186 lines
5.6 KiB
Markdown
186 lines
5.6 KiB
Markdown
# Colour Maximite 2 - CSUB Example and ARM Cortex-M7 Embedded Code
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This document demonstrates how to embed ARM Cortex-M7 machine code directly into your BASIC program on the Colour Maximite 2 using the `CSUB` directive. It covers a minimal example, explains the embedded machine code, and provides detailed instructions on writing, compiling, and embedding ARM Thumb-2 assembly or C routines.
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---
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## What is CSUB?
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`CSUB` allows embedding raw ARM machine code or compiled C/assembly routines into Colour Maximite 2 BASIC programs. These embedded routines appear as BASIC commands or functions and run natively on the ARM Cortex-M7 processor, enabling efficient, low-level operations beyond standard BASIC capabilities.
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According to the Colour Maximite 2 user manual:
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> `CSUB name [type [, type] …]`
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> hex [[ hex[…]]
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> hex [[ hex[…]]
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> `END CSUB`
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>
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> Defines the binary code for an embedded machine code program module written in C or ARM assembler. The module will appear in MMBasic as the command 'name' and can be used in the same manner as a built-in command.
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Multiple embedded routines can be used in a program, each defining a different module with a different name. The first hex word is a 32-bit word which is the offset in bytes from the start of the CSUB to the entry point of the embedded routine (usually the function `main()`). The following hex words are the compiled binary code for the module.
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---
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## CSUB Code Example
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```basic
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CSUB simple
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00000000 00004770
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END CSUB
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simple()
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PRINT " Returned successfully"
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````
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---
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## What Does This Thumb-2 Machine Code Do?
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### `CSUB simple`
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- Starts the definition of a machine-code subroutine named `simple`.
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- Tells the BASIC runtime that the following hex words are compiled machine code to execute when `simple()` is called.
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### `00000000`
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- The entry point offset for the CSUB.
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- A value of `0` indicates execution starts immediately at the next word.
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- This word is metadata only; it is **not executed** as machine code.
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### `00004770`
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- Encodes the Thumb instruction `BX LR` (Branch to Link Register).
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- This instruction **returns immediately** from the subroutine to BASIC.
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### `END CSUB`
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- Marks the end of the CSUB block.
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- Signals that the machine code definition is complete, and BASIC execution continues normally.
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### `simple()`
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- Calls the subroutine `simple`.
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- Execution jumps to the CSUB, reads the entry point offset, executes the `BX LR` instruction, and immediately returns.
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### `PRINT " Returned successfully"`
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- Prints a message to the BASIC console to indicate the subroutine returned successfully.
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---
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## ARM Thumb-2 Assembly Example Corresponding to the Machine Code
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```asm
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.syntax unified
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.thumb
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.global simple
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simple:
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movs r0, #42 @ Load immediate value 42 into register R0
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bx lr @ Return from subroutine
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```
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### Explanation
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* `.syntax unified`: Use modern unified ARM assembler syntax.
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* `.thumb`: Assemble for the Thumb instruction set used by Cortex-M7.
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* `.global main`: Declare the `main` symbol as global (entry point).
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* `movs r0, #42`: Move the immediate value 42 into register `R0` (standard register for function return values).
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* `bx lr`: Branch to the address stored in the Link Register (`LR`), returning control to the caller.
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---
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## How to Compile, Link, and Extract Binary
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### Step 1: Save Assembly Code
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Save the code above as `main.s`.
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### Step 2: Assemble to Object File
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```bash
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arm-none-eabi-as -mcpu=cortex-m7 -mthumb main.s -o main.o
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```
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* `-mcpu=cortex-m7`: Target Cortex-M7 CPU.
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* `-mthumb`: Use Thumb instruction set.
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### Step 3: Link to ELF Executable
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```bash
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arm-none-eabi-ld main.o -Ttext=0x0 -o main.elf
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```
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Or combine compiling and linking with GCC:
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```bash
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arm-none-eabi-gcc -mcpu=cortex-m7 -mthumb -nostartfiles -Wl,-Ttext=0x0 -o main.elf main.s
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```
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* `-nostartfiles`: Avoid linking standard startup code.
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* `-Ttext=0x0`: Load address set to 0.
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### Step 4: Extract Raw Binary
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```bash
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arm-none-eabi-objcopy -O binary main.elf main.bin
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```
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### Step 5: View Machine Code as Hex
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```bash
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xxd -e main.bin
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```
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Sample output:
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```
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00000000: 20420047
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```
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* `20420047` is the machine code word representing `movs r0, #42` and `bx lr`.
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---
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## Preparing the CSUB Block
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* The **first 32-bit word** is the entry point offset (usually `00000000`).
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* Following words are your compiled machine code in 32-bit hex words.
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Example:
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```basic
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CSUB myfunc
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00000000 20420047
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END CSUB
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myfunc()
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PRINT "Returned value in R0 was 42"
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```
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---
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## Passing Arguments and Returning Data
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* You can specify argument types in the CSUB definition, e.g.,
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```basic
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CSUB MySub integer, integer, string
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```
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* Up to 10 arguments are supported.
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* Variables or arrays passed are pointers to their data. This allows embedded routines to modify passed data directly.
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* Constants and expressions are passed as pointers to temporary memory containing their values.
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* Remember to call `routinechecks()` regularly in longer-running routines to keep USB and watchdog timers active, or keep your routine execution within a few milliseconds.
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---
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## Additional Tips and Verification
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* Use `arm-none-eabi-objdump -d main.elf` to disassemble and verify your machine code.
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* Compile C routines similarly, specifying `-mcpu=cortex-m7 -mthumb` in compiler flags.
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* Place CSUB blocks anywhere in your BASIC code; MMBasic will skip over them during execution.
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* Each hex word must be exactly eight hex digits and separated by spaces or new lines.
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* Errors in formatting or hex data will cause runtime errors in MMBasic.
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---
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## License
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This document and any accompanying code are released under the MIT License.
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