All true, but the question remains of when did the first 23FD did ship? Apparently the 2880 Block Multiplexor Channel along with the 2835/2305 were announced on February 10, 1970 for both the S/360 M85 and M195.
There are reliable sources that a 23FD did ship in January 1971 either in the M155 or in a 2835/2305; perhaps simultaneously to the same location. It could have shipped in a 2835/2305 for a S/360 M85 before January 1971 on a 2880 Block Multiplexor Channel but so far I can't find any information either way. Tom -----Original Message----- From: Sent: Tuesday, June 9, 2026 6:39 PM To: Jon Elson via cctalk <[email protected]> Subject: [cctalk] Re: Floppy disk The 360/85 had the microcode for its base instruction set in ROS (ROM) so it would not have needed a device like the 23FD to get started. It was also shipped starting in December 1969 which is before the date the 23FD is reputed to have been ready. A paper I found at https://dl.acm.org/doi/epdf/10.1145/1476936.1476969 describes instructions in the the 360/85 to load words into the writeable control store from main memory. The 370 systems on the other hand had no control store in ROS so needed a device to do the Initial Microcode Load (IML) to get the machine of the ground and I suspect the 23FD and later IML microcode sources where just part of the IML system as something would have to stuff the microcode into the control store. I see some 370 documents mention a "Service Processor" that loads the microcode, one candidate for this service processor would be one of the family of Universal Controllers that IBM used in a wide variety of machines dating back to the early to mid 1970s. Paul. On 2026-06-09 20:21, Jon Elson via cctalk wrote: > On 6/9/26 16:24, Tom Gardner via cctalk wrote: >> Lot's of mis-information which I think I correct below >> >> I'm pretty sure the first shipments of the 23FD was in January 1971 >> the with 2835 Storage Control Unit for the 2305 Fixed Head DASD and >> the S/370 M155; it might have been simultaneous but the 2835 did >> attach to existing S/360 M85 2880 Block Multiplexer channel and the >> M85 shipped in December 1969. It was followed later that year with >> the shipment in the S/370 M165 and then the rest of the S/370. FWIW, >> the 2835 SCU was very similar to the 3380 SCU, mainly differing due >> to the parallel data transfer from the drive. The 23FD was a read >> only device > > The 360/85 was EXTREMELY close to the 370/165, in fact it was the > prototype of the 165. Looking through principles of operation from > the two models shows instruction timings are totally identical. The > model 165 had a 23FD to load microcode and diagnostics. The 360/85 > had read-only 360 microcode, but emulators and diagnostics were > loadable. I can only assume that was done via a 23FD. The major > difference between 360/85 and 370/165 hardware was that microcode was > partially read-only in the 85, with 500 words or writeable control > store, implemented in 16-bit MST4 chips. The 165 had all microcode in > writable ECL static RAM in 64-bit chips. The /85 cache (storage > buffer) was implemented in the same 16-bit chip technology, the 165 > was done with the 64-bit chips. There were some slight differences in > the 360 vs. 370 processor status word, also. > > Jon > >
