[Stage1] Fix linker to force main() at base address

[Stage1] Add basic write-to-screen utilities for VGA mode 3

stage1/CMakeLists.txt

@@ -1,4 +1,4 @@
-add_executable(stage1 stage1.c)
+add_executable(stage1 stage1.c screen.c)
 target_compile_options(stage1 PUBLIC -ffreestanding)
 set_target_properties(stage1 PROPERTIES LINK_DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/linker.ld)
 target_link_options(stage1 PRIVATE -T${CMAKE_CURRENT_SOURCE_DIR}/linker.ld)

stage1/include/screen.h

@@ -0,0 +1,7 @@
+#ifndef __OS3_ST1_SCREEN_H
+#define __OS3_ST1_SCREEN_H
+
+extern void write_string(const char*, int);
+
+
+#endif

stage1/linker.ld

@@ -3,6 +3,9 @@ SECTIONS
 {
 	. = 0x10000;
 	.text : AT(0x10000){
+		stage1/CMakeFiles/stage1.dir/stage1.c.obj(.text);
+	}
+	.othertext : {
 		*(.text);
 	}
 	.data : {

stage1/screen.c

@@ -0,0 +1,39 @@
+#include "include/screen.h"
+#include "../os3_common/datatypes.h"
+#include "../os3_common/packing.h"
+
+int x = 0;
+int y = 0;
+
+void write_character(char c) {
+	os_u16 color_byte = 0x0A;
+	os_u16* vram = (os_u16*) 0xB8000;
+	os_u16* cursor_pos = vram + ((y * 80) + x);
+	*cursor_pos = c | (color_byte << 8);
+}
+
+void write_string(const char* str, int max_len) {
+	int i = 0;
+	do {
+		char c = *str;
+		if (c == 0) {
+			return;
+		}
+
+		if (c == '\n') {
+			y = (y + 1) % 25;
+			x = 0;
+			str++;
+			continue;
+		}
+
+		write_character(c);
+		x++;
+		if (x >= 80) {
+			y = (y + 1) % 25;
+			x = 0;
+		}
+		str++;
+		i++;
+	} while(i < max_len || max_len == 0);
+}

stage1/stage1.c

@@ -2,10 +2,16 @@
 // Created by barba on 06/10/2022.
 //
 
+#include "../os3_common/datatypes.h"
+#include "../os3_common/packing.h"
+#include "include/screen.h"
+
 int main() {
+	write_string("OS3 Booted into stage 1!\n", 0);
+	write_string("Looking for stage 2....", 0);
 	while(1) {
 
 	};
 
-    return 0;
-}
+	return 0;
+}