Add mmu

hal/paging.go

@@ -1,14 +1,25 @@
 package hal
 
+const PageSize = 1024
+
+var defaultRegister uint16 = 0
+
 type Page struct {
-	Referenced bool
-	PageNumber uint16
+	Referenced      bool
+	RegisterAddress *uint16
 }
 
-type PageTable []*Page
+// PageTable uses key as page number
+type PageTable map[uint16]*Page
 
 type Address uint16
 
+type MMU struct {
+	PageTable   *PageTable
+	LoadedPages []*Page
+	Registers   []float64 // 4k registers
+}
+
 func (address Address) PageNumber() uint16 {
 	return uint16(address) >> 10
 }
@@ -17,14 +28,55 @@ func (address Address) Offset() uint16 {
 	return uint16(address) & 0x03FF
 }
 
-func (table *PageTable) Find(address Address) {
+func (table PageTable) Find(address Address) {
 	// 10 bit for one register in a page
 	// 6 bit to address the page
 	// 000010 | 0000000011
 	// -> Page 2, offset 3
 
 	// Find the page with that number
+	page := table[address.PageNumber()]
+
+	// Page not loaded in "physical" register
+	if page.RegisterAddress == nil {
+		// Page Error
+		// TODO: Logging
+
+	}
+}
+
+func (mmu *MMU) Load(page *Page) {
+	if len(mmu.LoadedPages) >= 4 {
+		for index, loadedPage := range mmu.LoadedPages {
+			if loadedPage.Referenced {
+				continue
+			}
+			loadedPage.RegisterAddress = nil
+			mmu.LoadedPages = append(mmu.LoadedPages[:index], mmu.LoadedPages[index+1:]...)
+			break
+		}
+
+		// Everything is referenced, we need to remove one
+		if len(mmu.LoadedPages) == 4 {
+			mmu.LoadedPages = mmu.LoadedPages[:2]
+		}
+	}
+
+	page.RegisterAddress = mmu.findEmptyRegisterAddress()
+
+	mmu.LoadedPages = append(mmu.LoadedPages, page)
+}
+
+func (mmu *MMU) findEmptyRegisterAddress() *uint16 {
+	var i uint16
+	for i = 0; i < uint16(len(mmu.Registers)/PageSize); i++ {
+		for _, loadedPage := range mmu.LoadedPages {
+			if *loadedPage.RegisterAddress == i {
+				continue
+			}
+			return &i
+		}
+	}
 
-	// Check if it is already loaded
-	// Load it if not
+	return &defaultRegister
 }