9/20/6lecture 3 - instruction set - al1 address decoding for memory and i/o
TRANSCRIPT
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
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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
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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
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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
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The 68000 Memory Space 23 address lines
223 words with UDS* and LDS*
This is 8M words or 16M bytes
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Address Map When
implementing a system the designer creates a memory map.
Map would include where RAM, ROM and I/O are.
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Full address decoding Each addressable
location within the memory components responds to only a single unique address.
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Partial Address Decoding Some of address lines are unused Least complex and most inexpensive Each component will actually respond to
several addresses
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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
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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
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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
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PROMS A PROM can also be use to
implement logic functions Can use it to do address
decoding
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Example of PROM use Decoder design must be cheap and versitle.
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PROM System Advantage-
Ability to select blocks of differing size
Versitility
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FPGA, PLA, PAL Programmable
Logic Arrays AND plane – OR
plane Programmable
Array Logic Limited PLA
FPGA – A network of CLBs
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PAL vs PLA In a PAL the
ouput’s connection to product terms is fixed
More limited logic equation support