From 3b1e7cfbd6ef2e520702a90c4d0ac8e102f19d9a Mon Sep 17 00:00:00 2001
From: Julian Blake Kongslie
Date: Sun, 4 Apr 2021 14:53:31 -0700
Subject: Add indirect memory operations.

---
 asm.rb      |  1 +
 hdl/top.sv  | 36 ++++++++++++++++++++++++++++++------
 mem/mem.hex |  5 +++--
 3 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/asm.rb b/asm.rb
index b883510..c3a0125 100755
--- a/asm.rb
+++ b/asm.rb
@@ -1,6 +1,7 @@
 #!/usr/bin/ruby -w
 
 OPCODES = {
+  "i"     => 0x080,
   "acc="  => 0x000,
   "ladd"  => 0x100,
   "store" => 0x200,
diff --git a/hdl/top.sv b/hdl/top.sv
index 4cb5e30..00d0cab 100644
--- a/hdl/top.sv
+++ b/hdl/top.sv
@@ -70,11 +70,14 @@ bit [7:0] operand;
 bit [DATA_BITS-1:0] acc;
 bit [DATA_BITS-1:0] idx;
 
+bit [ADDR_BITS-1:0] address;
 bit [DATA_BITS-1:0] sign_extended_operand;
 
 enum
     {   FETCH
     ,   DECODE
+    ,   INDIRECT
+    ,   INDIRECTED
     ,   AGEN
     ,   MEMORY
     ,   HALT
@@ -116,14 +119,17 @@ always_ff @(posedge clk) begin
                     `ifdef DEBUG $display("\tdecode %x:%x", opcode, operand); `endif
                     case (opcode)
                         'h0: acc = sign_extended_operand;
-                        'h1: state = AGEN;
-                        'h2: state = AGEN;
+                        'h1, 'h2: begin
+                            address = {7'b0, operand[6:0]};
+                            state = operand[7] ? INDIRECT : AGEN;
+                        end
                         'h3: if (acc != sign_extended_operand) ++pc;
                         'h4: pc = pc + sign_extended_operand;
                         'h5: begin
                             mem_write_data = acc % 10 + 'h30;
                             acc = acc / 10;
-                            state = AGEN;
+                            address = {7'b0, operand[6:0]};
+                            state = operand[7] ? INDIRECT : AGEN;
                         end
                         'hf: begin
                             if (operand[0]) ++acc;
@@ -139,6 +145,24 @@ always_ff @(posedge clk) begin
                 end
             end
 
+            INDIRECT: begin
+                mem_valid = 1;
+                mem_write = 0;
+                mem_address = address;
+                state = `mem_ready ? INDIRECTED : INDIRECT;
+            end
+
+            INDIRECTED: begin
+                if (`mem_ready) begin
+                    mem_valid = 0;
+                    mem_write = 0;
+                end
+                if (`mem_read_valid) begin
+                    address = `mem_read_data;
+                    state = AGEN;
+                end
+            end
+
             AGEN: begin
                 mem_valid = 0;
                 mem_write = 0;
@@ -147,19 +171,19 @@ always_ff @(posedge clk) begin
                 case (opcode)
                     'h1: begin
                         mem_valid = 1;
-                        mem_address = {2'b0, idx + sign_extended_operand};
+                        mem_address = address + idx;
                         state = `mem_ready ? MEMORY : AGEN;
                     end
                     'h2: begin
                         mem_valid = 1;
-                        mem_address = {2'b0, idx + sign_extended_operand};
+                        mem_address = address + idx;
                         mem_write = 1;
                         mem_write_data = acc;
                         state = `mem_ready ? FETCH : AGEN;
                     end
                     'h5: begin
                         mem_valid = 1;
-                        mem_address = {2'b0, idx + sign_extended_operand};
+                        mem_address = address + idx;
                         mem_write = 1;
                         state = `mem_ready ? FETCH : AGEN;
                     end
diff --git a/mem/mem.hex b/mem/mem.hex
index d30c5b6..07023a8 100644
--- a/mem/mem.hex
+++ b/mem/mem.hex
@@ -3,7 +3,7 @@
 // Reset state is acc=0 idx=0
 
 000 // loop: acc= 0     # Print from 0x70 until we find a 0x00 byte
-170 // ladd 0x70
+170 // ladd i 0x6a
 300 // ifeq 0
 402 // jmp fib
 f44 // ++idx tx
@@ -63,8 +63,9 @@ f04 // ++idx
 f40 // tx
 f80 // halt
 
-@6c
+@6a
 
+070 // 6a: constant 0x070
 000 // 6b: LSB of ASCII rep
 000 // 6c: USB of ASCII rep
 000 // 6d: USB of ASCII rep
-- 
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