C code:

mainoff = -mainoff;

Results in the following asm code:

clr	c
clr	a
subb	a,_mainoff //(0x50)
mov	_mainoff,a //(0xB0)
clr	a
subb	a,(_mainoff + 1) //(0xC3)
mov	(_mainoff + 1),a //(0x3D)

and if mainoff is 50000 (0xC350), its two compliment sould be
-50000 (0x3CB0) but the code preduces -49744 (0x3DB0).

and C code:

mainoff = ~(--mainoff);

Results in the following:

	dec	_mainoff  //(0x50)
	mov	a,#0xff   
	cjne	a,_mainoff,00124$ //Skips
	dec	(_mainoff + 1)  
00124$:
	mov	a,_mainoff //(0x4F)
	cpl	a  
	mov	_mainoff,a //(0xB0)
	mov	a,(_mainoff + 1) //(0xC3)
	cpl	a
	mov	(_mainoff + 1),a //(0x3C)

And my main question was is this asm is legal ?

_asm
	mov    _TL2,_mainoff 
	mov    _TH2,(_mainoff + 1)   
_endasm;

The reason i'm so strict about speed is because its a critical code section under tight timing...