Re: 6502 Multitasking OS announce

[ralph.mason@liffe.com (Ralph Mason)]

After a rethink ( and a quick brushup on 6502 asm :) )......

I think you can implement critical sections using the set & test that
the 6502 provides.  I think sal ( or sar ) are the best for this
purpose because no matter how many threads try concurrently to get the
cs only one will succeed.

The Idea is that in the non-entered state the cs memory contains the
lsb set.  One ( and only one ) process can then roll the bit off to
the carry flag.

to release it all you need to do is set that bit again.  I written a
couple of routines that do just that.  They also allow the same
process to get the same cs again ( otherwise you can deadlock ).


get_cs:

	jsr get_current_process ;make sure it's the righe process
	cmp cs_page+2,x
	beq inc_exit

	sal cs_page,x  ; try to get it
	bcc loop:

inc_exit:
	inc cs_page+1,x ; keep a count
	jsr get_current_process ; asumes that process is returned in A
	sta cs_page+2,x	

	ret


release_cs:
	jsr get_current_process ;make sure it's the righe process
	cmp cs_page+2,x
	bnz exit
	
	dec cs_page+1,x ; dec the count of who is using it
	bnz exit

	lda #0
	sta cs_page+2,x ; cleanup owner
	inc cs_page,x;  ; clear cs
	
exit:	
	ret

To use these functions you set X with the cs number you want and call
the function.

Of course you'll need some setup code to set all the sections to 1,
and process Id's & counts to zero  during the OS startup.

setup_cs

	lda #1
	ldx #0
loop:
	sta cs_page,x
	inx
	bnz loop:
	
loop_2:
	lda #0
	sta cs_page+1,x
	sta cs_page+2,x
	inx
	bnz loop_2:

If you find any instructions that are not exactly the right 6502
please excuse me.. it's been a while.

have fun
Ralph Mason