Question

Explain the advantages and disadvantages of using memory I/O and standard I/O.

Answer 1

Memory mapped I/O and standard I/O are the two methods for communicating microprocessor with peripherals.

Processor talks to both memory and peripheral using the same bus.
Two ways to talk to a peripheral.

1. Memory mapped I/O
   
2. Standard I/O
   

Memory mapped I/O: 

• Peripheral registers occupy addresses in same address space as memory.
  
• e.g. Bus has 16-bit address.
  
• Lower 32k addresses may correspond to memory.
  
• Upper 32k addresses may correspond to peripherals.
  
• Requires no special instructions.
  
• Assembly instructions involving memory like MOV and ADD work with peripherals as well.
  
• Standard I/O requires special instructions (e.g. IN, OUT) to move data between peripheral registers and memory.
  
• The advantage of memory mapped I/O is that microprocessor need not include special instructions for communicating with peripherals.
  

Standard I/O (I/O mapped I/O): 

• Additional pin (M/IO) on bus indicates whether a memory or peripheral access.
  
• Standard I/O is also known as I/O mapped I/O.
• e.g. Bus has 16-bit address
  
• All 64k addresses correspond to memory when M/IO set to 0.
  
• All 64 addresses correspond to peripherals when M/IO set to 1.
  
• No loss of memory addresses to peripherals.
  
• Simpler address decoding logic in peripherals possible.
  
• When the number of peripherals much smaller than address space then high order address bits can be ignored.
  
• Smaller and/or faster comparators.
  
• The advantage of standard I/O is no loss of memory addresses to the use as I/O addresses.