First working version! Basic hello world kernel.

boot/boot.s

@@ -0,0 +1,40 @@
+# Declare constants for creating a multiboot header
+.set ALIGN,	1<<0
+.set MEMINFO,	1<<1
+.set FLAGS,	ALIGN | MEMINFO
+.set MAGIC,	0x1BADB002
+.set CHECKSUM,	-(MAGIC + FLAGS)
+
+# Declare multiboot header
+.section .multiboot
+.align 4
+.long MAGIC
+.long FLAGS
+.long CHECKSUM
+
+# Create a 16 KiB stack
+.section .bootstrap_stack
+stack_bottom:
+.skip 16384
+stack_top:
+
+# Kernel entry point
+.section .text
+.global _start
+.type _start, @function
+_start:
+	# We are now in kernel mode!
+	# Setup the stack
+	movl $stack_top, %esp
+	
+	# Call the main kernel function
+	call brados_main
+	
+	# In case the function returns, enter an endless loop
+	cli
+	hlt
+.Lhang:
+	jmp .Lhang
+
+# Set the size of the _start symbol for debugging
+.size _start, . - _start

kernel/brados.c

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+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+// Make sure we are using the right compiler
+#if defined(__linux__)
+#error "You are using the wrong compiler."
+#endif
+
+// Color constants
+enum vgaColor
+{
+	COLOR_BLACK = 0,
+	COLOR_BLUE = 1,
+	COLOR_GREEN = 2,
+	COLOR_CYAN = 3,
+	COLOR_RED = 4,
+	COLOR_MAGENTA = 5,
+	COLOR_BROWN = 6,
+	COLOR_LIGHT_GREY = 7,
+	COLOR_DARK_GREY = 8,
+	COLOR_LIGHT_BLUE = 9,
+	COLOR_LIGHT_GREEN = 10,
+	COLOR_LIGHT_CYAN = 11,
+	COLOR_LIGHT_RED = 12,
+	COLOR_LIGHT_MAGENTA = 13,
+	COLOR_LIGHT_BROWN = 14,
+	COLOR_WHITE = 15
+};
+
+uint8_t makeColor(enum vgaColor fg, enum vgaColor bg)
+{
+	return fg | bg << 4;
+}
+
+uint16_t makeVGAEntry(char c, uint8_t color)
+{
+	uint16_t c16 = c;
+	uint16_t color16 = color;
+	return c16 | color16 << 8;
+}
+
+size_t strlen(const char *str)
+{
+	size_t len = 0;
+	while (str[len] != 0)
+		len++;
+	return len;
+}
+
+static const size_t VGA_WIDTH = 80;
+static const size_t VGA_HEIGHT = 24;
+
+size_t terminalRow, terminalCol;
+uint8_t terminalColor;
+uint16_t *terminalBuf;
+
+void terminalInit()
+{
+	terminalRow = terminalCol = 0;
+	terminalColor = makeColor(COLOR_LIGHT_GREY, COLOR_BLACK);
+	terminalBuf = (uint16_t *) 0xB8000;
+	
+	for (size_t x = 0; x < VGA_WIDTH; x++) {
+		for (size_t y = 0; y < VGA_HEIGHT; y++) {
+			const size_t index = y * VGA_WIDTH + x;
+			terminalBuf[index] = makeVGAEntry(' ', terminalColor);
+		}
+	}
+}
+
+void terminalSetColor(uint8_t color)
+{
+	terminalColor = color;
+}
+
+void terminalPutEntryAt(char c, uint8_t color, size_t x, size_t y)
+{
+	const size_t index = y * VGA_WIDTH + x;
+	terminalBuf[index] = makeVGAEntry(c, color);
+}
+
+void terminalPutChar(char c)
+{
+	terminalPutEntryAt(c, terminalColor, terminalCol, terminalRow);
+	if (terminalCol++ == VGA_WIDTH) {
+		terminalCol = 0;
+		if (terminalRow++ == VGA_HEIGHT)
+			terminalRow = 0;
+	}
+}
+
+void terminalWriteString(const char *data)
+{
+	size_t dataLen = strlen(data);
+	for (size_t i = 0; i < dataLen; i++)
+		terminalPutChar(data[i]);
+}
+
+void brados_main()
+{
+	terminalInit();
+	terminalWriteString("Welcome to BRaDOS!\n");
+}

linker.ld

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+/* Kernel entry point */
+ENTRY(_start)
+
+/* Define the locations of the object file sections */
+SECTIONS
+{
+	/* Sections begin at 1 MiB */
+	. = 1M;
+	
+	/* The multiboot header needs to come first, followed by program code */
+	.text BLOCK(4K) : ALIGN(4K)
+	{
+		*(.multiboot)
+		*(.text)
+	}
+	
+	/* Read-only data */
+	.rodata BLOCK(4K) : ALIGN(4K)
+	{
+		*(.rodata)
+	}
+	
+	/* Initialized read-write data */
+	.data BLOCK(4K) : ALIGN(4K)
+	{
+		*(.data)
+	}
+	
+	/* Uninitialized read-write data and stack */
+	.bss BLOCK(4K) : ALIGN(4K)
+	{
+		*(COMMON)
+		*(.bss)
+		*(.bootstrap_stack)
+	}
+	
+	/* Add other sections here */
+}