Unit 3 Flashcards
types of memory access modes
Sequential Access Memory (SAM): Access in a set order
→ Example: Magnetic Tape, CD-ROM
Random Access Memory (RAM): Direct access to any location
→ Example: DRAM, SRAM
types of memory
CPU Registers – Fastest, inside CPU
Main Memory – Directly accessed by CPU, holds active data
Secondary Memory – Large, slow, long-term storage
Cache Memory – Fast, small, stores frequently used data
What is the Memory Hierarchy, and what are its advantages?
Memory Hierarchy Structure:
Magnetic Tapes – Slowest, used for removable storage
Magnetic Disks – Used for backup storage
Main Memory – Direct communication with CPU
Cache Memory – Fastest memory, located between CPU & main memory
Advantages:
Faster access to data with cache memory
High performance with minimized total memory system cost
types of ROMs
Mask Programming
Paths in ROM are pre-programmed by the manufacturer.
Costly process.
Programmable ROM (PROM)
Contents can be changed off-line.
Fuses in the PROM are blown to define 0s and 1s.
Irreversible programming (e.g., CD-R).
Erasable PROM (EPROM)
Can be erased with ultraviolet light and reprogrammed.
UV light discharges internal gates to reset the ROM.
Electrically Erasable PROM (EEPROM)
Erased using electrical signals instead of UV light.
Memory Connection
Data & Address Buses:
RAM and ROM chips are connected to the CPU through the data and address buses.
Low-order lines of the address bus select the byte within chips.
Other address lines select the particular chip using chip select inputs.
Memory Capacity:
Configuration gives 512 bytes of RAM (128x4) and 512 bytes of ROM.
RAM Selection:
Seven low-order bits of the address bus select one of the 128 bytes in each RAM.
Address lines 8 and 9, through a 2x4 decoder, select the specific RAM chip.
ROM Selection:
Chip select for ROM is controlled by address line 10 (0 for RAM, 1 for ROM).
ROM is enabled only during read operations through its RD control line.
secondary memory devices
ypes of Secondary Memory Devices:
Magnetic Disks:
Circular plates coated with magnetic material.
Data is stored along concentric circles (tracks) divided into sectors.
Example: Hard disks.
Magnetic Tapes:
Store binary information in parallel tracks.
Data is accessed sequentially by moving the tape to the desired location.
Commonly used for backup and archival storage.
Optical Disks:
Use laser beams to read/write data in spiral tracks.
Examples: CD-ROM (read-only), CD-RW, DVD.
USB Drives:
Flash memory storage device with USB interface.
Removable, rewritable, and portable with capacities ranging from 8 GB to 2 TB.
Solid State Drives (SSD):
Use flash memory in IC assemblies for persistent data storage.
Faster than traditional hard drives (HDD) with a limited number of writes.
SD Cards:
Non-volatile flash memory card used in portable devices.
Available in several capacities: SDSC (64 MB - 2 GB), SDHC (2 GB - 32 GB), SDXC (32 GB - 2 TB), and SDUC (2 TB - 128 TB).
Different Mapping Techniques
Associative Mapping:
Both address and data are stored in the cache memory.
The CPU generates a 15-bit address and searches for a matching address in the cache.
If a match is found, the corresponding data is returned. If no match, data is fetched from main memory.
Direct Mapping:
The 15-bit CPU address is split into a 9-bit index field and a 6-bit tag field.
The cache uses only the index to access data, while the tag is compared with the tag in the cache.
If tags match, it’s a hit; if not, it’s a miss, and the data is fetched from main memory and stored in the cache.
Set-Associative Mapping:
Each cache line can store multiple memory words (i.e., two or more) under the same index.
The CPU address’s index is used to access the cache, and the tag is compared with multiple tags in the cache.
If a match is found, data is returned; if not, one of the tag-data items is replaced with a new value.
Page Replacement Techniques
FIFO (First In, First Out):
Replaces the page that has been in memory the longest.
Each time a page is loaded, its identification number is added to a FIFO stack.
When memory is full, the oldest page is replaced.
Advantage: Easy to implement.
Disadvantage: Pages may be replaced too frequently in certain cases.
LRU (Least Recently Used):
Replaces the page that has not been used for the longest time.
Pages are tracked with a counter; the page that has the highest count is replaced.
Advantage: More efficient than FIFO in certain scenarios.
Disadvantage: Harder to implement due to the need for counters (Aging Registers).
