Lab 3

Memory-Mapped I/O: Controlling External Hardware

Learning Objectives

Understand that peripherals can be mapped into the address space so that ordinary memory write instructions control external hardware.
Observe the MMIO output box turning green when the program writes to address 0xFFFF.

Wires to Watch

rs1_to_ramrd_to_mmio

Guided Questions

1.Address 0xFFFF is 65535 in decimal. LOADI only supports a 6-bit immediate (max 31 unsigned). How can you load 65535 into a register using the available instructions? Think about bitwise NOT.
2.After assembling and running the program, what colour does the MMIO box turn? What wire on the datapath lights up?
3.Modify the program so it writes 0 to 0xFFFF after writing 1. What colour should the MMIO box be at the end?
4.Why does this architecture only need a STORE instruction to control hardware, rather than a special-purpose output instruction?

Challenge

Combine Lab 2 and Lab 3: write a program that toggles the MMIO output five times using a loop. You will need to track the current output value and flip it between 0 and 1 on each iteration.

Need a hint?

To load 0xFFFF into a register: R0 is always 0. NOT R0 = 0xFFFF. Use NOT R1 R0 to get 0xFFFF into R1 in a single instruction.

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