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the opcode table first word consists of bits: <7> = FPD is legal for this opcode. <4:6> = number of specifiers for unimplemented opcodes (VAX subsets) <3> = unused <0:2> = number of specifiers The mask used to be 0x70. The convention is that x_M_y is a mask value <right-justified>, for a macro like: #define get_foo(x) (((x) >> x_V_foo) & x_M_foo) For a subset VAX (like the 3900), the unimplemented opcodes are those that are recognized on a full VAX but simply trap as reserved instructions (e.g., the H-floating instructions). This really only affected symbolic printing and decoding of these instructions.