;Explanatory note for AUTL.AS, the autoloader for DIY6502
;Copyright (c) 1996  Douglas Beattie Jr.
;
;
;     A device driver for the 6522 VIA to receive
;       parallel data from an IBM PC printer port.
;
;     This is a fast wire-to-wire interface for a printer port.
;
;     Assemble with MAS65 and load into EEPROM at $F800.  Hook
;     printer lines BUSY, STROBE, and D0..D7 to Port B of the VIA.
;
;     To download Intel HEX files into the board, simply type
;     at the DOS command prompt,  COPY S001.HEX LPT1:   and it is done.
;
;     Theory of operation:
;
;          This driver utilizes VIA port B as data,
;           and CB1 as printer STROBE (active low) which
;           should trigger the NMI. The data should be
;           latched and recovered by the interrupt service
;           routine.
;
;          Strangely enough, the port is configured entirely
;           for input, yet proper emulation of a line printer
;           is achieved using Handshake Output Mode.
;
;          The BUSY status line IS HELD HIGH by
;           CB2 during processing due to OUTPUT handshaking.
;           Just before exiting the interrupt service, port B
;           is WRITTEN to, effectively clearing BUSY.
;
;          BUSY is needed to restrain further input while
;           additional processing of received data takes place.
;
;     Additional processing includes translation from
;       Intel HEX format and properly placing the object
;       code within memory.
;
;     The Intel-format HEX load modules have the option of immediate
;       execution if a transfer address was specified in the
;       MAS65 assembler source file; e.g. END  START.
;
;     If the module executes, all registers are saved in a
;       software stack.  This allows new programs to preempt
;       currently running programs, nesting in execution, and
;       returning to the 'caller'.
;
;     The actual return address was pushed into the stack before
;       execution transfer and points to a register-restore routine
;       which fetches the previous state of all registers, including
;       processor status, from the software stack.  The next return
;       address is that of the most-recently preempted program.
;