From 0ae15275d34c55859d0a7b55acab132852fc7b71 Mon Sep 17 00:00:00 2001
From: uvok
Date: Thu, 22 Jan 2026 19:18:37 +0100
Subject: Implement sub instruction

---
 eater_cpu/eater_decoder.sv | 21 +++++++++++++++++++++
 eater_cpu/eater_types.sv   |  8 ++++++++
 2 files changed, 29 insertions(+)

diff --git a/eater_cpu/eater_decoder.sv b/eater_cpu/eater_decoder.sv
index 5bb8feb..ad0fd52 100644
--- a/eater_cpu/eater_decoder.sv
+++ b/eater_cpu/eater_decoder.sv
@@ -28,6 +28,7 @@ function CpuState insdep_state;
     NOP: insdep_state = PC_to_MAR;
     LDA: insdep_state = LDA_INS_to_MAR;
     ADD: insdep_state = ADD_INS_to_MAR;
+    SUB: insdep_state = SUB_INS_to_MAR;
     OUT: insdep_state = OUT_A_to_OUT;
     HALT_op: insdep_state = HALT_st;
 
@@ -56,6 +57,10 @@ always @(posedge clk_i) begin
     ADD_MEM_to_B: next_state = ADD_ALU_to_A;
     ADD_ALU_to_A: next_state = PC_to_MAR;
 
+    SUB_INS_to_MAR: next_state = SUB_MEM_to_B;
+    SUB_MEM_to_B: next_state = SUB_ALU_to_A;
+    SUB_ALU_to_A: next_state = PC_to_MAR;
+
     OUT_A_to_OUT: next_state = PC_to_MAR;
 
     HALT_st: next_state = HALT_st;
@@ -118,6 +123,22 @@ always_comb begin
         internal_flags.A_in = 1;
     end
 
+    SUB_INS_to_MAR: begin
+        internal_flags.INS_out = 1;
+        internal_flags.MAR_in = 1;
+    end
+
+    SUB_MEM_to_B: begin
+        internal_flags.RAM_out = 1;
+        internal_flags.B_in = 1; 
+    end
+
+    SUB_ALU_to_A: begin
+        internal_flags.ALU_out = 1;
+        internal_flags.ALU_subtract_nadd = 1;
+        internal_flags.A_in = 1;
+    end
+
     OUT_A_to_OUT: begin
         internal_flags.A_out = 1;
         internal_flags.OUT_in = 1;
diff --git a/eater_cpu/eater_types.sv b/eater_cpu/eater_types.sv
index 256f6ca..a39977e 100644
--- a/eater_cpu/eater_types.sv
+++ b/eater_cpu/eater_types.sv
@@ -32,6 +32,13 @@ typedef enum logic[7:0] {
     // ADD: c) ALU -> A
     ADD_ALU_to_A,
 
+    // SUB: a) LSB of INStruction into MAR
+    SUB_INS_to_MAR,
+    // SUB: b) MEM -> B
+    SUB_MEM_to_B,
+    // SUB: c) ALU -> A
+    SUB_ALU_to_A,
+
     // OUT: A -> OUT
     OUT_A_to_OUT,
 
@@ -43,6 +50,7 @@ typedef enum logic[3:0] {
     NOP = 'b0000,
     LDA = 'b0001,
     ADD = 'b0010,
+    SUB = 'b0011,
     OUT = 'b1110,
     HALT_op = 'b1111
 } eater_instruction;
-- 
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