openpower/isa/prefix_codes.mdwn

   1 <!-- https://libre-soc.org/openpower/prefix_codes/ -->
   2
   3 # [DRAFT] Prefix-code decode
   4
   5 VA-Form
   6
   7 * pcdec. RT,RA,RB,RC
   8
   9 Pseudo-code:
  10
  11     tree[0:63] <- (RB)
  12     mode <- tree[62:63]
  13     tree[62:63] <- 0
  14     ra_used <- 0b0
  15     in_bits[0:63] <- (RC|0)
  16     if in_bits = 0 then
  17         in_bits[0:63] <- 1
  18     orig_in_bits <- in_bits
  19     tree_index <- 1
  20     found <- 0b0
  21     hit_end <- 0b0
  22     do bit_length = 1 to 6
  23         in_bit <- in_bits[63]
  24         if in_bits = 1 then
  25             if ra_used | (_RA = 0) then
  26                 hit_end <- 0b1
  27                 leave
  28             ra_used <- 0b1
  29             in_bit <- (RA)[63]
  30             in_bits <- 0b1 || (RA)[0:62]
  31         else
  32             in_bits <- 0b0 || in_bits[0:62]
  33         # walk the binary tree in `tree` from parent to the selected child
  34         tree_index <- tree_index * 2
  35         if in_bit = 1 then
  36             tree_index <- tree_index + 1
  37         if tree_index < 64 then
  38             if tree[63 - tree_index] then
  39                 found <- 0b1
  40                 leave
  41     compressed_index <- 0
  42     do i = 0 to 127
  43         possible <- 1
  44         j <- i
  45         do while j >= 4
  46             j <- j / 2
  47             if tree[63 - j] then
  48                 possible <- 0
  49                 leave
  50         if i = tree_index then
  51             leave
  52         else if i >= 64 then
  53             compressed_index <- compressed_index + possible
  54         else if tree[63 - i] = 1 then
  55             compressed_index <- compressed_index + possible
  56     switch(mode)
  57         case(0):
  58             RT[0:63] <- tree_index
  59             if ¬found then
  60                 in_bits <- orig_in_bits
  61                 ra_used <- 0b0
  62         case(1):
  63             RT[0:63] <- tree_index
  64             if hit_end then
  65                 in_bits <- orig_in_bits
  66                 ra_used <- 0b0
  67         case(2):
  68             RT[0:63] <- compressed_index
  69             if ¬found then
  70                 in_bits <- orig_in_bits
  71                 ra_used <- 0b0
  72                 RT[0:63] <- tree_index
  73         default:
  74             RT[0:63] <- compressed_index
  75             if hit_end then
  76                 in_bits <- orig_in_bits
  77                 ra_used <- 0b0
  78     RS <- in_bits
  79     CR0 <- ra_used || (tree_index >= 64) || found || hit_end
  80
  81 Special Registers Altered:
  82
  83     CR0