Intro to Computer Engineering

Studio 8 - Assembly: Just the Basics

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Introduction

Click here to access the Canvas page with the repository for this studio.

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Computing Concepts

There are some pieces of “common knowledge” that most people have about computers:

  1. Computers just follow instructions.

  2. Programming is the process of creating a set of instructions that a computer will follow.

  3. These instructions are in a binary language that computers “read”.

  4. Computers follow instructions in an exact order.

  5. Computers have memory (RAM or ROM) to store information.

The fetch-decode-execute cycle

The fetch-decode-execute process is part of what ties these concepts together.

  1. You’ve seen in CSE 131 and CSE 132 that computers do just follow instructions.

  2. You’ve been creating programs in high level languages (Java and C), but these programming languages often express computation in complex ways that a processor can’t directly understand. So another program, a compiler, is used to break down these complex programs into simpler steps. For example, an equation that you might see in a Java program, like:

d = a + b + c;

May actually be broken down into a smaller, but equivalent set of steps, like:

d = a
d = d + b
d = d + c

The end result stored in d is the same, but each individual step is simpler. Compilers often do three things a) break your original set of steps into smaller, less complex steps, b) translate these individual steps into a new, different language, and c) build a long list (a compilation) of these simpler steps in the new language. This compilation in the new language may look something like this:

mov r5,r2	// r5 <- r2
add r5,r3	// r5 <- r5 + r3
add r5,r4	// r5 <- r5 + r4

In this translation the variables were renamed (from a,b,c,d, to r2, r3, r4, and r5) and the order and structure of operations was replaced. For example, the d=a is represented with mov to indicate that the information in a is being moved (copied) to d. The word add was used rather than the symbol + and the word add doesn’t seem to work quite like + — add seems to modify something by adding to it, like the += in C or Java.

The things on the left, the mov and the add, are called opcodes. They are a code for an operation. The things to the right of the opcodes, like the r5, r2, etc., are called operands. The operands are the data used by the operation. At the end of the lines (after the //) are comments.

Data can be stored in different places. The two most common are the computer’s RAM and a small set of special, internal variables called registers. The rX values above are the latter.

  1. This assembly language is then given to a program called an assembler. The assembler just translates the values into numbers, producing the binary code for the program. For simple examples like the one shown above, this process is just a simple substitution code — each individual part is replaced with a numeric code. In the examples given, a mov can be represented with the binary value 0010 1100, and add with 0000 1100; the four r variables can just be represented by the 4-bit equivalent of their numeric value, so r2=0010, r3=0011, etc. With these codes it’s easy to represent the entire program as numbers:
Original Assembly Instruction Binary Representation of Machine Code Hexadecimal Representation of Machine Code
mov r5,r2 0010 1100 0101 0010 0x2C52
add r5,r3 0000 1100 0101 0011 0x0C53
add r5,r4 0000 1100 0101 0100 0x0C54

We’ve taken our idea of a list of instructions (program) and converted it into a smaller set of steps that we can represent as binary numbers! This is the type of machine language that computers, like the Arduino’s AVR processor, read. In fact, this example is exactly the language the Arduino understands. Other computers, like the Intel processor in your desktop, may not use the same types of steps or the same numeric codes. (The above numbers will do very different things on a different type of computer)

  1. A computer’s processor reads this machine language and acts on it. It turns out it’s pretty easy to build a machine to do exactly this. Basically, the machine has to repeatedly do three things:
    • Fetch the next instruction it needs to perform.
    • Decode it: Figure out what the individual parts are and what they mean. For example, if the number starts with 0010 1100 the computer needs a way to identify that this means it should do a mov.
    • Execute it: Actually follow through on the operation the instruction is describing, like copying (moving) data from one location to another.
  2. The computer memories are an important part of this process. A memory stores data and the instructions themselves. For example, you can think of the partial program shown above as just being an array of 16-bit numbers. The order of data in the array corresponds to the order to do the steps of the program. The above program could be thought of as an Arduino array of ints:
int instructions[] = {
	0x2C52,
	0x0C53,
	0x0C54
};

The computer can keep track of which step it’s currently on by just using an index into this array. In fact, this is exactly what happens. The index that keeps track of which step the computer is currently on is called either the Instruction Pointer (IP) or the Program Counter (PC). Both terms are used and both are pretty clear. For example, the index “0” for the example above can be thought of as “pointing to” the first instruction (the mov that is represented as 2C52). Of course, we usually advance from one instruction to another, so this pointer would usually go from the instruction at index 0, then to the instruction at index 1, then to the instruction at index 2, etc. I.e., it’s usually “counting” through the program’s instructions.

Assembly

The full manual for the AVR assembly language instructions is available here. It includes detailed descriptions of nearly all aspects of how the AVR processor reads instructions, including all the opcodes, how they use operands, and how they should be converted to machine language. Lucky for us, AVR is a reduced instruction set computer (RISC) and has physical circuits for most instructions, which means that it has relatively few instructions available compared to a processor like the Intel processors in your desktop or laptop. As a comparison, the AVR has about 123 instructions and a manual that is about 190 pages long. A modern Intel processor has more than 500 instructions and a manual that is about 2200 pages long.

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The first assembly

  1. The Arduino IDE natively supports programming in assembly.

Open up simpleValues/simpleValues.ino from your repository.

You’ll notice that the code still looks like C. That’s because it is! Our assembly unit is going to be a combination of C and assembly. If you open the assembly.S tab you’ll see the first bit of assembly you’ll work on today.

Upload this program now. What does it print to Serial Monitor?

  1. The Arduino IDE automatically uses all the files in the sketch folder, including any assembly language or C-language files. If you look at assembly.h, you see a function prototype—just like within any other header file. We’ve included that header file in the main Arduino file, simpleValues.ino, so our C code can call our assembly functions.

This function, however, does not have a corresponding C source file. It’s written in assembly.

C code always follows certain rules, or conventions, when being converted into machine code. If we follow these rules we will be able to call our assembly language code from C or vice-versa without problems.

For example, Arduino C expects any function returning a single byte to place it in register 24. There are 32 “general purpose” registers on the Arduino (named r0 to r31) and each holds just one byte. There are also special registers called the status register and the previously mentioned program counter.

Therefore, if we change the value that gets put in register 24 when the function returns, we’ll change the function’s return value. The ldi instruction loads an immediate value into a register.

Currently, the giveMeMax function returns 1. Update it so it returns the maximum value that could be stored in a register.

Rerun the program. What does it print out now?

A cop-out

We strongly considered writing this unit entirely in assembly. Since C can compile into assembly (but usually go straight to machine code), it is theoretically possible. However, we ultimately decided against it.

Here’s why:

Actually, that name-mangling is the reason for a lot of the “weird” bits of our program. The header files we provide wrap all the function names in extern "C" {} to tell the compiler, no, these are not C++ functions that should be name-mangled, but C functions, which play nicely with assembly.

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Mimicking C

The line of assembly language implementation of this function had 4 main parts. In order to create a “C-style” function in assembly, you need all of them.

  1. A .global pseudo-op with a function name (identical to a C function prototype in a header file elsewhere).
  2. A function name as a label, indicating the beginning of a function’s code.
  3. The actual code of the function
  4. A ret operation to make the function return.

All lines of assembly code look something like this:

[label:] instruction [operand], [operand]

where square brackets ([]) indicate an optional part. For example, the line you edited had the instruction, ldi (load immediate), followed by two operands. The first operand indicated which register to put the result in and the second operand was a specific number to load (a literal — the value was literally 1).

Above it was a simple label, giveMeMax, which just names a line of the program so that other parts of the program can refer to it. In this case they will be able to treat the name giveMeMax like a function and the label specifies where the function’s code is at.

Notice that this is identical to the function called in simpleValues.ino (i.e., simpleValue.ino calls giveMeMax()). The .global pseudo-operation above the giveMeMax: label is used to indicate that the label may be used from other files, like the simpleValue.ino file.

Calling a function causes it to start running the function’s instructions. In this case, calling giveMeMax() will start executing instructions at the giveMeMax label. By default it will execute them one-at-a-time and in-order — the program counter “counts” through the instructions! It continues linearly by default, so we need to alter this in order to resume running the calling function (i.e., to get back to the next line of code in simpleValue.ino). We do this with a ret operation. The ret is much like a return statement in a Java or C program — it causes the function to return. However, unlike Java and C, ret can not be given an operand. If you want to return a value you have to use other instructions to marshall the value into the appropriate register.

Our header file has a prototype for another function called giveMeZero().

  1. simpleValues.ino has commented lines of code using this function. Uncomment them. If you try building your project now the compiler will give you an error: you have not defined your function yet.
  2. To remedy that, define your function in assembly. Make a label called giveMeZero after the giveMeMax code.
  3. You need to indicate to the compiler that this label will need to be accessible from another file. Above your new label, use the .global pseudo-op (with the label name as the operand) to tell the compiler that our new label is global to the entire program.
  4. Assembly runs line after line forever until it is told to not do the next line. If you called giveMeZero() in your C code (which you can — try it), the function will never return. Also try moving the giveMeZero label before giveMeMax.
  5. Make sure that giveMeZero is defined after giveMeMax, add the ret operation after your the label to make the function return, and try it again. What happens? Does it still return a value, even though you did not explicitly set one?
  6. Use the ldi instruction like you did in giveMeMax, but have it return a zero.

What does your program print now?

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Calling all functions

You can also call functions from assembly. If you call a C function (call functionName) in assembly your program will call that function. By following a calling convention, an agreed-upon standard of how functions should be handled in assembly, it’s possible to have your assembly functions have C-style returns (which you did earlier by writing to register 24) and accept C-style arguments.

We provide a C function that prints the state of all the registers on your Arduino: printRegs(). You can call it from C (just like any other C function) or you can call it from assembly. You will need to include a header file in the simpleValues.ino file to call it. It already has a #include line commented out. Simply remove the comment (//) to include the file so you can use printRegs from C or assembly).

  1. Modify one of your assembly functions to call printRegs (using the call instruction) before and after ldi. What is in r24 before and after? Do any of the status register bits change?

  2. There’s another function stub in assembly.h, addFour(). We want it to take its argument, add four, and return the added value.

    Modify simpleValues.ino to call and print the result of addFour() on a number of your choice. Since we have not written the addFour() definition yet, your program won’t compile.

  3. Write the basics of the addFour() function in assembly: the .global pseudo-op, the appropriate label, and a ret call. Don’t worry about actually doing anything yet, just make sure your program compiles.

  4. The AVR version of gcc (the software that handles C compilation for AVR) has conventions for what registers are used by the C compiler for what purposes. This manual, in Sections 3 through 6, describes the essentials, including the conventions for parameter passing and return values in function call.

    What register do you expect the single byte argument to appear in? Use call printRegs to verify your answer.

  5. We want to add 4 to this argument and return it. The add instruction is perfect for this (don’t worry about overflow or carry), but it can only operate on registers. To use it properly, you will need to use a second register besides the argument register. The AVR gcc documentation sections you looked at earlier has a list of caller-saved registers (that they refer to as call-used registers in the document). Table 5.1 has errors, so here is a corrected version, and we’ll stick with the caller-saved nomenclature. The caller-saved registers you can use freely in your assembly language functions that are called from C without worrying about breaking things.

    Use ldi to put 4 in a caller-saved register and add to properly add four to the function’s argument. Function arguments are stored automatically in register pairs. A function’s first argument is stored in registers r24 and r25. It second argument is stored in r23 and r22, etc. Arguments of types needing more than 2 bytes use more than 2 registers. E.g., if the first argument is of type long, it is stored in r22, r23, r24, and r25. Since the argument given to addFour() is a byte, it does not need the upper 8 bits allocated to the first argument (r25), and r24 contains the first argument. Since the return value is also a byte, your function should store this result in register 24. Normally this would require a mov` operation, but there should be a convenient “coincidence” here.

Random Errors

If, when running your code, your output has seemingly random and unexplainable errors, double-check that you are properly using caller-saved registers, if you use them at all. Failure to save and restore them properly can cause all kinds of problems.

  1. Your addFour() function should now work correctly. Test it a bit. What happens when you overflow a byte (like 255+4)? What happens to the status register’s carry flag (C)?

Stacking up

The call and ret operations (and the push and pop operations you’ll use for the assignment) all operate on the stack. This is a convention for using memory, though it does blur the line between datatype and convention1.

The stack’s primary purpose is keeping track of function calls: when you call a function, the computer puts the location of the next instruction on the stack, then jumps to the called function. When that function returns, the computer looks at this stored address in the stack and jumps to the correct point in the program to continue execution.

The way this works is interesting. The stack is simply a segment of memory that we block out arbitrarily (if it gets bigger than this segment you get a stack overflow). By keeping track of the latest entry in the stack with a stack pointer (just like we keep track of the current instruction with the instruction pointer), we can push new data onto the stack and pop it off the stack. A push will increment the stack pointer and put this data in the new location, and a pop will read the data then decrement the pointer. Thus a call will push the return address onto the stack and a ret instruction will pop this address off.

You can use the stack for arbitrary data storage as well, provided you clear that data off before the stack needs to be used elsewhere. The push and pop instructions let you push and pop arbitrary registers, but remember that the stack knows nothing about data that you put on it. If you don’t pop all the data you push, there will be extra data on the stack. When your program tries to ret or read from the stack next, it will read your data, and not the data it was expecting. This is called stack corruption. Keep track of your stack.

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I/O from the get go

NOTE: If you get to this point, consider this studio a success! You can consider yourself done. This section just delves deeper into assembly’s capabilities, and is completely optional.

You’ll deal with further memory manipulation (loading and storing variables, pointers, references, etc) in this module’s assignment, but we’re going to cover another aspect of output in assembly.

Like your desktop computer, which connects to multiple forms of output (like your monitor, speakers, wi-fi card, and rumble pads on your Xbox controller), your Arduino can also communicate with output pins—you’ve been working with these pins all year. The digitalWrite() command in Arduino is a C function, but we’re going to do something similar in AVR assembly.

AVR assembly provides two ways to access I/O pins. All of them are numbered and can be accessed using their appropriate address in I/O space, but they are also mapped to different memory addresses and can be modified by editing their values in memory space. These pins then have two addresses: their I/O space address and their memory space address.

The address you use depends on the commands you use to access them. If you use dedicated instructions for modifying I/O ports (like we will) you use their I/O space address. If you use memory modification instructions (like the sort we will use to modify variables and memory in the assignment), you use the memory space address.

Unfortunately, the constants given to us in avr/io.h are the memory space addresses. We will have to convert them with the _SFR_IO_ADDR() macro, which justs subtracts 0x20.

  1. Define the turnOnLight() function in assembly, then add some code to simpleValues.ino to call it.

  2. AVR I/O pins are grouped in banks: a large microcontroller could hypothetically have around 20 banks of 8 ports, but ours only has 3. For each bank there are three corresponding I/O registers, each controlling a single aspect of each pin. For example, the DDR register controls data direction (1 for output, 0 for input), while the PORT register will write output to an output pin or selectively enable the pull-up resistor for an input pin. The PIN register will read an input pin.

    The three banks, B, C, and D, correspond respectively to digital pins 13-8, analog pins 5-0 (two of C’s bits correspond to built-in timers), and digital pins 7-0—yup, the order is weird.

    We’ll be using the B bank to control pin 13, since that requires no additional wiring. Arduino pin 13 corresponds to bank B’s fifth pin.

    Use the sbi command (Set Bit in I/O, which writes a 1 to a single I/O bit in a single bank) to change the data direction of Arduino pin 13 to output: sbi _SFR_IO_ADDR(DDRB), PIN5.

    _SFR_IO_ADDR() converts a memory address of an I/O pin to it’s appropriate I/O address, so we are setting the the data direction (DDRx of DDRB) of the fifth pin (PIN5) on bank B (xxxB of DDRB) to output (1).

  3. Now set the correct bit of PORTB to 1 to turn on the pin. Remember to use _SFR_IO_ADDR().

    What happens when you upload this program to your Arduino? The LED built into Arduino pin 13 should flicker on and off while the program starts, but should then settle into a specific state.

  4. Now add a cbi instruction (Clear Bit of I/O register) to turn off the LED directly after you turn it on. Rerun the program. What happens to the LED? How long does it take for the change to occur? How does that compare to the 16MHz clock rate of the Arduino?

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Check out!

  1. Commit your code and get checked out by a TA.

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Key Concepts

This is a mental checklist for you to see what the Studio is desgined to teach you.

Generated at 2024-04-06 20:30:52 +0000.
Page written by Ben Stolovitz, edited by Doug Shook, Roger Chamberlain, and James Orr.

  1. Although you can consider any datatype to be a convention, or, at least, you can consider the implementation of a datatype to be a convention. E.g., IEEE floating point. ↩