On 2018-Jun-20 12:33:05 -0400, Paul Winalski wrote: >All of the System/360 series except the model 25 used separate channel >processors to perform I/O. The S360 architecture defined separate main CPU and I/O channel processors and the actual implementation varied between models. IBM stressed the compatibility between models so it can be difficult to determine what the actual implementation did in hardware vs microcode. At least the model 30 also emulated the channel processor using the main CPU. [1] confirms this for the multiplexor[2] channels and implies it for the selector[3] channels. The "CPU interference factors" in [5](p65) suggest the model 50 also emulated the channel processors. The idea of separate I/O processors was also used in the CDC6600. >term "memory") completely independently from the CPU. The S/360 model >25 was the last of the 360 series and was really a 16-bit minicomputer >microprogrammed to execute the S/360 instruction set. Note that most S360 machines were microcoded with the native ALU size varying between 8 and 32 bits. The model 25 was also the only S360 with writable microcode and there was a microcoded APL implementation for it so it "natively" executed APL. I'm not sure if there were any other novel microcode sets for it. Going back to Greg's question of actual I/O performance: A model 50 could support 3 selector channels, with a nominal rate of 800kBps each[6]. Since each selector channel could only perform a single I/O operation at a time, I believe the actual rate was effectively limited to the fastest device on the channel - which [5] indicates was 340kBps for a 7340-3 Hypertape at 3022bpi. That implies a total of 1020kBps of I/O. The "CPU interference" indicates that each byte transferred blocked the CPU for 0.95us, so 1020kBps of I/O would also steal 97% of the CPU-storage bandwidth. [1] http://bitsavers.org/pdf/ibm/360/funcChar/GA24-3231-7_360-30_funcChar.pdf [2] "multiplexer" channels were used for low speed devices - card readers, card punches, printers, serial communications. [3] "selector" channels were used for high speed devices - tape, DASD[4] [4] Direct Access Storage Device - IBM speak for "disk" [5] http://bitsavers.org/pdf/ibm/360/funcChar/A22-6898-1_360-50_funcChar_1967.pdf [6] https://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP2050.html -- Peter Jeremy