## About

### x86 assembler in Open Watcom

#### Inline

```c
int myCode(int);
#pragma aux myCode = \
"mov eax,ebx"    \
value [eax] \
modify [eax] \
parm [ebx];

int main(void) {
  printf("%d\n", myCode(10)); //=> 10
  return 0;
}
```

Labels should start with an alphanumeric character. Starting them with a `.` dot won't work.

#### External file

##### Stack style, for use with DOS extender

Compile with `wasm`

```asm
PUBLIC _myCode
PUBLIC myData_

.386
.model flat,c

.DATA
myData_:
  BYTE 32

.CODE

_myCode:
  push ebp
  mov ebp, esp
  
  mov eax, [ebp + 8]
  
  pop ebp
  ret

end
```

```c
extern int myCode(int);
#pragma aux (__cdecl) myCode

extern char myData;

int main(void) {
  printf("%d\n", myCode(myData)); //=> 32
}
```

### Examples

#### Protected Mode Keyboard interrupt handler in assembler called from Open Watcom C with shared variable

Two object files are linked using `wcl386`

```c
// main.c
extern void __cdecl far interrupt keyboardHandler();
extern char keyboardPressed;
void far (interrupt *int9Save)();

int main(void) {
  int9Save = _dos_getvect(9);
  _dos_setvect(9, keyboardHandler);

  while (keyboardPressed == 0) {
    printf("%d\n", keyboardPressed);
  }

  printf("wow\n");
  delay(200);

  _dos_setvect(9, int9Save);

  return 0;
}
```

```asm
; kb.asm
PUBLIC _keyboardPressed
PUBLIC _keyboardHandler


.386
.model flat,c

.DATA

_keyboardPressed db 0

.CODE

_keyboardHandler:
  cli
  push eax
  push ebx

  in al, 0x60
  mov bl, al

  ; acknowledge keyboard
  in al, 0x61
  mov ah, al
  or al, 0x80
  out 0x61, al
  xchg ah, al
  out 0x61, al

  and bl,0x80
  jne _keyboardHandler_notKeydown

  mov [_keyboardPressed], 1
_keyboardHandler_notKeydown:
  ; acknowledge interrupt
  mov al, 0x20
  out 0x20, al

  pop ebx
  pop eax
  sti
  iretd ; needed for 32 bit protected mode
```

#### Links

* https://users.pja.edu.pl/~jms/qnx/help/watcom/compiler-tools/cpasm.html
* https://tuttlem.github.io/2015/10/04/inline-assembly-with-watcom.html
* https://montcs.bloomu.edu/Information/LowLevel/Assembly/asm-stack.html
* https://en.wikipedia.org/wiki/X86_calling_conventions#stdcall
* https://blarg.ca/2018/04/16/using-watcoms-register-calling-convention-with-tasm
* https://github.com/open-watcom/open-watcom-v2/discussions/528#discussioncomment-180994
* https://stackoverflow.com/a/73243437

## References

### Further reading

* [Open Watcom 2.0 clib docs](https://open-watcom.github.io/open-watcom-v2-wikidocs/clib.html)
* [David Brackeen's 256 Color VGA Programming in C](http://www.brackeen.com/vga/index.html)
* [Ralf Brown's Interrupt List](http://www.ctyme.com/rbrown.htm)
* [FreeVGA project](http://www.osdever.net/FreeVGA/home.htm)

## Uncategorized/Unhandled

* https://wiki.osdev.org/User:Omarrx024/VESA_Tutorial