Hello everyone!
I need to write a service for Windows, but I don't know the logic behind it.
Does anyone have a skeleton of a FASM application that I could modify to suit my needs? I think I've figured out the list of API functions, but I'm still unclear on what format to specify in the "format pe64 xxx" line and how to set up a communication channel between the service and my application for data exchange. Thanks in advance for any information.

The format for .sys files is native.

revolution, apparently, I didn't explain the problem well. Basically, the scheme is this:
I have a user-mode application and a service to which I want to send commands using the ServiceControl() function. The service returns information to me either through the Clipboard or through Pipes. This is my personal service, which I create with the "SERVICE_WIN32_OWN_PROCESS = 0x10" flag, and immediately delete it when finished. If you run the "ProcessHacker" software, you'll see that all services with the "WIN32_OWN_PROCESS" flag have the *.exe extension, not *.sys. Therefore, I also need to create an EXE, but I'm not sure what format to specify in the "gui\console\native" code header. I suspect I need a GUI. Sorry for my English - this is a translator.

For .exe files that don't want a console attached use GUI.

I looked at the PE-header of one of the services in the system32 folder,
and rightly so—it turns out a GUI format is needed.

Here's a link with a description of all the services' functions:
https://learn.microsoft.com/en-us/windows/win32/services/service-functions

A Windows .exe GUI format just tells the loader not to attach a console and redirect stdin, stdout and stderr to the console. Other than that the behaviour is the same as for CONSOLE format.

That's true, but if you specify CONSOLE for the service, a console window will probably appear on the screen. Although this isn't certain, since starting with Windows 7, all services run in a private session(0), and I don't know whether the service console is passed to the user session(1). However, the GUI format definitely won't show any extra windows, unless you describe the window itself in the resources section.

IIRC background services can't open windows in the user's session, there isn't a base desktop for them to paint onto. Since a console is just another window then the console won't appear anywhere.

Can a console .exe successfully run as a service? Maybe the loader will detect that the console window failed to initialise and abort the load? One way to find out is to try it.

The service from session(0) can send a window to the user session(1,2,N) via WTSSendMessage(), but I think this is a useless function, since it is better to use IPC mechanisms for exchanging information.