CSE 378 Homework #4: Memory Mapped Character I/O

Supplement to the main assignment.

This page focuses primarily character IO. Once you understand it, you'll should have no problem understanding disk IO from the description of the block controller component in the SMOK documentation.

What To Do

The overall goal is to implement the character and disk input and output portion of the Cebollita I/O architecture. That archtitecture employs memory mapping to expose the character controller device that lets software write to the console and read from the keyboard. Understanding how it works requires understanding two different issues: memory mapping and I/O controllers.

Getting Started

Delete the Memory Mapped Unit(s) container from your model and introduce the real container. (Right-click / Add Component / Saved Container...) Immediately connect the block controller component inside of it to your memory component. It doesn't matter now, but it will later, so you might as well do it now. Now connect it to your machine so that it handles loads/stores to addresses 0x40000000-0x4000003F. Also make sure that you do NOT write to memory if the address is in that range.

(Character) I/O Controllers

Code that uses the character (and disk) controller is already implemented in ...\cebollita\apps\iolibs\iolib.c. Looking at it MAY help clarify things that aren't made clear by this description.

There are two parts to I/O controllers, the I and the O. Both sides are a little different than what we're accustomed to. On the input side, there is no instruction that causes a character to be typed - the user types one whenever s/he feels like. So, input is asynchronous with instruction execution; it happens when it happens. On the output side, output is slow, requiring many cycles to complete. Because of this, output as viewed by the software is demarcated by two events. The first is the start of an output operation. That is caused by the software telling the controller to perform an output, using a controller-specific command. The second event is output completion. This is signalled by the controller by raising one of the bits of its output to indicate that it is done. While an output operation is going on, it is a software error to tell the controller to start another one (meaning the result is undefined).

To accomplish these tasks, the controller has a command set (more or less a set of instructions that it knows how to execute) that it accepts as its input. When the software wants the controller to start an output, it sends it a command to the character controller (by writing some data to it) indicating what it wants (i.e., "write a character" and the character).

The controller also has an output. The output indicates the current state of the controller, e.g., "ready to accept a new output command" and/or "a keyboard input character is available" (or neither).

Given this, it is the software's responsibility to use the controller correctly. For output, the software must do this:

1. Wait for the controller to indicate that it is ready to accept an output command. This is done by sitting in a loop reading the controller output until it indicates that the controller is ready.
2. Send a command indicating that the software wants to output a character, and what the character is.
The character will appear on the console after some unpredictable number of cycles.

When the software wants to read a character, it must do this:

1. Sit in a loop reading the controller output, until the controller indicates that a keyboard input character is available.
2. When the value the software reads from the controller indicates that input is available, the character is included as part of what was read - the "key is available" is a higher-order bit of the controller output that was read in the loop, and the character itself is in bits 0-7.
3. Because the software has read that character, it sends a command to the controller telling it to turn off the "keyboard input available" bit in its output. That way the software can tell the next time a character is typed.

The details of how this work depend on the controller. We'll be using SMOK's Character Controller Device. Its documentation tell you what the controller's command set is, and what the controllers output bits mean.

Memory Mapping

Okay, we know how the software can read and write characters, except for one thing: how does it send commands to the controller, and how does it read the controller's output?

We could create new opcodes for this: RFCC (Read From Character Controller) and WTCC (Write To Character Controller). That would work, but the downside is that we'd need a different pair of opcodes for each controller attached to our system. Besides being cumbersome, that would make it impossible to attach controllers that were designed after the processor was built (since the processor wouldn't have the needed opcodes).

Instead of using new opcodes, we note that sending commands to the controller is basically a store operation: "Take the contents of this register and send it to..." Similarly, reading the controller's output is basically a load: "Get ... and put it in this register." So, instead of special opcodes, we'll just use sw and lw.

How can that work? How does the machine know we want to talk with the character controller, rather than memory, on a sw or lw? The answer is that we use a special address to indicate that we really mean the controller, not memory. In particular, Cebollita uses the address 0x40000000. A sw to that address should take the register contents and store it to the character controller (and not anywhere in memory), and a lw from that address puts the character controller's output into a register.

To get this to work is relatively simple: you just need to add enough control to notice that the instruction is a lw or sw and that the address is 0x40000000. Under those conditions, you route the data to/from the character controller, and (even if it's a sw) you do NOT cause the memory interface to do a write to ram.

Building Test Applications

First, I recommend that you compile, assemble, and link by hand.

To test the character device, you can use the program ...\cebollita\apps\tests\chartest.c. It calls printString() and readString() to read/write from/to the terminal. Compile and assemble it as always. After that, you'll need to link it with ...\cebollita\apps\iolibs\iolib.c (well, with the .o for that file, I mean). Make sure to put the prologue first in the command that links, otherwise the initial value of the PC in your SMOK model (which is almost certainly 0) will not be pointing at the first instruction that should be executed.

To test the disk controller, you can use ...\cebollita\apps\tests\blocktest.c. It needs to be linked with iolib.c (I mean, .o) as well. Additionally, to run it you need to connect a "virtual disk" to the block controller in the memory map container (right-click / Properties). That's simply a file that the block controller will read/write. Note that the file will be altered by writes, so don't use anything valuable. (I copy the file ...\cebollita\apps\blocktest_diskfile_master to blocktest_diskfile, and connect the latter to the block controller, but you can use any file you want (that is at least something like 3KB in size).)

Running Applications

You don't have to do anything special to run a test application in SMOK - just do what you always have. However, for reasons that will be explained in a moment, nothing you type in the terminal window will be echoed (unless some piece of software being executed by your SMOK machine prints it). That's normal. What's more, it's correct behavior.

If you want to compare what your SMOK machine is doing by running the same application in the Cebollita simulator, a few complications arise. They come up because, in the SMOK world, you're talking to raw hardware. When you press a key while speaking to raw hardware, all that happens is that the key's value is delivered to the hardware - no character appears on the screen, or anything else, until some software has read the character and done something with it. The iolib.c file assumes that this is what is going on -- raw hardware -- as that's pretty much what happens in SMOK.

Unfortunately, when running the Cebollita simulator, Java and/or Cygwin and/or Windows are between your keyboard and the simulator, and they one of them does "helpful" things like put the character on the screen, etc. That gets in the way of what iolib.c is trying to do, so it has to be turned off.

To turn it off, you must have things set up correctly:

1. You must run in a Cygwin shell (or, more accurately, you must not run in a DOS shell).

2. You must have environment variable CYGWIN set to "tty" before the Cygwin shell is launched. Did I mention BEFORE. You can't do it in your .bash_profile, for instance, you have to do it the raw Windows way.

3. Before running the Cebollita simulator, you must turn off Cygwin's buffering and echoing of keyboard input:
   stty -echo -icanon min 1
   After running, you must turn it on:
   stty echo icanon
It's easiest to automate this last step. The shell script cebrun.sh in the distribution does that for you. To invoke it, type ./cebrun.sh app-name in the cygwin shell (while in directory apps, which contains the cebrun.sh script). The script will try to run tests/app-name.exe. So, you might say ./cebrun.sh chartest.

If the simulator terminates abnormally, you might find that your Cygwin shell seems to have died - what you type doesn't show up. Everything's fine, you just need to type the second command above.