From a92eb11f4b77bd34db3eb529ec8e3877afdc8d65 Mon Sep 17 00:00:00 2001
From: cspark <git@cspark.dev>
Date: Tue, 18 Nov 2025 03:07:40 +0000
Subject: [PATCH] Decode instructions using unions

---
 main.c | 48 +++++++++++++++++++++++-------------------------
 1 file changed, 23 insertions(+), 25 deletions(-)

diff --git a/main.c b/main.c
index 3bd3b3b..2653a3e 100644
--- a/main.c
+++ b/main.c
@@ -105,24 +105,19 @@ void overture_condition_jump(enum overture_condition_mode mode, uint8_t r0, uint
 	};
 }
 
-
-typedef struct overture_decoded_instruction {
-	uint8_t overture_mode : 2;
-	uint8_t left_operand : 3;
-	uint8_t right_operand : 3;
-	uint8_t combined_immediate : 6;
-} overture_decoded_instruction;
-
-overture_decoded_instruction overture_decode_instruction(uint8_t current_instruction) {
-	overture_decoded_instruction instruction = {
-		.overture_mode = current_instruction >> 6,
-		.left_operand =  (current_instruction << 2) >> 5,
-		.right_operand =  (current_instruction << 5) >> 5,
-		.combined_immediate =  (current_instruction << 2) >> 2,
+typedef union overture_instruction {
+	uint8_t instruction;
+	union {
+		struct {
+			uint8_t combined_immediate : 6;
+			uint8_t overture_mode : 2;
+		};
+		struct {
+			uint8_t right_operand : 3;
+			uint8_t left_operand : 3;
+		};
 	};
-
-	return instruction;
-};
+} overture_instruction;
 
 void overture_fetch_instruction(uint8_t program_counter, uint8_t* current_instruction, uint8_t rom[256]) {
 	*current_instruction = rom[program_counter - 1];
@@ -144,7 +139,7 @@ typedef struct overture {
 	uint8_t output;
 	bool output_enable;
 
-	uint8_t current_instruction;
+	overture_instruction current_instruction;
 } overture;
 
 void sleep_hz(int hz) {
@@ -211,7 +206,7 @@ uint8_t* multiplex_destination(uint8_t operand, overture* cpu) {
 	return destination;
 }
 
-void overture_execute_instruction(overture_decoded_instruction instruction, overture* cpu) {
+void overture_execute_instruction(overture_instruction instruction, overture* cpu) {
 	uint8_t* source_register = multiplex_source(instruction.left_operand, cpu);
 	uint8_t* destination_register = multiplex_destination(instruction.right_operand, cpu);
 
@@ -252,9 +247,13 @@ void cpu_diagnostics(overture* cpu) {
 
 	printf("input: %d\n", cpu->input);
 	printf("output: %d\n", cpu->output);
-	printf("Output enabled?: %d\n", cpu->output_enable);
+	printf("Output enabled?: %d\n\n", cpu->output_enable);
 
-	printf("Current instruction: %d\n\n", cpu->current_instruction);
+	printf("Current instruction: %d\n", cpu->current_instruction.instruction);
+	printf("Mode: %d\n", cpu->current_instruction.overture_mode);
+	printf("Left Operand: %d\n", cpu->current_instruction.left_operand);
+	printf("Right Operand: %d\n", cpu->current_instruction.right_operand);
+	printf("Combined Immediate: %d\n\n", cpu->current_instruction.combined_immediate);
 }
 
 void overture_cycle(overture* cpu, uint8_t rom[256]) {
@@ -263,10 +262,9 @@ void overture_cycle(overture* cpu, uint8_t rom[256]) {
 	};
 	cpu->program_counter += 1;
 	cpu->output_enable = false;
-	overture_fetch_instruction(cpu->program_counter, &(cpu->current_instruction), rom);
+	overture_fetch_instruction(cpu->program_counter, &(cpu->current_instruction.instruction), rom);
 
-	overture_decoded_instruction instruction = overture_decode_instruction(cpu->current_instruction);
-	overture_execute_instruction(instruction, cpu);
+	overture_execute_instruction(cpu->current_instruction, cpu);
 
 	cpu_diagnostics(cpu);
 }
@@ -301,7 +299,7 @@ int main() {
 		.output = 0,
 		.output_enable = true,
 
-		.current_instruction = 0
+		.current_instruction.instruction = 0
 	};
 
 	overture_start_clock(&cpu, rom);