Chapter 8 Memory Interface Microprocessors Chapter 8 Memory Interface prepared by Dr. Mohamed A. Shohla
Chapter Overview Memory Devices Address Decoding 8088 and 80188 (8-Bit) Memory Interface 8086, 80186, 80286, and 80386SX (16-Bit) Memory Interface Cache Memory
Memory Devices Address Connections Data Connections Selection Connections
Semiconductor Memory Types
Semiconductor Memory RAM Misnamed as all semiconductor memory is random access Read/Write Volatile Temporary storage Static or dynamic
Memory Cell Operation
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
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
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
Read Only Memory (ROM) Permanent storage Microprogramming (see later) Nonvolatile Microprogramming (see later) Library subroutines Systems programs (BIOS) Function tables
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
Address Decoding Simple NAND Gate Decoder A decoder used to select a 2K x 8 EPROM for memory locations FF800H-FFFFFH. 2K-8
Sample Decoder Circuit The 74LS138, 3-to-8 line decoder
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.
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.
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.