9/20/6 Lecture 3 - Instruction Set - Al 1

Address Decoding for Memory and I/O

9/20/6 Lecture 3 - Instruction Set - Al 2

Address Decoding Address Decoding Designs

Full Address Decoding Partial Address Decoding Block Address Decoding

Implementation Random, Decoders, PROM, FPGA

9/20/6 Lecture 3 - Instruction Set - Al 3

Address Decoding Required for a microcomputer where memory

and I/O support are essential Needed for embedded system when on chip

microcontroller memory is not sufficient

9/20/6 Lecture 3 - Instruction Set - Al 4

The Memory Space 2 basic approaches

Memory mapped system – main memory and I/O space are just different addresses or regions – or memory mapped I/O (MMIO) Addressing is the same pins for memory and I/O Advantage – less pin and hardware complexity

Port Mapped I/O – have unique pins (signals) that differentiate memory and I/O address spaces Advantage – If limited memory, memory is memory Advantage – Large I/O space

9/20/6 Lecture 3 - Instruction Set - Al 5

Other architectures Harvard Architecture

Separate memory spaces for instructions and data Requires pin(s) to differentiate I/O is MMIO

Check these out on www.wikipedia.com

9/20/6 Lecture 3 - Instruction Set - Al 6

The 68000 Memory Space 23 address lines

223 words with UDS* and LDS*

This is 8M words or 16M bytes

9/20/6 Lecture 3 - Instruction Set - Al 7

Address Map When

implementing a system the designer creates a memory map.

Map would include where RAM, ROM and I/O are.

9/20/6 Lecture 3 - Instruction Set - Al 8

Full address decoding Each addressable

location within the memory components responds to only a single unique address.

9/20/6 Lecture 3 - Instruction Set - Al 9

Example of full address decoding

9/20/6 Lecture 3 - Instruction Set - Al 10

Ex continued

9/20/6 Lecture 3 - Instruction Set - Al 11

Partial Address Decoding Some of address lines are unused Least complex and most inexpensive Each component will actually respond to

several addresses

9/20/6 Lecture 3 - Instruction Set - Al 12

Partial Address decoding example

9/20/6 Lecture 3 - Instruction Set - Al 13

Block Address decoding Compromise between full and partial. Don’t decode all of address lines but do

decode more than the bare minimum. Less repeated addresses for each populated

device

9/20/6 Lecture 3 - Instruction Set - Al 14

Designing the decode logic Multiple methods of implementing the decode

logic One method is of course to implement it with

“random logic” – i.e., AND gates, OR gates, inverters, NAND gates, NOR gates

Advantage – speed Disadvantage – possibly the number of chips

9/20/6 Lecture 3 - Instruction Set - Al 15

Decoders USE m-line-to-n-line decoders Decode an m-bit input into one of n outputs

where n = 2m

Popular 74LS138 – 3-to-8 decoder Another 74LS154 – 4-to-16 decoder

9/20/6 Lecture 3 - Instruction Set - Al 16

Decoder Truth table

9/20/6 Lecture 3 - Instruction Set - Al 17

Example of decoder use

9/20/6 Lecture 3 - Instruction Set - Al 18

Implementation

9/20/6 Lecture 3 - Instruction Set - Al 19

PROMS A PROM can also be use to

implement logic functions Can use it to do address

decoding

9/20/6 Lecture 3 - Instruction Set - Al 20

Example of PROM use Decoder design must be cheap and versitle.

9/20/6 Lecture 3 - Instruction Set - Al 21

PROM Programming

9/20/6 Lecture 3 - Instruction Set - Al 22

PROM System Advantage-

Ability to select blocks of differing size

Versitility

9/20/6 Lecture 3 - Instruction Set - Al 23

FPGA, PLA, PAL Programmable

Logic Arrays AND plane – OR

plane Programmable

Array Logic Limited PLA

FPGA – A network of CLBs

9/20/6 Lecture 3 - Instruction Set - Al 24

PAL vs PLA In a PAL the

ouput’s connection to product terms is fixed

More limited logic equation support

9/20/6 Lecture 3 - Instruction Set - Al 25

Special devices There are also special chips specifically

designed for address decoding Some may be designed for a specific family

of chips