Branch sync and push, reformated and made the OS look more "official"

kernel/arch/i386/boot.s

@@ -0,0 +1,72 @@
+.set ALIGN, 1<<0    /* align loaded modules on page boundaries */
+.set MEMINFO, 1<<1  /* provide memory map */
+.set FLAGS, ALIGN | MEMINFO /* Multiboot flag */
+.set MAGIC, 0x1BADB002 /* Magic number lets bootloader find the header */
+.set CHECKSUM, -(MAGIC + FLAGS) /* checksum of above, to prove we are multiboot (sanity check I think) */
+
+/* so this basically declares the multiboot header that defines this program as a kernel,
+   and the following values are in the multiboot standard. The bootloader will look for the
+   first 8KiB of the kernel file, aligned with the 32-bit boundary, this means that it won't
+   work with 64bit until I keep on learning or I create my own bootloader (without multiboot)
+*/
+.section .multiboot
+.align 4
+.long MAGIC
+.long FLAGS
+.long CHECKSUM
+
+/* This is where my knowledge is not high enough but here is the summary:
+        - esp is NOT in the multiboot standard so the kernel has to provide it.
+          this basically means that it doesn't define the value of the stack register
+          (whatever that means)
+        - This allocates some room for a stack by creating a symbol at the bottom of it
+          then allocate 16 KiB and then creating a symbol on top.
+        - The stack grows downwards on x86.
+
+And many more, apparently it's re-aligned to System-V ABI standard as well? Not sure, but hopefully I can
+continue learning
+*/
+.section .bss
+.align 16
+stack_bottom:
+.skip 16384 # 16 KiB
+stack_top:
+
+/* linker specifies a _start as the entry point to the kernel and the bootloader will jump to the position
+however it doesn't make any sense to return to this after the bootloader is gone
+*/
+.section .text
+.global _start
+.type _start, @function
+_start:
+        /* We are now in... no printf and other things, kernel has FULL CONTROL of the cpu. No security also lol */
+
+        /* To set up a stack, this sets the esp to register to the point of the stack because it grows downwards on
+        on x86 systems. this is done in assembly because languages like C can't function without a stack */
+        mov $stack_top, %esp
+
+        /* so we need to initialize some critical processor states before the high-level kernel is entered.
+        interestingly a lot of things are done at the boot level, we should load GDT, enable paging, and add runtime
+        support (for C++). Crazy!
+        */
+
+        /* time to enter the kernel,
+        according to the guide, this is a well defined call as the stack was originally 16-byte aligned, we pushed a 
+        multiple of 16-bytes to the stack (since 0 bytes were pushed so far), so alignment has been perserved.
+        */
+        call kernel_main
+
+        /* If there is nothing else to do, the computer will be in an infinite loop 
+                - Disable interrupts with cli (they wil be disabled by the bootloader)
+                - Wait for next interrupt to arrive with halt instruction (hlt)
+                - Jump to hlt instruction if it ever wakes up due to a non-maskable
+                  interrupt occuring or due to system management mode.
+        */
+        cli
+1:      hlt
+        jmp 1b
+
+/* set the size of _start symbol to the current location minus its start.
+Useful for debugging or if I implement call tracing
+*/
+.size _start, . - _start

kernel/arch/i386/linker.ld

@@ -0,0 +1,25 @@
+ENTRY(_start) /* Entry point symbol */
+
+SECTIONS {
+        /* Load address in memory: 2MB for kernel */
+        . = 2M;
+
+        /* Multiboot header must be in the first 8KB of the file and 32-bit aligned */
+        .text BLOCK(4K) : ALIGN(4K) {
+                *(.multiboot)
+                *(.text)
+        }
+
+        .rodata BLOCK(4K) : ALIGN(4K) {
+                *(.rodata)
+        }
+
+        .data BLOCK(4K) : ALIGN(4K) {
+                *(.data)
+        }
+
+        .bss BLOCK(4K) : ALIGN(4K) {
+                *(COMMON)
+                *(.bss)
+        }
+}

kernel/drivers/video/vga.c

@@ -0,0 +1,61 @@
+#include "../../include/video/vga.h"
+#include <string.h>
+
+size_t term_row;
+size_t term_col;
+uint8_t term_color;
+uint16_t* term_buf = (uint16_t*)VGA_MEMORY;
+
+void
+term_init(void)
+{
+        term_row = 0;
+        term_col = 0;
+        term_color = vga_entry_color(VGA_COLOR_WHITE, VGA_COLOR_BLACK);
+
+        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;
+                        term_buf[index] = vga_entry(' ', term_color);
+                }
+        }
+}
+
+void
+term_setcolor(uint8_t color)
+{
+        term_color = color;
+}
+
+void
+term_putentryat(char c, uint8_t color, size_t x, size_t y)
+{
+         const size_t index = y * VGA_HEIGHT + x;
+         term_buf[index] = vga_entry(c, color);
+}
+
+void
+term_putchar(char c)
+{
+        term_putentryat(c, term_color, term_row);
+        if (++term_col == VGA_WIDTH) {
+                term_col = 0;
+                if (++term_row == VGA_HEIGHT) {
+                        term_row = 0;
+                }
+        }
+}
+
+void
+term_write(const char* data, size_t size)
+{
+        for (size_t i = 0; i < size; i++) {
+                term_putchar(data[i];
+        }
+}
+
+void
+term_writestr(const char* data)
+{
+        term_write(data, strlen(data));
+}

kernel/include/drivers/video/vga.h

@@ -0,0 +1,51 @@
+#ifndef VGA_H
+#define VGA_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+/* hardware text mode const */
+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,
+}
+
+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;
+}
+
+/* using mode 3 of VGA 80x25 */
+#define VGA_WIDTH 80
+#define VGA_HEIGHT 25
+#define VGA_MEMORY 0xB8000 /* VGA memory location */
+
+void term init(void);
+void term_setcolor(uint8_t color);
+void term_putentryat(char c, uint8_t color, size_t x, size_t y);
+void term_putchar(char c);
+void term_write(const char* data, size_t size);
+void term_writestr(const char* data);
+
+#endif #VGA_H

kernel/include/lib/string.h

@@ -0,0 +1,8 @@
+#ifndef STRING_H
+#define STRING_H
+
+#include <stddef.h>
+
+size_t strlen(const char* str);
+
+#endif // STRING_H

kernel/init/kernel.c

@@ -0,0 +1,10 @@
+#include "../include/drivers/video/vga.h"
+#include <string.h>
+
+void
+kernel_main(void)
+{
+        term_init();
+
+        term_writestr("Hello World!\n");
+}

kernel/lib/string.c

@@ -0,0 +1,11 @@
+#include "../include/lib/string.h"
+#include <stddef.h>
+#include <stdint.h>
+
+size_t
+strlen(const char* str)
+{
+        size_t len = 0;
+        while (str[len]) len++;
+        return len;
+}