ating data types, no bit reordering or input checking is performed during load or store instructions as it is in S_floating.The Alpha 21164 also supports VAX floating point formats. VAX is short for Virtual Address eXtension. VAX systems feature the operating system, VMS that support virtual memory. VAX floating-point numbers are stored in one set of formats in memory (datum) and in a second set of formats in registers. The floating-point load and store instructions convert between these formats purely by rearranging bits; no rounding or range checking is done by the load and store instruc-tions. Alpha processors support F, G, and some D floating point formats. F_floating data types are much like IEEE S_floating in that they are also four contiguous bytes, the 31st bit is the signed bit, bits 30 through 23 represent exponents, and bits 22 through 0 represent the fraction. However, instead of producing an equivalent T_floating data type, it produces an equivalent VAX G_floating data type in the registerThe G_floating operand occupies 64 bits in a register. According to Digital Equipment Company, the bits are as follows:The form of a G_floating datum is sign magnitude with bit 15 the sign bit, bits *14:4* an excess 1024 binary exponent, and bits *3:0* and *63:16* a normal-ized 53-bit fraction with the redundant most significant fraction bit not repre-sented. Within the fraction, bits of increasing significance are from 48 through 63, 32 through 47, 16 through 31, and 0 through 3. The 11-bit exponent field encodes the values 0 through 2047. An exponent value of 0, together with a sign bit of 0, is taken to indicate that the G_floating datum has a value of 0 (9, 2-5).D_floating data types are only partially supported in Alpha architecture.For backward compatibility, exact D-floating arithmetic may be provided via software emulation. D_floating format compatibility in which binary files of D_floating numbers may be processed, but without the...
