I ask my friend Joe Chisolm what he remembered from his days working on
Sigma/CP-V, I'm not sure he is on this list, here is what he sent me back:
" If I remember correctly the device controller and tape unit will stop at
100 bytes. However, on the next read the tape unit will advance the tape
until it sees a inter-record gap and then start reading after that. So the
extra 100 bytes is ignored. The IRG on 800 bpi is just blank erased tape.
Since it's NRZI there is no clock recovery from the tape. For 1600 BPI
(PE) it's different. There is a specific pattern but I dont remember now.
6250 BPI works the same way but I dont remember how the inter-record gap is
done. But it's still detected by the tape unit.
The 800 bpi units had an erase head which would erase the tape before the
write was done. This made sure the IRG was blank and could be detected.
If there was a bad spot on the tape the driver would get a write error.
back up over the error record (back to the IRG) and issue a write tape
mark, which would erase some amount of tape and do a TM. then backup over
the TM and write again. When doing the MPC drives I had to do a work
around because you could erase too much tape and the read would timeout
thinking it was a blank tape."
On Sun, Mar 10, 2024 at 6:59 AM Steve Martin <steve@xxxxxxxxxxxx> wrote:
Got a question regaring Sigma I/O. (I'm churning through a long-time
project of writing a simulator for Sigma 7 under Linux.)
Say the CPU issues an SIO for input from a tape. The SIO specifies 100
bytes be read, but the next record to be read on the tape is 200 bytes
long. I know that this could cause a length mismatch status back to the
CPU, but what happens to the extra 100 bytes in the record? Do they get
thrown out? Does the IOP or the CPU buffer them and use them for the next
read? Or something else?
Thanks to any who might have the answer.
