Lecture 4 - Memory

Question 1

What is the general understanding of Read Only Memory (ROM)?

Answer 1

• Will keep its contents when power is off
• Contents cannot be changed
• Used for system boot
• Types: ROM, PROM, EPROM, EEPROM/Flash

Question 2

What is ROM?

Answer 2

• ROM - Read Only Memory
• Mask (data) supplied by customer
• Mass production
• Can never be changed

Question 3

What is PROM?

Answer 3

• PROM Programmable ROM
• programmable ROM
• comes “empty” (E.g. all bits are 1)
• can be programmed (“burned”) by supplying certain voltages to a specific pin
• “Burning” of fuses (wires) within chip, setting individual bits to 0
• Once a bit is flipped, it can never be changed

Question 4

What is EPROM?

Answer 4

• EPROM Erasable Programmable ROM
• Original state can be restored, ~ 20mins UV exposure

Question 5

What is EEPROM?

Answer 5

• EEPROM (Flash) Electronically Erasable Programmable ROM
• Can be electrically programmed/erased many times (~ 10,000)
• “In circuit programming” (e.g. for system parameters in computer system)
• Alternative solution: static RAM + battery backup → runs out after some time

Question 6

How is ROM implemented?

Answer 6

REFER TO SLIDES

Question 7

What is Random Access Memory (RAM)?

Answer 7

• Will lose its contents when power is off
• Contents can be changed any time
• Can store programs or data
• Writing data to a location (“address”) overwrites old data contents
• Types: Static, Dynamic
  Static RAM
  – Static RAM keeps data as long as power is on
  Dynamic RAM
  – Dynamic RAM “leaks” memory contents over short time period, needs periodic “refresh” → periodical read/write all cells

Question 8

How is RAM implemented - DRAM AND SRAM

Answer 8

REFER TO SLIDES

Question 9

What is the memory for RAM?

Answer 9

Mem = 1 cell = 1 bit (=1 flip-flop)

Question 10

What does 1 Byte equate to?

Answer 10

8 bits

Question 11

What does 8 data lines equate to?

Answer 11

1 Data Bus

Question 12

What happens if there is 2 bytes?

Answer 12

• 2 Outputs coming together on the same line!
• Only one device is allowed to send data at a time
  ➢ Enable/disable mechanism via tri-state gates required

REFER TO SLIDES

Question 13

How is memory controlled?

Answer 13

REEFER TO SLIDES

Question 14

What are some of the main memeory sizes?

Answer 14

kilobyte (KB = 2^10 Bytes = 1’024 Bytes),
megabyte (MB = 2^10 KB = 2^20 Bytes),
gigabyte (GB = 2^10 MB = 2^30 Bytes)
terabyte (TB = 2^10 GB = 2^40 Bytes)

Question 15

How is memory size calculated?

Answer 15

size = number of cells * cell size

EX: A Memory module has 2M cells with 16bits per cell.
What is its size in MB? -> 2M * 2 Bytes = 4MB (as 16 bits is 2 bytes)

Question 16

What is address space?

Answer 16

• The number of cells in a memory module determines the number of address lines required.
• Memory module sizes are always powers of 2
• The number of bits in a memory cell determines the number of data lines required.
• One data line is required per bit
• Typical memory cell sizes are 4, 8, 16, 32, 64

EX:
* How many address lines has a 4M * 1Byte memory module?
4M = 2^2 *2^20 = 2^22 → 22
* How many data lines has this module?
1Byte = 8 bits