SOFTWARE OPTIMIZATION TECHNIQUES - Software Design for Six-Sigma: A Roadmap for Excellence - page 450

Memory

RAM Hybrid ROM

DRAM SRAM NVRAM Flash EEPROM EPROM PROM Masked

FIGURE 17.4 Memory classification in embedded systems.

The fastest possible memory that is available is desired for a real-time system;
however, cost should be considered as well. The following is a list of memory in
order of fastest to slowest while still considering cost:

1. Internal CPU memory
2. Registers
3. Cache
4. Main memory
5. Memory on board external devices

TABLE 17.2 Memory Types and Attributes

Max Erase
Cycles

Cost
(per Byte) Type Volatile? Writeable? Erase Size Speed

SRAM Yes Yes Byte Unlimited Expensive Fast

DRAM Yes Yes Byte Unlimited Moderate Moderate

Masked
ROM

No No n/a n/a Inexpensive Fast

PROM No Once, with a

device
programmer

n/a n/a Moderate Fast

EPROM No Yes, with a

device
programmer

Entire Chip Limited

(consult
datasheet)

Moderate Fast

EEPROM No Yes Byte Limited

(consult
datasheet)

Expensive Fast to read,

slow to
erase/write

Flash No Yes Sector Limited

(consult
datasheet)

Moderate Fast to read,

slow to
erase/write

NVRAM No Yes Byte Unlimited Expensive

(SRAM

Fast

+

battery)