unit 6 - memory types

UNIT 6 - MEMORY TYPES

Six-Transistor Static RAM Cell

DYNAMIC RAM MEMORIES

TYPES OF RAM

---DYNAMIC RAM (CAPACITOR TYPE STRUCTURE)

COMPARED WITH:

---STATIC RAM (FLIP-FLOP TYPE STRUCTURE)

ADVANTAGES OF DYNAMIC RAM

1. HIGHER DENSITY (APPROX. ¼ THE PHYSICAL SIZE OF STATIC RAM; USES LESS TRANSISTORS PER BIT: 1 OR 3. VS. 6)

2. LOWER POWER CONSUMPTION (< .05 MILLIWATT/BIT FOR DYNAMIC RAM VS. .2 MILLIWATT/BIT FOR STATIC RAM, EXCEPT FOR CMOS STATIC RAM)

3. LESS EXPENSIVE PER BIT THAN STATIC RAM. (HOWEVER, NEED FOR “REFRESH CIRCUITRY” ADDS TO COST)

DISADVANTAGES OF DYNAMIC RAM

1. POSSIBLE DISADVANTAGE: DYNAMIC RAM IS OFTEN ORGANIZED AS 2N

X 1

2. NEED FOR REFRESH CYCLES (SIMILAR TO “DUMMY READS”)

---COST

---COMPLEXITY

---MAY SLOW DOWN THE PROCESSOR SOMEWHAT (FROM 1% FOR A HARDWARE APPROACH TO 10% FOR A SOFTWARE APPROACH).

Typical One-Transistor DRAM Cell

TYPICAL % TIME REQUIRED FOR DRAM REFRESH CYCLES

CONSIDER A 256K X 1 DRAM CHIP

- 18 BIT ADDRESS INPUT - 9 BIT ROW ADDRESS (NO. OF ROW = 29 = 512)

- 9 BIT COLUMN ADDRESS (NO. COLUMNS = 28 = 512)

ASSUME A 2 MILLISEC MAXIMUM REFRESH INTERNAL

ASSUME A 100 NANOSECOND MEMORY CYCLE TIME FOR READ, WRITE, OR REFRESH

TOTAL TIME TO REFRESH ALL 512 ROWS: 512 X 100 X10-9 SEC = .0512 X 10-3 SEC

THIS MUST BE DONE EVERY 2 MILLISECONDS

==> PERCENTAGE OF TIME THAT MUST BE USED FOR MEMORY REFRESH IS

.0512 MILLISECONDS = 2.56 % 2 MILLISECONDS