative

Question 15

which type doesn’t have line bits?

Answer 15

FA–> the block could be anywhere in the cache

Question 16

which type does have line bits?

Answer 16

DM–> line bits determine exactly where the block is located in the cache

Question 17

which type has the block replacement algorithm?

Answer 17

FA

Question 18

what is the block replacement algorithm?

Answer 18

cache = full, line must be evicted to make space for new line of data

Question 19

3 methods to determine which block to replace for FA

Answer 19

1. LRU (least recently used)
2. FIFO (first in first out)
3. Random (no favoritism)

Question 20

**DM

0 1 1 0

what is the tag bit?

Answer 20

0 –> msb

Question 21

**DM

0 1 1 0

what is the line bit?

Answer 21

1 –> next bit to left of msb

Question 22

**DM

0 1 1 0

what are the block offset bits?

Answer 22

1 0 –> last two bits after tag and line

• 0 is technically lsb

Question 23

what are the two different policies for writing operations for cache?

Answer 23

1. write-through: CPU writes are cached while also written to main memory
   - stalls the CPU until write is done
2. write-back: CPU writes are cached, but not immediately written to main memory
   - main memory contents become
   “stale”
   - value = evicted or modified
   (“dirty”) –> then written to main
   memory

Question 24

how big is a block in physical memory?

Answer 24

one word

Question 25

principle of locality

Answer 25

programs tend to use data and instructions in memory that have addresses near or equal to those they have used recently

Question 26

temporal locality

Answer 26

recently referenced blocks are likely to be referenced again in the near future

Question 27

spatial locality

Answer 27

blocks with nearby addresses tend to be referenced close together in time