From 30e5e25f17d5fab59315e7246828912b29ada5c0 Mon Sep 17 00:00:00 2001
From: uvok
Date: Mon, 19 Jan 2026 18:18:34 +0100
Subject: eater: Implement leftover states and state transitions

---
 eater_cpu/eater_decoder.sv | 71 ++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 59 insertions(+), 12 deletions(-)

(limited to 'eater_cpu/eater_decoder.sv')

diff --git a/eater_cpu/eater_decoder.sv b/eater_cpu/eater_decoder.sv
index 2246826..4e8973c 100644
--- a/eater_cpu/eater_decoder.sv
+++ b/eater_cpu/eater_decoder.sv
@@ -12,6 +12,8 @@ CpuState internal_state;
 CpuState next_state;
 CpuControlFlags internal_flags;
 
+wire [3:0] actual_instruction = instruction_i[7:4];
+
 assign flags_o = internal_flags;
 
 initial begin
@@ -21,25 +23,38 @@ initial begin
     // internal_flags = '{default: '0};
 end
 
+function CpuState insdep_state;
+    case (actual_instruction)
+    LDA: insdep_state = LDA_INS_to_MAR;
+    ADD: insdep_state = ADD_INS_to_MAR;
+    OUT: insdep_state = OUT_A_to_OUT;
+
+    default: insdep_state = INIT;
+    endcase
+endfunction
+
 // next-state machine
 always @(posedge clk_i) begin
     next_state = INIT;
 
     case (internal_state)
-    INIT: begin
-        next_state = PC_to_MAR;
-    end
-    PC_to_MAR: begin
-        next_state = MEM_to_INS;
-    end
 
-    MEM_to_INS: begin
-        next_state = PC_inc;
-    end
+    INIT: next_state = PC_to_MAR;
 
-    PC_inc: begin
-        next_state = PC_to_MAR;
-    end
+    PC_to_MAR: next_state = MEM_to_INS;
+
+    MEM_to_INS: next_state = PC_inc;
+
+    PC_inc: next_state = insdep_state();
+
+    LDA_INS_to_MAR: next_state = LDA_MEM_to_A;
+    LDA_MEM_to_A: next_state = PC_to_MAR;
+
+    ADD_INS_to_MAR: next_state = ADD_MEM_to_B;
+    ADD_MEM_to_B: next_state = ADD_ALU_to_A;
+    ADD_ALU_to_A: next_state = PC_to_MAR;
+
+    OUT_A_to_OUT: next_state = PC_to_MAR;
 
     default: begin
        next_state = INIT; 
@@ -73,8 +88,40 @@ always_comb begin
     PC_inc: begin
         internal_flags.PC_count = 1;
     end
+
+    LDA_INS_to_MAR: begin
+        internal_flags.INS_out = 1;
+        internal_flags.MAR_in = 1;
+    end
+
+    LDA_MEM_to_A: begin
+        internal_flags.RAM_out = 1;
+        internal_flags.A_in = 1;
+    end
+
+    ADD_INS_to_MAR: begin
+        internal_flags.INS_out = 1;
+        internal_flags.MAR_in = 1;
+    end
+
+    ADD_MEM_to_B: begin
+        internal_flags.RAM_out = 1;
+        internal_flags.B_in = 1; 
+    end
+
+    ADD_ALU_to_A: begin
+        internal_flags.ALU_out = 1;
+        internal_flags.A_in = 1;
+    end
+
+    OUT_A_to_OUT: begin
+        internal_flags.A_out = 1;
+        internal_flags.OUT_in = 1;
+    end
+
     default: begin
     end
+
     endcase
 end
 
-- 
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