First step for memory management

core/main.c

@@ -4,6 +4,7 @@
 #include "interrupt.h"
 #include "io.h"
 #include "irq.h"
+#include "mem.h"
 #include "multiboot.h"
 #include "pit.h"
 #include "types.h"
@@ -19,6 +20,7 @@ void cpuid(int code, uint32_t *a, uint32_t *d)
 // Multiboot information available here : https://www.gnu.org/software/grub/manual/multiboot/multiboot.html#kernel_002ec
 void kmain(unsigned long magic, unsigned long addr)
 {
+    unsigned long upper_mem = 0;
     initVGA(BLACK, GREEN);
     printString("Setting up IDT\n");
     gdtSetup();
@@ -35,6 +37,7 @@ void kmain(unsigned long magic, unsigned long addr)
             printString("KB mem_upper = ");
             printInt((unsigned)mbi->mem_upper);
             printString("KB\n");
+            upper_mem = mbi->mem_upper;
         }
 
         /* Is boot_device valid? */
@@ -52,6 +55,12 @@ void kmain(unsigned long magic, unsigned long addr)
         }
     }
 
+    if(upper_mem == 0){
+        printString("Cannot get upper phy mem bound. Using default value 32MB\n");
+        upper_mem = 32 * 1024;
+    }
+    memInit(upper_mem);
+
     printString("Setting up IRQ handlers\n");
     irqSetRoutine(IRQ_KEYBOARD, keyboard_handler);
     irqSetRoutine(IRQ_TIMER, timer_handler);

core/mem.c

@@ -0,0 +1,26 @@
+#include "mem.h"
+#include "vga.h"
+
+struct mem_desc *page_desc = (struct mem_desc *)&__ld_kernel_end;
+
+int memInit(unsigned long upper_mem)
+{
+	printString("Free Mem going from ");
+	printInt((unsigned long)&__ld_kernel_end);
+	printString(" to ");
+	printInt(upper_mem * 1024);
+	printString("\n");
+	unsigned long memdesc_end =
+	    (unsigned long)page_desc +
+	    ((upper_mem) / (PAGE_SIZE / 1024)) * sizeof(struct mem_desc);
+	uint lastUsed = (memdesc_end >> PAGE_SHIFT) + 1;
+	for (uint i = 0; i < (upper_mem / (PAGE_SIZE / 1024)); i++) {
+		struct mem_desc *mem = &page_desc[i];
+		if (i < lastUsed)
+			mem->ref = 1;
+		else
+			mem->ref = 0;
+		mem->phy_addr = i * PAGE_SIZE;
+	}
+	return 0;
+}

core/mem.h

@@ -0,0 +1,18 @@
+#pragma once
+#include "types.h"
+
+
+#define PAGE_SHIFT 12
+#define PAGE_SIZE (1 << PAGE_SHIFT)
+
+// Defined in linker.ld script
+extern uint32_t __ld_kernel_begin;
+extern uint32_t __ld_kernel_end;
+
+struct mem_desc{
+	unsigned long phy_addr;
+	unsigned long ref;
+};
+
+
+int memInit(unsigned long upper_mem);

linker.ld

@@ -16,6 +16,7 @@ SECTIONS
 	.text BLOCK(4K) : ALIGN(4K)
 	{
 		*(.multiboot)
+      __ld_kernel_begin = .;
 		*(.text)
 	}
 
@@ -38,6 +39,8 @@ SECTIONS
 		*(.bss)
 	}
 
+   __ld_kernel_end = .;
+
 	/* The compiler may produce other sections, by default it will put them in
 	   a segment with the same name. Simply add stuff here as needed. */
 }