openpower/isafunctions/fpfromint.mdwn

   1 # A.3 FLoating-Point Convert from Integer Model
   2
   3 The following describes algorithmically the operation of the Floating
   4 Convert From Integer instructions.
   5
   6 <!-- Power ISA Book I Version 3.0B Section A.3 page 782 -->
   7
   8     def Round_Float( tgt_precision, sign, exp, frac, round_mode ):
   9        inc <- 0
  10        if tgt_precision = "single-precision" then
  11            lsb <- frac[23]
  12            gbit <- frac[24]
  13            rbit <- frac[25]
  14            xbit <- frac[26:63] > 0
  15         else # tgt_precision = "double-precision"
  16             lsb  <= frac[52]
  17             gbit <= frac[53]
  18             rbit <= frac[54]
  19             xbit <= frac[55:63] > 0
  20
  21         if round_mode  = 0b00  then           # Round to Nearest
  22             if lsb = 1 and gbit = 1              then inc <- 1
  23             if lsb = 0 and gbit = 1 and rbit = 1 then inc <- 1
  24             if lsb = 0 and gbit = 1 and xbit = 1 then inc <- 1
  25         end
  26         if round_mode  = 0b10  then           # Round toward + Infinity
  27             if sign = 0 and gbit = 1 then inc <-1
  28             if sign = 0 and rbit = 1 then inc <-1
  29             if sign = 0 and xbit = 1 then inc <-1
  30         end
  31         if round_mode  = 0b11  then           # Round toward - Infinity
  32             if sign = 1 and gbit = 1 then inc <-1
  33             if sign = 1 and rbit = 1 then inc <-1
  34             if sign = 1 and xbit = 1 then inc <-1
  35         end
  36         if tgt_precision = "single-precision" then
  37             frac[0:23] <- frac[0:23] + inc
  38         else # tgt_precision = "double-precision"
  39             frac[0:52] <- frac[0:52] + inc
  40         if carry_out = 1 then exp <- exp + 1
  41         FPSCR[FR] <- inc
  42         FPSCR[FI] <- gbit | rbit | xbit
  43         FPSCR[XX] <- FPSCR[XX] | FPSCR[FI]
  44
  45     def INT2FP(FR, cvt, RN):
  46         if cvt = "sint2double" then
  47             tgt_precision = "double-precision"
  48             sign       <- FR[0]
  49         if cvt = "sint2single" then
  50             tgt_precision <- "single-precision"
  51             sign       <- FR[0]
  52         if cvt = "uint2double" then
  53             tgt_precision <- "double-precision"
  54             sign       <- 0
  55         if cvt = "uint2single" then
  56             tgt_precision <- "single-precision"
  57             sign       <- 0
  58
  59         result = [0] * 64
  60         exp        <- 63
  61         frac[0:63] <- FR
  62
  63         if frac[0:63] = 0 then
  64             # Zero Operand
  65             FPSCR[FR] <- 0b00
  66             FPSCR[FI] <- 0b00
  67             FPSCR[FPRF] <- "+ zero"
  68         else
  69             if sign = 1 then frac[0:63] <- ¬frac[0:63] + 1
  70             # do the loop 0 times if FR = max negative 64-bit integer or
  71             #                     if FR = max unsigned 64-bit integer
  72             do while frac[0] = 0
  73                 frac[0:63] <- frac[1:63] || 0b0
  74                 exp <- exp - 1
  75             Round_Float( tgt_precision, sign, exp, frac0:63, RN )
  76             if sign = 0 then FPSCR[FPRF] <- "+normal number"
  77             if sign = 1 then FPSCR[FPRF] <- "-normal number"
  78             result[0]    <- sign
  79             result[1:11] <- exp + 1023     # exp + bias
  80             result[12:63] <- frac[1:52]
  81
  82         return result
  83