call c 32bit code

Makefile

@@ -1,6 +1,7 @@
 all:
 	as -o boot.o boot.s
-	ld -o boot.bin --oformat binary -e init boot.o
+	gcc -nostdinc -nostdlib -ffreestanding -c main.c -o theObjectCode.o
+	ld -o boot.bin -Ttext 0x0 --oformat binary -e init boot.o theObjectCode.o
 
 run:
 	qemu-system-x86_64 boot.bin

boot.s

@@ -1,15 +1,58 @@
 .code16 # tell the assembler that we're using 16 bit mode
+.include "gdt.s"
+
+.section .text
+
 .global init # makes our label "init" available to the outside
+.globl mainentry
 
 init: # this is the beginning of our binary later.
+
   #jmp init # jump to "init"
   #ljmpw $0xFFFF, $0 # jumps to the "reset vector", doing a reboot
-  jmp printhelloworld
+  call printhelloworld
+
+  jmp switchto32bit
+
+switchto32bit:
+  # Load Global Descriptor Table
+  lgdt gdt32ptr
+
+     # Load Control Register CR0
+     # Enable Protected Mode (Bit0 in CR0)
+     # Store Control Register CR0
+     movl %cr0, %eax
+     orl $(1<<0), %eax
+     movl %eax, %cr0
+
+     # Setup all Segment Registers
+     # and reload Code Segment, Instruction Pointer
+     movw $0x0010, %ax
+     movw %ax, %ds
+     movw %ax, %es
+     movw %ax, %fs
+     movw %ax, %gs
+     movw %ax, %ss
+     ljmp $0x0008, $JumpToPM
+
+.code32
+JumpToPM:
+   call mainentry
+
+   ljmpw $0xFFFF, $0 # jumps to the "reset vector", doing a reboot
 
 printhelloworld:
-  mov $0xe37, %ax
+  mov $0x0e, %ah # function mode -> display char
+
+  mov $'S', %al
   int $0x10 # video bios interrupt
-  mov $0xe38, %ax
+  mov $'T', %al
+  int $0x10 # video bios interrupt
+  mov $'A', %al
+  int $0x10 # video bios interrupt
+  mov $'R', %al
+  int $0x10 # video bios interrupt
+  mov $'T', %al
   int $0x10 # video bios interrupt
 
 .fill 510-(.-init), 1, 0 # add zeroes to make it 510 bytes long

gdt.s

@@ -0,0 +1,27 @@
+#GDT
+
+gdt_start:
+INITSEG = 0x0050
+INITOFF = 0x0000
+
+.section .rodata
+
+gdt:
+   # Null Descriptor
+   .word 0x0000, 0x0000
+   .byte 0x00, 0b00000000, 0b00000000, 0x00
+   # Code Descriptor
+   .word 0xffff, (INITSEG * 0x10 + INITOFF)
+   .byte 0x00, 0b10011010, 0b11001111, 0x00
+   # Data Descriptor
+   .word 0xffff, (INITSEG * 0x10 + INITOFF)
+   .byte 0x00, 0b10010010, 0b11001111, 0x00
+gdt_end:
+
+gdt32ptr:
+   .word (gdt_end - gdt - 1)
+   .long (gdt + INITSEG * 0x10 + INITOFF)
+
+gdt64ptr:
+   .word (gdt_end - gdt - 1)
+   .quad (gdt + INITSEG * 0x10 + INITOFF)

main.c

@@ -0,0 +1,18 @@
+void mainentry() {
+    int x = 5;
+
+    char* video_memory = (char*) 0xb8000;
+    *video_memory = 'X';
+
+    // Here , the star following the type means that this is not a variable to hold
+    // a char (i.e. a single byte ) but a pointer to the ADDRESS of a char ,
+    // which , being an address , will actually require the allocation of at least
+    // 32 bits .
+    char * video_address = (char*)(0xb8000);
+    // If we ’d like to store a character at the address pointed to , we make the
+    // assignment with a star - prefixed pointer variable . This is known as
+    // dereferencing a pointer , because we are not changing the address held by
+    // the pointer variable but the contents of that address .
+    *video_address = 'x';
+    // Just to emphasise the purpose of the star , an ommision of it , such as:
+}