1. Chapter 8 Memory Interface
Microprocessors
Chapter 8
Memory Interface
prepared by
Dr. Mohamed A. Shohla

2. Chapter Overview Memory Devices Address Decoding
8088 and (8-Bit) Memory Interface
8086, 80186, 80286, and 80386SX (16-Bit) Memory Interface
Cache Memory

3. Memory Devices Address Connections Data Connections
Selection Connections

4. Semiconductor Memory Types

5. Semiconductor Memory RAM
Misnamed as all semiconductor memory is random access
Read/Write
Volatile
Temporary storage
Static or dynamic

6. Memory Cell Operation

7. Dynamic RAM Bits stored as charge in capacitors Charges leak
Need refreshing even when powered
Simpler construction
Smaller per bit
Less expensive
Need refresh circuits
Slower
Main memory
Essentially analogue
Level of charge determines value

8. Static RAM Bits stored as on/off switches No charges to leak
No refreshing needed when powered
More complex construction
Larger per bit
More expensive
Does not need refresh circuits
Faster
Cache
Digital
Uses flip-flops

9. SRAM v DRAM Both volatile Dynamic cell Static
Power needed to preserve data
Dynamic cell
Simpler to build, smaller
More dense
Less expensive
Needs refresh
Larger memory units
Static
Faster
Cache

10. Read Only Memory (ROM) Permanent storage Microprogramming (see later)
Nonvolatile
Microprogramming (see later)
Library subroutines
Systems programs (BIOS)
Function tables

11. Types of ROM Written during manufacture Programmable (once)
Very expensive for small runs
Programmable (once)
PROM
Needs special equipment to program
Read “mostly”
Erasable Programmable (EPROM)
Erased by UV
Electrically Erasable (EEPROM)
Takes much longer to write than read
Flash memory
Erase whole memory electrically

12. Address Decoding Simple NAND Gate Decoder A decoder used to select a
2K x 8 EPROM for memory locations FF800H-FFFFFH.
2K-8

13. Sample Decoder Circuit
The 74LS138, 3-to-8 line decoder

14. Example: Design a 64K-8 EPROM interface for the 8088 microprocessor using EPROM chips (8K x 8). The ROM memory starts at address F0000H-FFFFFH.

15. Basic 8088 Memory Interface
Interfacing EPROM to the 8088
A 32K-8 EPROM interface for the 8088 using EPROM chips (4K x 8). The ROM memory starts at address F8000H-FFFFFH.

16. Interfacing RAM to the 8088
A 512K-8 RAM interface for the 8088 using RAM chips (32K x 8). The ROM memory starts at address 00000H.