9 track tapes and block sizes

[Chuck Guzis]

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.

The problem arose if the program either failed to keep up (e.g.
competing programs demanding the CPU) or if an ECS transfer occurred.
Where I got involved was with a 4-CPU configuration sharing 4MW of ECS,
using the ECS for swap space.  ECS transfers completely swamp the
read/write pyramid, so the PP gets locked out and can't service the
drive.  Recovery was doing a backward-read to the BOT or preceding IRG.
Lather, rinse, repeat.

It was fun to watch and we never solved that one to anyone's
satisfaction.  I don't know if 1LT survived in NOS.

Long NRZI reads/writes are easy, as all that was needed was to keep a
XOR of the frames as they're read or written for the LRC.  PE and GCR
tapes would be a bit thornier, I'd imagine.

--Chuck