This document discusses addressing memory and ports through the use of decoders. It describes how 3-to-8 decoders are used as address decoders to enable specific ROM, RAM, or port devices based on the address. Parallel ROMs are addressed using decoders, with a 12-bit address allowing up to 4KB of memory. RAM decoders allow up to 8 RAM chips to be addressed using 11 address lines. Port decoders translate memory addresses to select signals for input/output ports, allowing ports to be accessed using memory instructions. Memory banks for the 8086 use address lines A1-A19 to select memory within the bank, and A0 and a bus enable line to separately address the lower and upper banks.

1. Addressing Memory and Ports
Reference: Microprocessors and
Interfacing – Douglas V Hall – Chapter 7

2. Address Decoder
• 3to8 decoders like 74LS138 serve as the
address decoders.
• Produces a signal which enables the ROM,
RAM, or port device, which is enabled for a
particular address
• Makes sure that only one device is enabled at
a time to put data in the bus.

3. Parallel ROMs with decoder

4. ROM address decoding
• Memory size=212
*8 bit =4096*8bit=4KByte

5. RAM decoder
• Toadd eight 2K*8 RAMs
• We need 211
address line
• A0-A10 will be used for this
• A11, A12, and A13 are the
selectors

6. Port Decoder

7. Memory Mapped I/O
• Using a decoder which translates memory
addressing to chip select signals for port
device is called Port Decoder
• Advantage: Any instruction that references
memory, can be used to input data from or
output data to ports
• Disadvantage: Some of the system memory
space is used up

8. 8086 Memory Bank

9. Memory Bank
• Odd-even memory bank is used to read or
write 16 bit at a time
• Address A1-A19 are used to select desired
memory within the bank
• A0 is used for selecting lower bank and Bus
High Enable is used for selecting upper bank
• It also prevents unwanted writing of data in
adjacent memory location during byte
operation

10. Addressing Decoding
Type Address Data Type BHE A0 Bus Cycles
Data lines
used
Even 0000 Byte 1 0 One D0-D7
Even 0000 Word 0 0 One D0-D15
Odd 0001 Byte 0 1 One D8-D15
Odd 0001 Word 0 1 First D8-D15
1 0 Second D0-D7

11. Thanks…….