cutemouse rs232 uart I/O access overview ctmouse 2.1beta4 6/2008:

irqhandler
	outb 20,20
	inb 3fd 	; LSR: get status
	inb 3f8		; RBR: fetch data, flush receive buffer
	; now check if LSR showed break / frame overrun
	; call mouse protocol parser

enableuart
	outb 3fb,80	; LCR: DLAB on
	outw 3f8,96	; divisor
	outw 3fb,0f02h	; LCR, MCR 7,8N1 DTR RTS OUTX
	inb 3f8		; RBR flush receive buffer
	outw 3f9,1	; IER, FCR: enable DR IRQ, disable FIFO
	inb 21
	outb 21, previous and not bit for our IRQ (enable IRQ)

disableuart
	inb 21
	outb 21, previous or bit for our IRQ (disable IRQ)
	; now keep DTR RTS on and disable driver, avoid PnP ID
	outw 3fb,300	; LCR, MCR: DLAB off DTR RTS on OUTx off
	outb 3f9,0	; IER: IRQ off

searchcom
	comloop
		checkcom
			call detectmouse for each addr
	; LCRSET is 3 here (LCR 0 - noparity 0 3)
	; default in enableuart is DLAB off, 7, 8N1 (00000010b)
	; ... and MCR 00001111b DTR/RTS/OUTx all on

checkuart
	inw 3fc		; MSR, LSR: modem control, line status
	inb 3fd		; LSR: backup
	cli
	inb 3fb		; LCR: line ctrl reg
	outb 3fb,10111111b	; LCR: DLAB on, 8S2
	inb 3fb
	outb 3fb,00000010b	; LCR: no DLAB, 7N1
	inb 3fb
	sti
	; now see if LCR bits actually changed as intended
	inb 3f9		; IER: IRQ enable reg (zap pending IRQ?)
	outb 3fb,...	; restore LCR

detectmouse
	checkuart	; takes addr si
	inw 3fb		; LCR, MCR: backup
	outw 3fb,0	; LCR, MCR: no DLAB, DTR/RTS/OUT2 off
	outw 3f9,0	; IER, FCR: IRQ off, FIFO off
	outw 3fb,80	; LCR: DLAB
	outw 3f8,96	; divisor 1200 baud
	outb 3fb,LCRSET	; usually 8N1 or 7N1, both with DLAB off
	inb 3f8		; RBR: flush receive buffer
	; now wait 1 tick
	outb 3fc,00000011b	; MCR: DTR/RTS on, OUT2 off
	; wait again, give mouse time to send pnp data
	inb 3fd		; LSR
	inb 3f8		; RBR: receive a byte
	; ... possibly more inb to receive some more bytes
	outw 3fb,...	; restore LCR, MCR