There were four models of the IBM System/360 Model 91. They differed by their main memory configuration, all using IBM's
2395 Processor Storage. The 91K had 2 MB, using one 2395 Model 1. Both the 91KK and the 91L came with 4 MB of main memory: the former used a pair of 2395 Model 1s, the latter a single 2395 Model 2. The 6 MB KL was equipped with one Model 1 and one Model 2 IBM 2395s.
Models built There were only 15 Model 91s ever produced, four of which were for IBM's internal use. After quoting from Pugh et al, William H. Blair says "Many disagree on the number of 360/91s that IBM built or sold. I have read and heard it authoritatively stated that the number was 10, 11, 12, 14, 15, or 20." As for those delivered to customers, "a 360/85 was delivered from when a 91 was ordered until it was ready."
Differences from standard System/360 behaviour Because of the emphasis on speed, there were some minor differences in the system's behaviour: • Floating point divide results could differ in the least significant bit. • Some exceptions were imprecise. • The handling of floating point underflow and overflow was different, although arguably better. • Memory stores could occur out of sequence. A memory barrier instruction was added to allow control of this where necessary. ==IBM and NASA==