Computer Organization Chapter 7: Memory Organization

Memory Hierarchy

• Consists of Main Memory, Cache Memory, and Auxiliary Memory
• Main Memory and Auxiliary Memory are categorized based on access time and cost
• Cache Memory is the fastest and most expensive

Main Memory

• Also known as Random Access Memory (RAM)
• Used for temporary storage of programs and data
• Read and write operations are performed by RAM
• Volatile, meaning it loses all stored information when power is interrupted or turned off
• RAMs come with word capacities of 1K, 4K, 8K, 16K, etc. and word sizes of 1, 4, or 8-bits
• Capacity metrics: 1K = 1024, 1M = 1,048,576, 1G = 1,073,741,824

SRAM vs DRAM

• SRAM (Static RAM):
• Lower access time, making it faster
• Costlier than DRAM
• Requires constant power supply
• Less storage capacity available due to complex internal circuitry
• Low packaging density
• No need to refresh periodically
• DRAM (Dynamic RAM):
• Higher access time, making it slower
• Less expensive than SRAM
• Offers reduced power consumption
• Large storage capacity available due to small internal circuitry
• High packaging density
• Needs to refresh periodically due to capacitor used as storage element

Read-Only Memory (ROM)

• A memory device that stores permanent binary information
• Information is specified by the designer and embedded in the unit
• Once the pattern is established, it stays within the unit even when power is turned off and on again
• A ROM that can be programmed is called a PROM
• ROMs are used to store fixed-type information, such as tables for various functions, fixed data, and instructions
• ROMs can be used to design combinational logic circuits

Memory Address Map

• Example: Combining several 1K x 8 PROMs to produce a 4K x 8 PROM
• Solution: 1K x 8 PROM has 10 address lines because 2^10 = 1024 (1K)
• We need 4 number of 1K x 8 PROM chips to make a 4K x 8 PROM

Auxiliary Memory

• Also known as Secondary Memory
• Used to store large chunks of data at a lesser cost per byte than primary memory
• Non-volatile, meaning it does not lose data when the device is powered down
• Not directly accessible by the CPU; accessed via input/output channels
• Common forms of auxiliary memory devices include flash memory, optical discs, magnetic disks, and magnetic tapes

Magnetic Disks

• Circular plate of metal or plastic coated with magnetized material
• Often both sides of the disk are used, and several disks may be stacked on one spindle with read/write heads available on each surface
• Bits are stored in the magnetized surface in spots along concentric circles called tracks
• The tracks are commonly divided into sections called sectors

Magnetic Tape

• Magnetic tape transport consists of electrical, mechanical, and electronic components
• The tape itself is a strip of plastic coated with a magnetic recording medium
• Bits are recorded as magnetic spots on the tape along several tracks
• Magnetic tape units can be stopped, started, or rewound, but cannot be started or stopped fast enough between individual characters
• A tape unit is addressed by specifying the record number and the number of characters in the record
• Sequential data access mechanism
• Less expensive and less powerful than magnetic discs