Memory Mapped I/O

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

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.

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.

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)

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

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)

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.