Function Name: OPEN

Purpose: Open logical file
Call address: $FFC0 (hex) 65472 (decimal)
Preparation:

	Registers:	none
	Memory:		system map
	Flags:		none
	Calls:		SETLFS, SETNAM, SETBNK

	Registers:	.A = error code (if any)
			.X used
			.Y used
	Memory:		setup for I/O
			STATUS, RSSTAT updated
	Flags:		.C = 1 -> error

Description: OPEN prepares a logical file for I/O operations. It creates a unique entry in the Kernal logical file tables LAT ($362), FAT ($36C) and SAT ($376) using its index LDTND ($98) and data supplied by the user via SETLFS. There can be up to then logical files OPENed simultaneously. OPEN performs device-specific opening tasks for serial, cassette and RS-232 devices, including clearing the previous status and transmitting any given filename or command string supplied by the user via SETNAM and SETBNK. The I/O status is updated appropriately and can be read via READST.
The path to OPEN is through an indirect RAM vector at $31A. Applications may therefor provide their own OPEN procedures or supplement the system's by redirecting this vector to their own routine.

EXAMPLE: OPEN 1,8,15,"IO"

	LDA	#length		;fnlen
	LDX	#filename
	JSR	$FFBD		;SETNAM
	
	LDX	#0		;fnbank (RAM0)
	JSR	$FF68		;SETBNK

	LDA	#1		;la
	LDX	#8		;fa
	LDY	#15		;sa
	JSR	$FFBA		;SETLFS

	JSR	$FFC0		;OPEN

	BCS	error

filename .BYTE 'IO'
length   = 2