Initial setup, Basic helloworld kernel from wiki.osdev.org bare bones

src/boot.s

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+/*
+ * Multiboot 
+ */
+.set ALIGN,    1<<0             /* align loaded modules on page boundaries */
+.set MEMINFO,  1<<1             /* provide memory map */
+.set FLAGS,    ALIGN | MEMINFO  /* this is the Multiboot 'flag' field */
+.set MAGIC,    0x1BADB002       /* 'magic number' lets bootloader find the header */
+.set CHECKSUM, -(MAGIC + FLAGS) /* checksum of above, to prove we are multiboot */
+
+.section .multiboot
+.align 4
+.long MAGIC
+.long FLAGS
+.long CHECKSUM
+ 
+
+.section .bss
+.align 16
+stack_bottom:
+.skip 16384 # 16 KiB
+stack_top:
+ 
+
+.section .text
+.global _start
+.type _start, @function
+_start:
+
+ 
+
+	mov $stack_top, %esp
+ 
+
+
+	call kernel_main
+
+	cli
+1:	hlt
+	jmp 1b
+ 
+
+.size _start, . - _start

src/kernel.c

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+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include "vga_colors.h"
+#include "string.h"
+static const size_t VGA_WIDTH = 80;
+static const size_t VGA_HEIGHT = 25;
+
+size_t kterm_row;
+size_t kterm_column;
+uint8_t kterm_color;
+uint16_t* kterm_buffer;
+
+
+static inline uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg) {
+    return fg | bg << 4;
+}
+
+static inline uint16_t vga_entry (unsigned char uc, uint8_t color) {
+    return (uint16_t) uc | (uint16_t) color << 8;
+}
+
+void init_kterm () { 
+    kterm_row = 0;
+    kterm_column = 0;
+    kterm_color = vga_entry_color ( VGA_COLOR_LIGHT_GREY , VGA_COLOR_BLACK);
+    kterm_buffer = (uint16_t*) 0xB8000;
+    for (size_t y = 0; y < VGA_HEIGHT; y++ ){
+        for( size_t x = 0; x < VGA_WIDTH; x++){
+            const size_t index = y * VGA_WIDTH + x;
+            kterm_buffer[index] = vga_entry(' ', kterm_color);
+
+        }
+    }
+}
+
+
+void kterm_setcolor(uint8_t color){
+    kterm_color = color;
+}
+
+void kterm_putat (char c, uint8_t color, size_t x, size_t y ) {
+    const size_t index = y * VGA_WIDTH + x;
+
+    kterm_buffer[index] = vga_entry(c, color);
+
+}
+
+void kterm_put (char c) {
+    kterm_putat ( c, kterm_color, kterm_column, kterm_row);
+    if(kterm_column++ == VGA_WIDTH ){
+        kterm_column = 0;
+        if(kterm_row++ == VGA_HEIGHT)
+            kterm_row = 0;
+    }
+
+}
+
+void kterm_write(const char* data, size_t size) {
+    for(size_t i = 0; i < size; i++){
+        kterm_put(data[i]);
+    }
+}   
+
+
+void kterm_writestring(const char* data ){
+    kterm_write(data, strlen(data));
+}
+
+
+void kernel_main (void) {
+    /** initialize terminal interface */ 
+    init_kterm();
+
+    kterm_writestring("K: Hello world!\n");
+
+    
+}   

src/linker.ld

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+/* The bootloader will look at this image and start execution at the symbol
+   designated as the entry point. */
+ENTRY(_start)
+ 
+/* Tell where the various sections of the object files will be put in the final
+   kernel image. */
+SECTIONS
+{
+	/* Begin putting sections at 1 MiB, a conventional place for kernels to be
+	   loaded at by the bootloader. */
+	. = 1M;
+ 
+	/* First put the multiboot header, as it is required to be put very early
+	   early in the image or the bootloader won't recognize the file format.
+	   Next we'll put the .text section. */
+	.text BLOCK(4K) : ALIGN(4K)
+	{
+		*(.multiboot)
+		*(.text)
+	}
+ 
+	/* Read-only data. */
+	.rodata BLOCK(4K) : ALIGN(4K)
+	{
+		*(.rodata)
+	}
+ 
+	/* Read-write data (initialized) */
+	.data BLOCK(4K) : ALIGN(4K)
+	{
+		*(.data)
+	}
+ 
+	/* Read-write data (uninitialized) and stack */
+	.bss BLOCK(4K) : ALIGN(4K)
+	{
+		*(COMMON)
+		*(.bss)
+	}
+ 
+	/* 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. */
+}

src/string.h

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+#include <stddef.h>
+size_t strlen(const char* str){
+    size_t len = 0;
+    while(str[len]){
+        len++;
+    }
+    return len;
+}

src/vga_colors.h

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+enum vga_color {
+	VGA_COLOR_BLACK = 0,
+	VGA_COLOR_BLUE = 1,
+	VGA_COLOR_GREEN = 2,
+	VGA_COLOR_CYAN = 3,
+	VGA_COLOR_RED = 4,
+	VGA_COLOR_MAGENTA = 5,
+	VGA_COLOR_BROWN = 6,
+	VGA_COLOR_LIGHT_GREY = 7,
+	VGA_COLOR_DARK_GREY = 8,
+	VGA_COLOR_LIGHT_BLUE = 9,
+	VGA_COLOR_LIGHT_GREEN = 10,
+	VGA_COLOR_LIGHT_CYAN = 11,
+	VGA_COLOR_LIGHT_RED = 12,
+	VGA_COLOR_LIGHT_MAGENTA = 13,
+	VGA_COLOR_LIGHT_BROWN = 14,
+	VGA_COLOR_WHITE = 15,
+};