Memory Interfacing
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Microprocessor Programming - Memory Interfacing, Study notes of Microprocessors
Description about Memory Interfacing , Memory, Types of RAM memory, Types of ROM, Memory Interfacing, The 74LS138, 3-to-8 line decoder .
Typology: Study notes
2010/2011
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Memory Interfacing
Memory
- (^) Memory consists of a number of storage locations,
each of which is identified by a unique address
- (^) The ability of the CPU to identify each location is
known as its addressability
- (^) Each location stores a word i.e. the number of bits
that can be processed by the CPU in a single
operation. Word length may be typically 8,16, 24,
32, 64 or as many as 128 bits.
- (^) Memory Can be classified into two categories
- (^) RAM
- (^) ROM
Types of ROM
• Programmable Read Only Memory (PROM)
- Empty of data when manufactured
- May be permanently programmed by the user
• Erasable Programmable Read Only Memory (EPROM)
- Can be programmed, erased and reprogrammed
- (^) The EPROM chip has a small window on top allowing it to be
erased by shining ultra-violet light on it
- After reprogramming the window is covered to prevent new
contents being erased
- Access time is around 45 – 90 nanoseconds
Types of ROM
- (^) Electrically Erasable Programmable Read Only Memory
(EEPROM)
- (^) Reprogrammed electrically without using ultraviolet light
- (^) Must be removed from the computer and placed in a special machine to
do this
- (^) Access times between 45 and 200 nanoseconds
- (^) Flash ROM
- (^) Similar to EEPROM
- (^) However, can be reprogrammed while still in the computer
- (^) Easier to upgrade programs stored in Flash ROM
- (^) Used to store programs in devices e.g. modems
- (^) Access time is around 45 – 90 nanoseconds
Memory Interfacing
MEMORY ADDRESS MAP
• Address space assignment to each memory
chip on a me
• Example:
RAM 1
RAM 2
RAM 3
RAM 4
ROM
0000 - 007F
0080 - 00FF
0100 - 017F
0180 - 01FF
0200 - 03FF
Component
Hexa
address
0 0 0 x x x x x x x
0 0 1 x x x x x x x
0 1 0 x x x x x x x
0 1 1 x x x x x x x
1 x x x x x x x x x
Address bus
Example: Design a 64K-8 EPROM interface for the 8086 microprocessor using EPROM chips (8K x 8). The ROM memory starts at address F0000H-FFFFFH.