From 085e5ceac41833f197c6cebda1c0ec925c59fb8b Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sun, 22 May 2022 17:09:03 -0700
Subject: Two-cycle memory arbiter, enabling 16 PDP-8s @ 50MHz.

---
 hdl/defs.svh       |  2 +-
 hdl/mem_arbiter.sv | 24 +++++++++++-------------
 2 files changed, 12 insertions(+), 14 deletions(-)

(limited to 'hdl')

diff --git a/hdl/defs.svh b/hdl/defs.svh
index 3206764..f4590c8 100644
--- a/hdl/defs.svh
+++ b/hdl/defs.svh
@@ -5,7 +5,7 @@
 
 `define PDP_ADDRESS_BITS    15
 
-`define NUM_PDPS            2
+`define NUM_PDPS            16
 
 `define UART_BYTE_BITS      8
 
diff --git a/hdl/mem_arbiter.sv b/hdl/mem_arbiter.sv
index a48f96d..1404a54 100644
--- a/hdl/mem_arbiter.sv
+++ b/hdl/mem_arbiter.sv
@@ -34,6 +34,12 @@ module mem_arbiter
         end else begin
             if (ram_ready) ram_valid = 0;
 
+            if (!ram_valid && hold_valid[selector]) begin
+                ram_valid = 1;
+                ram_data = hold_data[selector];
+                hold_valid[selector] = 0;
+            end
+
             if (hold_valid == 0) begin
                 if (command_ready && command_valid) begin
                     hold_valid[0] = 1;
@@ -54,25 +60,17 @@ module mem_arbiter
             end
 
             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;
+                    if (j > `NUM_PDPS)
+                        j -= `NUM_PDPS+1;
+                    if (hold_valid[j]) begin
+                        selector = j;
+                        break;
                     end
                 end
-                if (ram_valid && !ram_was_valid) begin
-                    if (selector == `NUM_PDPS)
-                        selector = 0;
-                    else
-                        ++selector;
-                end
             end
 
-
             command_ready = hold_valid == 0;
             for (int i = 0; i < `NUM_PDPS; ++i)
                 pdp_ready[i] = hold_valid == 0;
-- 
cgit v1.2.3