[Info-vax] Backup TK50 tapes

[glen herrmannsfeldt]

Johnny Billquist <bqt at softjar.se> wrote:

(snip, someone wrote)
>>> I believe that is correct.  However I purposely restricted VMS backup
>>> tapes to a maximum of 32K because that meant I could copy savesets to
>>> disk.

>> Is that a VMS limit or DEC hardware limit?
 
> If the 32K really is a limit, I believe it would be in VMS, in treating 
> a 16-bit number as signed here.
> The controllers that I know by DEC have 16 bit fields for record lengths 
> in hardware, but it is unsigned.

Hmm, just like IBM. The S/360 instruction set has instructions for
doing signed halfword (16 bit) arithmetic, but not for unsigned.
(Actually, it is load halfword that is important.) One more instruction
would allow zeroing the high bits, but was not worth doing in the OS/360
days.

The channel hardware allows 16 bit unsigned.

-- glen