How Can I Create Custom Assembly Language Instructions?

Introduction

When it comes to programming, assembly language is the language that sits closest to the hardware. It is often used for low-level system programming, such as writing device drivers, operating systems, and firmware. However, the inherent nature of assembly language is to be as close to the hardware as possible, with most instructions being predefined and fixed. This might pose a challenge when trying to introduce custom instructions. In this article, we will explore the options available to create custom or user-defined assembly language instructions, including macro facilities and other creative techniques.

Custom Assembly Instructions in Theory

The nature of assembly language is to be as close to the hardware as possible, with no room for user-defined elements, not even functions. The instructions available are predefined, such as the JMP (Jump) and PUSH (Push) instructions. These are used to achieve functionality similar to higher-level constructs like function calls. For example, a JMP and PUSH instruction sequence is often used to approximate a function call by saving the return address on the stack before jumping to the target function.

Why Custom Instructions Are Rare

There is a facility to redefine instructions through microcode, but this is typically not available to users of assembly language. Microcode is a low-level set of instructions that control the operation of a microprocessor. Redefining instructions at the microcode level is highly risky and can result in incompatible behavior on different CPUs or even a completely non-functional CPU. While some experimentation at this level can be done by chip manufacturers to test new features or fix bugs, it is not a scalable or practical approach for regular users.

Note: Tweaking microcode directly is not recommended for users as it can lead to a 'zombie' deadlocked or completely non-functional CPU. Hence, custom instructions defined at this level are generally not available or advisable for users.

User-Defined Instructions Through Macro Facility

Instead of attempting to redefine core instructions, users can leverage the macro facility available in most assemblers. A macro is a way to introduce abbreviations for sequences of operations, allowing users to define their own "instructions". These macros are essentially user-defined sequences of instructions that can be expanded in place. This makes it possible to create a more organized and simpler way of writing assembly code.

Implementing Macros

To create a custom instruction using a macro, follow these steps:

Define the macro with a name and parameters in your assembly code. Write the sequence of instructions that the macro should expand to. Use the macro name in your code instead of the expanded sequence.

For example, if you frequently need to perform a sequence of operations like loading a value into a register, performing a series of calculations, and storing the result, you can define a macro to encapsulate these operations. This not only simplifies your code but also improves code readability and maintainability.

Example

Here's a simple example in ARM assembly to demonstrate the use of macros to create a custom instruction:

    .macro CUSTOM_INSTRUCTION reg1, reg2, value
tldr %1, value
tmov %2, %1
t.endm

In this example, the macro `CUSTOM_INSTRUCTION` takes three arguments: the destination and source registers (`reg1` and `reg2`) and an impermissible value (`value`). The macro loads the value into `reg1` and then moves it to `reg2`. You can use it in your code like this:

CUSTOM_INSTRUCTION r0, r1, 12345678

This will expand to:

    ldr r0, 12345678
    mov r1, r0

Undefined Instructions and Environmental Methods

Some assembly architectures intentionally leave certain instructions undefined. This can be done to give users more freedom or to allow the system to handle the instruction erroneously in a flexible way. For example, if the instruction is undefined and the operation causes an error, the operating system can intercept the error and execute custom code, giving users the ability to define their own behavior for such instructions.

This technique can be risky and should be used with caution. It requires careful handling of errors and guarantees of compatibility with the operating system. However, it can be a powerful tool if used correctly.

Conclusion

Create custom assembly language instructions by leveraging the macro facility available in most assemblers. This method not only simplifies your code but also improves its readability and maintainability. Additionally, be aware that some architectures intentionally leave certain instructions undefined, allowing users to define their own behavior through error handling.

While direct redefinition of core instructions through microcode is not a practical or advisable approach for most users, the use of macros and undefined instructions can provide powerful tools for creating custom functionality in assembly language.