From: Jacob Lifshay <programmerjake@gmail.com>
Date: Wed, 21 Sep 2022 22:41:55 +0000 (-0700)
Subject: add description of prefix-code decode
X-Git-Tag: opf_rfc_ls005_v1~335
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=68fdcecd75fed86414e0b116e1ea174236c573be;p=libreriscv.git

add description of prefix-code decode
---

diff --git a/openpower/prefix_codes.mdwn b/openpower/prefix_codes.mdwn
index 3158de09a..1a6ffb21d 100644
--- a/openpower/prefix_codes.mdwn
+++ b/openpower/prefix_codes.mdwn
@@ -11,11 +11,39 @@ This is useful for Huffman codes, and other prefix-codes, which are used a lot i
 
 `pcdec RT,RA,RB,RC,imm`
 
-if `imm` is 1 TODO FIXME
+    |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:
+
+```
+t[i] = (RA >> i) & 0x1
+
+                       |
+           +-----------+-----------+
+           |                       |
+           0                       1
+           |                       |
+     +---t[2]----+           +---t[3]----+
+     |           |           |           |
+     0           1           0           1
+     |           |           |           |
+  +t[4]-+     +t[5]-+     +t[6]-+     +t[7]-+
+  |     |     |     |     |     |     |     |
+  0     1     0     1     0     1     0     1
+  |     |     |     |     |     |     |     |
+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
 
-VA-Form
+sorta-VA-Form
 
 * pcdec RT,RA,RB,RC,imm