Conference noted::hackers_v1

143.0. "GT40 bootstrap code" by REX::MINOW () Tue Jul 30 1985 09:48

A while back in some notesfile (CTNOTES or the somewhat belly-up
HACKERS), someone was asking for the GT40 down-line loader format
It is just the standard PDP-11 absolute loader.  However, it uses
six-bit data (ignoring random carriage-returns).

While cleaning out my office, I came upon a paper copy of the
bootstrap.  The relevant parts follow the form feed.  Note that
I'm just typing them in -- and haven't assembled or tested them.

This was taken from Jack Burness's GT40 bootstrap, "BOOTVT S09"
Copyright 1973, 1974, Digital Equipment Inc.

With hope that this reaches the requestor...

Martin.

;
; GET8 gets an eight-bit byte (absldr calls it to read binary data):
; Uses r2, r4 (permanently).  Returns the 8-bit byte in r0.
;
get8:	jsr	pc,getsix		; Get a six-bitter
	mov	r0,-(sp)		; save
	br	getp84			; and off we go
;
; GET84 is entered when the "which byte" counter overflows
get84:	clr	r2			; reset the counter
getp84:	tst	(r2)+			; next position
	add	get8tb-2(r2),pc		; off we go.
;
; Dispatch table, don't move it.
get8tb:	.word	get81-get8tb
	.word	get82-get8tb
	.word	get83-get8tb
	.word	get84-get8tb
;
get81:	jsr	pc,getsix		; get a character now
	mov	r0,r4			; save for a second
	asl	r0
	asl	r0			; shift to left of byte
	aslb	r0
	rolb	@sp			; pack them in
	aslb	r0
	rolb	@sp			; a good 8 bit thing
	mov	(sp)+,r0		; pop and return now
	rts	pc
;
get82:	asl	r0			; worst case, shift 4
	asl	r0
	aslb	r0
	rolb	r4
	aslb	r0
	rolb	r4
	aslb	r0
	rolb	r4
	mov	r4,r0
	mov	(sp)+,r4
	rts	pc
;
get83:	rol	r0
	rol	r0
	ror	r4
	rorb	r0
	ror	r4
	rorb	r0			; final character assembled
	tst	(sp)+			; fudge stack
	rts	pc
;
; Getsix gets one byte from the DL11E
;
getsix:	jsr	pc,getkb		; Any traffic to host?
	tstb	@#DLICSR		; Any traffic from host?
	bpl	getsix			; Loop until we have work to do
	jsr	r5,getbyt		; Get the byte
	  br	getsix			; Ignore <cr>, etc.
	rts	pc			; And return
;
; Getbyt is called from interrupt and non-interrupt modes to obtain
; one byte and strip off garbage.  Result in r0.  Return:
;	 (r5)	if the byte is not alphanumeric
;	2(r5)	if the byte is alphanumeric.
;
getbyt	mov	@#DLIBUF,r0		; Get the byte from the DL11E
	bic	@#177600,r0		; Remove junk and parity
	cmpb	r0,#137			; Is it too big?
	bhi	1$			; If so, return to (r5)
	cmpb	r0,#40			; Is it too small
	blo	1$			; Return to (r5) if so.
	tst	(r5)+			; Alphanumeric, return to 2(r5)
1$:	rts	r5			; Exit.
	.end

Thank you for evoking memories of working with Jack.  I have a moving-company
box full of listings of some work he did while a contractor here.  Most of
his comments end with the word "now", and "off we go!" is another of his
stylistic "signatures".  He was also known for profane and obscene commentary:
he was so annoyed that Decus stripped out his juiciest comments from Lunar
Lander that he submitted a useful program that had an executable profane .ASCII
string - if removed the program failed to function!

re .0

Twas I that twas asking, Martin. Thank you kindly.

Peter Q.

When they submitted the moon lander sources (after censorship), Jack
was reported to have chuckled that they missed the "diagonal ones".

Peter, If you'd like, I could xerox the rest of the absldr -- I can't
seem to find the papertape anywhere, though.

Martin.