As far as I know there is no fastcall conventions in Turbo C.
It did appear in Borland C++ (Turbo C++?), but that's past the Turbo C lifetime.

I really want to write this program using __fastcall, as it would be a little more efficient. So, how do I handle the stack? I want this for the 16-bit x86 microprocessors.

You can write programs with whatever conventions available from your compiler, but you must stick to the original conventions used in the libraries you're trying to replace.
And fastcall was not standardized anyway. It can differ across the compilers, so there's no point in using it in public APIs.

With Turbo C it would be correct to push all the arguments on stack in reverse order (for cdecl function), and expect the returned results in AX, or DX:AX.
When returning the structures the actual method depends on the structure size, small structures are packed into AX or DX:AX registers, larger structures are passed via statically allocated buffer, returning pointer in DX:AX for far pointers or AX for near pointer. Turbo C usually just copies structure bodies, without implementing any kind of RVO.
You are free to clobber all registers except BP, and you must restore SI/DI if you clobbered those when "register variables" optimization is enabled.

Thank you.

Really, you should check with docs for the actual compiler you will be using, since there are no "standard C libraries" outside of the context of specific compiler's environment.
Bitsavers has manuals (user and reference guides) for many tools from Borland for sure.

I have OW 2.0 and Borland C/C++ 1.0 (I think) and 3.0. I didn't install the source code for OW, and I don't know about Borland. I want to analyze the source code for functions such as puts() and getchar() to see what I should better and what I don't need to do. Where can I find the spurce code for the Borland compilers? BTW, the library would be implementation-specific but should have the same names and parameters.

I really want to code much of the library in assembler and declare it as __fastcall. How do I use the parameters defined by __fastcall? BTW, I have Pacific C. Should I code for it? I remember discovering that it pushes the last parameter to a function in AX or DX:AX.

Just write the function in C, then disassemble it, and you will be able to figure out which parameters go where from it. If not, then I question your ability to write the entire function in assembler in the first place.

Ringding wrote on 2022-11-24, 16:24:

Just write the function in C, then disassemble it, and you will be able to figure out which parameters go where from it. If not, then I question your ability to write the entire function in assembler in the first place.

What? I know full well how to write x86 assembly but basically zero experience in disassembly. They are different disciplines

maxtherabbit wrote on 2022-11-24, 22:51:

What? I know full well how to write x86 assembly but basically zero experience in disassembly. They are different disciplines

Actually, there's no need to disassemble anything.
Most C compilers provide an option for ASM output, eg. "gcc -S"

Actually just declaring the function and calling it from somewhere and looking at the resulting assembler code should give you everything you need.

Most C compilers provide an option for ASM output, eg. "gcc -S"

Yes, but disassembly is often more readable, at least to me, because it is not littered with hard-to-decipher assembler macros and other magic that allows the linker to turn the entire thing into something that actually works.