add sv.bc warm words for pospopcount bug #672

[libreriscv.git] / openpower / prefix_codes.mdwn

diff --git a/openpower/prefix_codes.mdwn b/openpower/prefix_codes.mdwn
index 40c9054511b0cdae2b35c34877fcb0ba1882a13c..71119b6f6981d62e0e6f150b066b41a7e5a5c689 100644 (file)
--- a/openpower/prefix_codes.mdwn
+++ b/openpower/prefix_codes.mdwn
@@ -14,20 +14,21 @@ Links:
 
 * <https://gist.github.com/kirlf/2eb242f225f9bfed4ecbfc8e1e2f5f71>
 * <https://en.m.wikipedia.org/wiki/Huffman_coding>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=933>
 
 # Prefix-code decode description
 
-`pcdec RT,RA,RB,RC,imm`
+`pcdec. RT,RA,RB,RC,imm`
 
     |0   |6   |11  |16  |21  |26  |31   |
     | PO | RT | RA | RB | RC | XO | imm |
 
 if `imm` is 1, at most one code-word is decoded -- useful for things like DEFLATE that alternate code words with other bits, or use multiple binary code trees. if `imm` is 0, it decodes multiple code-words
 
-The binary code tree is encoded in `RA` like so:
+The binary code tree is encoded in `RB` like so:
 
 ```
-t[i] = (RA >> i) & 0x1
+t[i] = (RB >> i) & 0x1
 
                        |
            +-----------+-----------+
@@ -47,46 +48,9 @@ t[8]  t[9]  t[10] t[11] t[12] t[13] t[14] t[15]
 and so on for t[16..]
 ```
 
-Decoding a code word works by walking on the tree from the root to the children, matching each passed 0 or 1 to the next read input bit in RB in LSB to MSB order. When `t[i]` is set, then a valid code word was read and `i` is written to the next byte of output in RT in LSB to MSB order. When no set `t[i]` is encountered, and there are still input bits left, then the code word is >6-bits, so SO/OV/OV32 are set, and decoding stops.
-
-# [DRAFT] Prefix-code decode
-
-sorta-VA-Form
-
-* pcdec RT,RA,RB,RC,imm
-
-Pseudo-code:
-
-    tree[0:63] <- (RA)
-    in_bits[0:63] <- (RB)
-    in_pos <- (RC)
-    decoded_in_pos <- in_pos
-    output <- [0] * 64
-    out_byte <- 0
-    decoded[0:7] <- 1
-    overflow <- 0
-    do while in_pos <u 64
-        # walk the binary tree in `tree` from parent to the selected child
-        # XXX should this be "in-_bits" not "bitstream"?
-        decoded <- decoded * 2 + bitstream[63 - in_pos]
-        in_pos <- in_pos + 1
-        if decoded >=u 64 then
-            overflow <- 1
-            break
-        if tree[63 - decoded] then
-            decoded_in_pos <- in_pos
-            output[56 - 8 * out_byte:63 - 8 * out_byte] <- decoded
-            decoded <- 1
-            out_byte <- out_byte + 1
-            # XXX if one? if "imm"?
-            if one | (out_byte >=u 8) then
-                break
-    RT <- output
-    RS <- decoded_in_pos
-
-Special Registers Altered:
-
-    SO OV OV32
+Decoding a code word works by walking on the tree from the root to the children, matching each passed 0 or 1 to the next read input bit in RA in LSB to MSB order. When `t[i]` is set, then a valid code word was read and `i` is written to the next byte of output in RT in LSB to MSB order. When no set `t[i]` is encountered, and there are still input bits left, then the code word is >5-bits, so SO/OV/OV32 are set, and decoding stops.
+
+[[!inline pages="openpower/isa/prefix_codes" quick="yes" raw="yes" ]]
 
 # [DRAFT] Prefix-code encode