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) & (gbit = 1)              then inc <- 1
  23             if (lsb = 0) & (gbit = 1) & (rbit = 1) then inc <- 1
  24             if (lsb = 0) & (gbit = 1) & (xbit = 1) then inc <- 1
  25         if round_mode  = 0b10  then           # Round toward + Infinity
  26             if (sign = 0) & (gbit = 1) then inc <-1
  27             if (sign = 0) & (rbit = 1) then inc <-1
  28             if (sign = 0) & (xbit = 1) then inc <-1
  29         if round_mode  = 0b11  then           # Round toward - Infinity
  30             if (sign = 1) & (gbit = 1) then inc <-1
  31             if (sign = 1) & (rbit = 1) then inc <-1
  32             if (sign = 1) & (xbit = 1) then inc <-1
  33
  34         # increase fraction, record the top bit as a 'carry out'
  35         if tgt_precision = 'single-precision' then
  36             tmp        <- [0]*25
  37             tmp[1:24]  <- frac[0:23]
  38             tmp[0:24]  <- tmp[0:24] + inc
  39             carry_out  <- tmp[24]
  40             frac[0:23] <- tmp[1:24]
  41         else # tgt_precision = 'double-precision'
  42             tmp        <- [0]*54
  43             tmp[1:53]  <- frac[0:52]
  44             tmp[0:53]  <- tmp[0:53] + inc
  45             carry_out  <- tmp[53]
  46             frac[0:52] <- tmp[1:54]
  47         if carry_out = 1 then exp <- exp + 1
  48         # TODO, later
  49         # FPSCR[FR] <- inc
  50         # FPSCR[FI] <- gbit | rbit | xbit
  51         # FPSCR[XX] <- FPSCR[XX] | FPSCR[FI]
  52
  53     def INT2FP(FR, cvt):
  54         if cvt = 'sint2double' then
  55             tgt_precision = 'double-precision'
  56             sign       <- FR[0]
  57         if cvt = 'sint2single' then
  58             tgt_precision <- 'single-precision'
  59             sign       <- FR[0]
  60         if cvt = 'uint2double' then
  61             tgt_precision <- 'double-precision'
  62             sign       <- 0
  63         if cvt = 'uint2single' then
  64             tgt_precision <- 'single-precision'
  65             sign       <- 0
  66
  67         frac   <- [0] * 64
  68         result <- [0] * 64
  69         exp    <- 63
  70         frac   <- FR[0:63]
  71
  72         if frac[0:63] = 0 then
  73             # Zero Operand
  74             # TODO, FPSCR
  75             #FPSCR[FR] <- 0b00
  76             #FPSCR[FI] <- 0b00
  77             #FPSCR[FPRF] <- '+ zero'
  78             result <- [0] * 64
  79         else
  80             if sign = 1 then frac[0:63] <- ¬frac[0:63] + 1
  81             # do the loop 0 times if FR = max negative 64-bit integer or
  82             #                     if FR = max unsigned 64-bit integer
  83             do while frac[0] = 0
  84                 frac[0:63] <- frac[1:63] || 0b0
  85                 exp <- exp - 1
  86             # round to nearest
  87             Round_Float( tgt_precision, sign, exp, frac, 0b00 )
  88             # TODO, FPSCR
  89             #if sign = 0 then FPSCR[FPRF] <- '+normal number'
  90             #if sign = 1 then FPSCR[FPRF] <- '-normal number'
  91             result[0]    <- sign
  92             result[1:11] <- exp + 1023     # exp + bias
  93             result[12:63] <- frac[1:52]
  94
  95         return result
  96