From eada089ee9952b7e58a74c120841f7ad537f6329 Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sun, 27 Mar 2022 17:00:36 -0700
Subject: A more-fair memory arbiter that actually works.

---
 hdl/mem_arbiter.sv | 65 ++++++++++++++++++++++++++++--------------------------
 1 file changed, 34 insertions(+), 31 deletions(-)

(limited to 'hdl/mem_arbiter.sv')

diff --git a/hdl/mem_arbiter.sv b/hdl/mem_arbiter.sv
index c34faa8..a48f96d 100644
--- a/hdl/mem_arbiter.sv
+++ b/hdl/mem_arbiter.sv
@@ -32,47 +32,50 @@ module mem_arbiter
                 hold_valid[i] = 0;
             selector = 0;
         end else begin
-            automatic bit ram_was_valid = ram_valid;
-
             if (ram_ready) ram_valid = 0;
 
-            if (command_ready && command_valid) begin
-                hold_valid[0] = 1;
-                hold_data[0] = command_data;
-                hold_data[0].tag = 0;
-            end
-            for (int i = 0; i < `NUM_PDPS; ++i) begin
-                if (pdp_ready[i] && pdp_valid[i]) begin
-                    hold_valid[i+1] = 1;
-                    hold_data[i+1].address[`RAM_ADDRESS_BITS-1:`PDP_ADDRESS_BITS] = i;
-                    hold_data[i+1].address[`PDP_ADDRESS_BITS-1:$clog2(`RAM_LINE_WORDS)] = pdp_data[i].address;
-                    hold_data[i+1].write = pdp_data[i].write;
-                    hold_data[i+1].data = pdp_data[i].data;
-                    hold_data[i+1].mask = pdp_data[i].mask;
-                    hold_data[i+1].tag = i+1;
+            if (hold_valid == 0) begin
+                if (command_ready && command_valid) begin
+                    hold_valid[0] = 1;
+                    hold_data[0] = command_data;
+                    hold_data[0].tag = 0;
+                end
+                for (int i = 0; i < `NUM_PDPS; ++i) begin
+                    if (pdp_ready[i] && pdp_valid[i]) begin
+                        hold_valid[i+1] = 1;
+                        hold_data[i+1].address[`RAM_ADDRESS_BITS-1:`PDP_ADDRESS_BITS] = i;
+                        hold_data[i+1].address[`PDP_ADDRESS_BITS-1:$clog2(`RAM_LINE_WORDS)] = pdp_data[i].address;
+                        hold_data[i+1].write = pdp_data[i].write;
+                        hold_data[i+1].data = pdp_data[i].data;
+                        hold_data[i+1].mask = pdp_data[i].mask;
+                        hold_data[i+1].tag = i+1;
+                    end
                 end
             end
 
-            for (int i = 0; i < `NUM_PDPS+1; ++i) begin
-                automatic int j = selector + i;
-                if (j > `NUM_PDPS) j = j - (`NUM_PDPS+1);
-                if (!ram_valid && hold_valid[j]) begin
-                    ram_valid = 1;
-                    ram_data = hold_data[j];
-                    hold_valid[j] = 0;
+            if (hold_valid != 0) begin
+                automatic bit ram_was_valid = ram_valid;
+                for (int i = 0; i < `NUM_PDPS+1; ++i) begin
+                    automatic int j = selector + i;
+                    if (j > `NUM_PDPS) j = j - (`NUM_PDPS+1);
+                    if (!ram_valid && hold_valid[j]) begin
+                        ram_valid = 1;
+                        ram_data = hold_data[j];
+                        hold_valid[j] = 0;
+                    end
+                end
+                if (ram_valid && !ram_was_valid) begin
+                    if (selector == `NUM_PDPS)
+                        selector = 0;
+                    else
+                        ++selector;
                 end
             end
 
-            if (ram_valid && !ram_was_valid) begin
-                if (selector == `NUM_PDPS)
-                    selector = 0;
-                else
-                    ++selector;
-            end
 
-            command_ready = !hold_valid[0];
+            command_ready = hold_valid == 0;
             for (int i = 0; i < `NUM_PDPS; ++i)
-                pdp_ready[i] = !hold_valid[i+1];
+                pdp_ready[i] = hold_valid == 0;
         end
     end
 
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