write-up on twin L0 cache/buffer

diff --git a/3d_gpu/architecture/6600scoreboard.mdwn b/3d_gpu/architecture/6600scoreboard.mdwn
index 1ee935f4931bea0cc54e99497049e98ddb9715a3..0b0328888c85f0e6a911513d04398cd5be4b5e8e 100644 (file)
--- a/3d_gpu/architecture/6600scoreboard.mdwn
+++ b/3d_gpu/architecture/6600scoreboard.mdwn
@@ -341,6 +341,36 @@ Twin L0 cache/buffer design
 [Flaws](https://bugs.libre-soc.org/show_bug.cgi?id=216#c24)
 in the above were detected, and needed correction.
 
+Notes:
+
+* The flaw detected above is that for each pair of LD/ST operations
+  coming from the Function Unit (to cover mis-aligned requests),
+  the Addr[4] bit is **mutually-exclusive**.  i.e. it is **guaranteed**
+  that Addr[4] for the first FU port's LD/ST request will **never**
+  equal that of the second.
+* Therefore, if the two requests are split into left/right separate L0
+  Cache/Buffers, the advantages and optimisations for XOR-comparison
+  of bits 12-48 of the address **may not take place**.
+* Solution: merge both L0-left and L0-right into one L0 Cache/Buffer,
+  with twin left/right banks in the same L0 Cache/Buffer
+* This then means that the number of rows may be reduced to 8
+* It also means that Addr[12-48] may be stored (and compared) only once
+* It does however mean that the reservation on the row has to wait for
+  *both* ports (left and right) to clear out their LD/ST operation(s).
+* Addr[4] still selects whether the request is to go into left or right bank
+
+Other than that, the design remains the same, as does the algorithm to
+merge the bytemasks.  This remains as follows:
+
+* PriorityPicker selects one row
+* For all rows greater than the selected row, if Addr[5:48] matches
+  then the bytemask is "merged" into the output-bytemask-selector
+* The output-bytemask-selector is used as a "byte-enable" line on
+  a single 128-bit byte-level read-or-write (never both).
+
+Twin 128-bit requests (read-or-write) are then passed directly through
+to a pair of L1 Caches.
+
 [[!img twin_l0_cache_buffer.jpg size="600x"]]
 
 # Multi-input/output Dependency Cell and Computation Unit