[Info-vax] Backup TK50 tapes

[Johnny Billquist]

On 2013-02-26 03:44, glen herrmannsfeldt wrote:
> 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.

Well, the VAX as such have instructions for dealing with 16-bit values 
as both signed and unsigned.
But even if it hadn't have had that, it's just a bit of convenience. You 
can solve that by just a few more instructions.
So, if there really is a limit to 32K in VMS, it's in the software. From 
a hardware point of view, the maximum block size is 64K-1.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol