While the GE 645 hardware was being designed and debugged in Phoenix, a system was put in place where a GE 635 could be used to run a simulator known as the 6.36, so that development and checkout of Multics could occur in parallel. This process involved creating a tape on the CTSS system which would be inputted to GECOS on the 635 system in MIT so that it would run under the 6.36 simulator; the resulting output would be carried back via tape to CTSS for debugging/analysis. This simulated environment was replaced by the first 645 hardware in 1967. The GECOS operating system was fully replaced by Multics in 1969 with the Multics supervisor separated by protection rings with "gates" allowing access from user mode.
While the GE 645 hardware was being designed and debugged in Phoenix, a system was put in place where a GE 635 could be used to run a simulator known as the 6.36, so that development and checkout of Multics could occur in parallel. This process involved creating a tape on the CTSS system wh.ich would be inputted to GECOS on the 635 system in MIT so that it would run under the 636 simulator; the resulting output would be carried back via tape to CTSS for debugging/analysis. This simulated environment was replaced by the first 645 hardware in 1967. The GECOS operating system was fully replaced by Multics in 1969 with the Multics supervisor separated by protection rings with "gates" allowing access from user mode.