Continued adding next instructions

src/machine/cpu.odin

@@ -30,10 +30,19 @@ cycle :: proc(s: ^System) {
     case 0x0:
         switch opcode {
             case 0x00E0: op_cls(s)
+            case 0x00EE: op_ret(s, opcode)
         }
     case 0x1: op_jp_add(s, opcode)
+    case 0x2: op_call_add(s, opcode)
+    case 0x3: op_skip_on_condition(s, opcode, true)
+    case 0x4: op_skip_on_condition(s, opcode, false)
+    case 0x5: op_skip_reg_condition(s, opcode, true)
     case 0x6: op_ld(s, opcode)
     case 0x7: op_add(s, opcode)
+
+	// todo 0x8 with last bit for ALU
+
+	case 0x9: op_skip_reg_condition(s, opcode, false)
     case 0xA: op_set(s, opcode)
     case 0xD: op_draw(s, opcode)
     }
@@ -54,12 +63,68 @@ op_cls :: proc(s: ^System) {
     s.display = {}
 }
 
+// RET: Return from a subroutine.
+op_ret :: proc(s: ^System, opcode: u16) {
+	// decrement sp to the previous used slot
+	// current sp will always be empty
+	s.sp -= 1
+
+	// restore the pc from where we called from
+	s.pc = s.stack[s.sp]
+}
+
 // JP addr: Jump to location nnn
 op_jp_add :: proc(s: ^System, opcode: u16) {
     addr := (opcode & 0x0FFF)
     s.pc = addr
 }
 
+// Call addr: Call subroutine at nnn.
+op_call_add :: proc(s: ^System, opcode: u16) {
+    addr := (opcode & 0x0FFF)
+
+	// Save current pc to stack for return location
+	s.stack[s.sp] = s.pc
+	// advance sp for next free slot
+	s.sp += 1
+
+	s.pc = addr
+}
+
+// SE: Skip next instruction if Vx = kk.
+// SNE: Skip next instruction if Vx != kk.
+op_skip_on_condition :: proc(s: ^System, opcode: u16, condition: bool) {
+    // Extract the register indices to get values
+    vx_index := (opcode & 0x0F00) >> 8
+	register_val := s.v[vx_index]
+
+	// Extract value to compare
+    op_val := u8(opcode & 0x00FF)
+
+	if (register_val == op_val) == condition {
+		s.pc += 2
+	}
+}
+
+// SE Vx, Vy:  Skip next instruction if Vx = Vy
+// SNE Vx, Vy: Skip next instruction if Vx != Vy.
+op_skip_reg_condition :: proc(s: ^System, opcode: u16, condition: bool) {
+    // Extract the register indices
+    vx_index := (opcode & 0x0F00) >> 8
+    vy_index := (opcode & 0x00F0) >> 4
+
+    // Read the actual value from the registers.
+    rx := s.v[vx_index]
+    ry := s.v[vy_index]
+
+	if (rx == ry) == condition {
+		s.pc += 2
+	}
+}
+
+
+// continue Bnnn
+
 // LD: Load value kk into x register
 op_ld :: proc(s: ^System, opcode: u16) {
     register := (opcode & 0x0F00) >> 8
@@ -83,34 +148,44 @@ op_set :: proc(s: ^System, opcode: u16) {
 
 // DRAW: Display n-byte sprite starting at memory location I at (Vx, Vy), set VF = collision.
 op_draw :: proc(s: ^System, opcode: u16) {
-    x_register := (opcode & 0x0F00) >> 8
-    y_register := (opcode & 0x00F0) >> 4
+    // Extract the register indices that hold the sprite's X and Y screen position
+    vx_index := (opcode & 0x0F00) >> 8
+    vy_index := (opcode & 0x00F0) >> 4
 
-    // Handle wrapping
-    x_coord := (s.v[x_register] & 0x03F)
-    y_coord := (s.v[y_register] & 0x1F)
+    // Read the actual coordinates from those registers.
+    // Wrap to screen bounds: 64 wide (0x3F mask), 32 tall (0x1F mask)
+    screen_x := s.v[vx_index] & 0x3F
+    screen_y := s.v[vy_index] & 0x1F
 
+    // VF is the collision flag — clear it before drawing
     s.v[0xF] = 0
 
-    n_bytes := (opcode & 0x000F)
+    // Number of rows in the sprite (1 byte per row, each byte = 8 pixels)
+    sprite_height := opcode & 0x000F
 
-    for row in 0..<n_bytes {
-        sprite_row := s.memory[s.i + row]
-        // log.debugf("sr: 0x%X", sprite_row)
+    for row in 0..<sprite_height {
+        // Each byte in memory is one row of 8 pixels, MSB is the leftmost pixel
+        sprite_row_bits := s.memory[s.i + row]
 
         for col in 0..<8 {
-            mask := u8(0x80) >> u8(col)
+            // Shift a 1 from the MSB right to isolate the bit at this column
+            bit_mask := u8(0x80) >> u8(col)
 
-            if sprite_row & mask != 0 {
-                // this bit is set Xor the pixel
-                y_pos := ((y_coord + u8(row)) % 32)
-                x_pos := ((x_coord + u8(col)) % 64)
+            // Only draw if this bit in the sprite is set
+            if sprite_row_bits & bit_mask != 0 {
+                // Wrap the draw position if the sprite goes off-screen
+                pixel_x := (screen_x + u8(col)) % 64
+                pixel_y := (screen_y + u8(row)) % 32
 
-                pixel := &s.display[y_pos][x_pos]
+                pixel := &s.display[pixel_y][pixel_x]
+
+                // CHIP-8 XORs pixels — if we're about to unset a lit pixel, that's a collision
                 if pixel^ == 1 {
                     s.v[0xF] = 1
                 }
-                pixel^ = pixel^ ~ 1
+
+                // XOR the pixel: toggles it on if off, off if on
+                pixel^ ~= 1
             }
         }
     }