openpower/isa/prefix_codes.mdwn

   1 <!-- https://libre-soc.org/openpower/prefix_codes/ -->
   2
   3 # [DRAFT] Prefix-code decode
   4
   5 VA2-Form
   6
   7 * pcdec. RT,RA,RB,RC,once
   8
   9 Pseudo-code:
  10
  11     tree[0:63] <- (RB)
  12     ra_used <- 0b0
  13     in_bits[0:63] <- (RC|0)
  14     if in_bits = 0 then
  15         in_bits[0:63] <- 1
  16     final_in_bits <- in_bits
  17     final_ra_used <- ra_used
  18     output <- [0] * 64
  19     out_byte <- 0
  20     decoded[0:7] <- 1
  21     so_bit <- 0b0
  22     early_stop <- 0b0
  23     do while out_byte < 8
  24         in_bit <- in_bits[63]
  25         if in_bits = 1 then
  26             if ra_used | (_RA = 0) then
  27                 early_stop <- 0b1
  28                 if decoded != 1 then  # error: stopped in the middle of a code
  29                     so_bit <- 0b1
  30                 leave
  31             ra_used <- 0b1
  32             in_bit <- (RA)[63]
  33             in_bits <- 0b1 || (RA)[0:62]
  34         else
  35             in_bits <- 0b0 || in_bits[0:62]
  36         # walk the binary tree in `tree` from parent to the selected child
  37         decoded <- decoded[1:7] || in_bit
  38         if decoded <u 64 then
  39             if tree[63 - decoded] then
  40                 final_in_bits <- in_bits
  41                 final_ra_used <- ra_used
  42                 output[56 - 8 * out_byte:63 - 8 * out_byte] <- decoded
  43                 decoded[0:7] <- 1
  44                 out_byte <- out_byte + 1
  45                 if once then
  46                     leave
  47         else
  48             so_bit <- 0b1
  49             early_stop <- 0b1
  50             leave
  51     RT <- output
  52     RS <- final_in_bits
  53     CR0 <- final_ra_used || (output = 0) || early_stop || so_bit
  54
  55 Special Registers Altered:
  56
  57     CR0