;
; swap file de^, user/flags a, for the file presently attached
; to buffer hl^.  Return de a pointer to the previous file, and
; a as the previous flags lo byte setting.
; This permits use of a single buffer for fast file transfers,
; using the .BDUMP and .BLOAD routines to control user access
; during file read/writes.
; a,f,d,e
.bfswap::
	push	b
	push	h
	mov	b,m;		existing flags byte
	mov	m,a;		set revised flags byte
	inx h	! inx h;	advance to size
	inx h	! inx h;	advance to fcbptr
	mov a,m	! inx h ! mov c,m;	existing fcb
	mov m,d	! dcx h	! mov m,e;	revised fcb
	mov d,c	! mov e,a;	de := previous fcb
	mov	a,b;		a := previous flags
	pop	h
	pop	b
	ret