FORTRAN 77 on PDP-11

[Sean 'Captain Napalm' Conner]

It was thus said that the Great der Mouse once stated:
> 
> > ;assuming A[0] = MSW, and that each "number"
> > ;is 7 bytes long
> ...
> > 		movw	#7,r3
> > 		clrw	r6		; clear carry over
> > .loop:		clrw	r4		; clear tmp registers
> > 		clrw	r5
> > 		movb	(r0)[r3],r4	; get unsigned byte into signed word
> > 		movb	(r1)[r3],r5
> > 		mulw	r4,r5		; do signed multiplication
> > 		addw	r6,r5		; add in previous "carry over"
> > 		movb	r5,(r2)[r3]	; save result
> > 		rotl	-8,r5,r6	; save "carry over"
> > 		bitcw	#00FF,r6	; make sure "carry over" is between 0 - 255
> > 		sobgeq	r3,.loop	; loop
> 
> * The comment says 7 bytes long; the code actually does 8 (7..0)

  You're right---I wanted 8 bytes, but wrote 7.  

> * Why not use movzbw instead of clrw-and-movb?

  Because I didn't know about it.  I never really *did* any VAX assembly
programming, but I do have a book on it, and I really like the instruction
set.

> * It's canonically bicw, not bitcw.

  Ah.

> * You need bicw #ff00,r6, not #00ff.

  Oh ... it *clears* bits in the destination that are set in the source.

> * movzbw r6,r6 does what your bi[t]cw tries to, and is shorter.

  Ah.

> * If you'd store the numbevrs little-endian, the VAX way, instead of
>   big-endian, you could do it much more cheaply with emul - especially
>   if you do use a multiple of 4 bytes.

  Well, I was trying to show how to do it if the MUL instruction was only
signed in nature and you wanted to do an unsigned multiply of multi-byte
values (remember, the target is a PDP-11).

> > Get's around the fact that the MUL instruction is signed only, even
> > though the longer number is unsigned.
> 
> Provided you have integer overflow traps turned off, the difference
> doesn't matter; the only difference between signed and unsigned
> multiply is what constitutes arithmetic overflow.

  Um ... no ... you're right.  I think.  

  [ rest snipped ]

  -spc (Is it obvious I never programmed assembly on a VAX? 8-/