DEC RK11-C Disk Controller - on ebay...or is it?

No, 'Format' is a separate bit in the CSR from 'Read/Write-All', and they do different things. The RK11 always re-writes the header word of each sector when it writes a sector in normal operation; when 'Format' is set on a Write operation, it merely supresses the 'read header word and check/compare' function (which normally precedes any disk operation, to make sure the head's at the right place). Format/Write then goes ahead and writes the header word of the sector, just as in normal operation. (It is possible to set 'Format' on a Read operation; that just reads in the sector header words into memory.) Although in theory one could use 'Read/Write-All/Write' to format packs, the Unix V6 RK pack formatter: https://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/mdec/rkf.s sets _both_ 'Format' and 'Read/Write-All', and _doesn't_ set up the sector header words in the memory buffer, arguing that even with 'Read/Write-All' on, the hardware is still generating the sector header word contents. (I'm too lazy to check the RK11-C and RK11-D engineering drawings to confirm that.) Noel