Re: 9 track tapes and block sizes

[Paul Koning via cctalk]

> On Oct 5, 2020, at 12:30 AM, Chuck Guzis via cctalk <cctalk@classiccmp.org> 
> wrote:
> 
> On 10/4/20 8:58 PM, jim stephens via cctalk wrote:
> 
>> I had not seen your earlier no tape gap mentions.
> 
> The old CDC 6000 SCOPE 1LT driver.  Since SCOPE user programs use
> circular buffering, The PP overlay 1LT simply emptied the CM buffer and
> wrote it, 12-bit word by 12-bit word to the drive controller, so long as
> the CPU program kept the buffer filled.

At least on NOS, that's not how long block handling works.  It uses 1MT (the 
main tape driver) with 1LT as a helper in a separate PP.  Each reads a portion 
of the long block into its PP memory, hand off to the other PP to continue 
reading while in parallel the first PP transfers the piece it read into central 
memory.  This kind of ping-pong algorithm is needed because the block transfer 
operations (both I/O and memory copy) are blocking.  They can be split up into 
individual word transfers but that isn't how the problem is handled in this 
case, and possibly that wouldn't be fast enough.

        paul