Memory Mapped I/O

1
Memory Mapped I/O
2
What is Memory Mapped I/O?

• Instead of having special methods for accessing
  the values to be read or written, just get them
  from memory or put them into memory.
• The device is connected directly to certain main
  memory locations.
• Two types of information to/from the device
• Status
• Value read/write

3
Why use Memory Mapped I/O

• Makes programming simpler.
• Do not have special commands to access I/O
  devices.
• Just use lw and sw.
• Takes some memory locations
• Very few compared to the size of main memory.

4
Which Memory?

• Memory addresses xffff0000 and above are used for
  I/O devices.
• The device controller has registers.
• These registers are given a memory address.
• Recall that the processor is attached to a bus,
  memory is attached to the bus, I/O devices are
  attached to the bus.
• When the bus sees certain addresses, it knows
  they are not memory addresses, but are addresses
  for accessing I/O devices.

5
Communicating with the Keyboard

• The keyboard has 2 registers associated with it
• Receiver control at address xffff0000
• Receiver data at address xffff0004
• The receiver control will have a 1 in the
  rightmost bit when there is a value ready to be
  read. It will have a 0 in that bit otherwise.
• The receiver data will have the character pressed
  on the keyboard (only when the receiver control
  has a 1 in the rightmost bit)

6
Polling

• To read, you go do you have something now? Now?
  Now? Now? Ok now read the value.
• li t4, 0xffff0000 rec. control addr
• again lw t1,0(t4) get rec. control value
• andi t1, t1, 1 get rightmost bit
• beqz t1, again if not ready check again
• lw t0, 4(t4) get char. from rec. data

7
Display

• Again 2 registers
• Transmitter control (0xffff008)
• Transmitter data (0xffff000c)
• li t4, 0xffff0000
• again lw t1, 8(t4)
• andi t1,t1,1
• beqz t1,again
• sw t0, 12(t4)

8
Working with Memory Mapped I/O

• When using memory mapped I/O on Spim, you must
  check the Mapped I/O box in the options.
• A real MIPS system (not one simulated by Spim)
  will have many more control/data register pairs
  for all of the devices.
• Must make sure you empty the data register before
  key is pressed again.
• Accessing the data register resets the status
  register.