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compatible with unmodified user mode PDP-11 code. The name "VAX-11", used on early models, was chosen to highlight this capability. The VAX ISA is considered a
user mode PDP-11 code (thus the -11 in VAX-11), offering an upward compatible path for existing customers. The first machine in the series, the VAX-11/780
OpenVMS
of VAX/VMS, which used certain RSX-11M user-mode utilities before native VAX versions had been developed. By the V3.0 release, all compatibility-mode utilities
Addressing mode
Digital Equipment Corporation (DEC) VAX, treat registers and literal or immediate constants as just another addressing mode. Others, such as the IBM System/360
aimed at the high-end time-sharing market. The early VAX CPUs provided a PDP–11 compatibility mode under which much existing software could be immediately