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This file is a historical part of how I turned C versions into ASM.

Porting History

I learned ASM programming a long time ago (on a platform far, far away) so I was very rusty in every way. I couldnt just jump straight in. (initially, on a Nucleo STM32F401RE board)

The embedded ARM platform has very little feedback, other than the magic blinky light, and to have the light blink is a multi step process. When I tried to just grab asm from another ARM platform, it didnt work, and I had no idea which part(s) needed help.

Modular testing

Eventually I decided to pick some working C code, and port it piece by piece. This has the advantage of modularity: If I only added a small module at one time, I knew which code needed to be fixed. So, "gpio_toggle()" became "asm_gpio_toggle", and so on.

Start by Streamlining

The original C code used the libopencm3 library, which has thousands and thousands of defines, because it supports possibly 100 hardware varients. This makes its define structure kind of function-oriented, which makes it challenging to port to ASM.

Because of this, I decided to pull in all the defines actually used in the C code, into the source itself, and remove the #includes for them.

Once I had done this, and the C code still compiled, I knew I had all I needed to port the functionality, in the one file.

After that, it was just a matter of piecemeal porting from C to ASM.

Simplify nested defines

As mentioned, libopencm3 supports many many hardware varients. As such, they have a lot of nested #defines. If they are constant based, AND you change your source code file to be named ".S" instead of ".s", you might actually be able to use a lot of them as-is.

Some of them are overly difficult to truely understand, unless you unwind them. Example:

#define PERIPH_BASE        (0x40000000U)
#define PERIPH_BASE_AHB1   (PERIPH_BASE + 0x20000)
#define GPIO_PORT_A_BASE   (PERIPH_BASE_AHB1 + 0x0000)
#define GPIOA              GPIO_PORT_A_BASE

Really??? All that, just for "GPIOA = 0x40020000" ? Technically, since theres no extensive use of variables and types, you can actually use the above as-is, as long as you name your source file with ".S"

However, if you want to preserve the original nesting, AND you want to use traditional ".s", you can do it with

.equ PERIPH_BASE,       (0x40000000U)
.equ PERIPH_BASE_AHB1,  (PERIPH_BASE + 0x20000)
.equ GPIO_PORT_A_BASE,  (PERIPH_BASE_AHB1 + 0x0000)
.equ GPIOA,              GPIO_PORT_A_BASE

Porting this code to other hardware

Lets say you have a similar-but-different ARM32 hardware, and you want to have your own ASM blinker.

One way to approach this, is similar to how I did the original port: Find C code that does the blink, then substutite functions bit by bit.

But, if you cant, or dont want to do that, then the strategy would be:

  1. Get technical spec on your hardware, with full register address details

  2. Create a version of stm32f4_regs_inc.s for your own hardware, substututing values as appropriate.

For example, my board focuses around GPIOA, but many other boards instead use GPIOB for the onboard LED(s)

  1. Adjust the mainline ASM code to do the setup and blink calls appropriate for your hardware

GDB tips

I should first say that there is an alleged GUI tool to help with this, called "inspire", I think. I used just straight gdb on command line.

That being said, gdb has some features that werent there when I learned it. There is a "layout" command, that almost turns it into a Text UI. It gives you multi-"window" display, with options to show register state, original source, or mixed src+asm. Example

layout split