From 1230afc0aaef07b03f09d3725f459df07b982de8 Mon Sep 17 00:00:00 2001
From: Andreas Baumann <mail@andreasbaumann.cc>
Date: Sat, 8 Jul 2017 12:43:53 +0200
Subject: added dedicated kernel entry to avoid address reordering under
 compiler optimization affect the entry poin 0x8800 of kernel_main (now
 kernel_entry)

---
 doc/LINKS.TODO          |  3 +++
 src/Makefile            |  8 ++++++--
 src/README              |  1 +
 src/drivers/video/vga.c |  2 +-
 src/kernel/entry.c      | 16 ++++++++++++++++
 5 files changed, 27 insertions(+), 3 deletions(-)
 create mode 100644 src/kernel/entry.c

diff --git a/doc/LINKS.TODO b/doc/LINKS.TODO
index 9c26e69..092c620 100644
--- a/doc/LINKS.TODO
+++ b/doc/LINKS.TODO
@@ -96,3 +96,6 @@ http://bos.asmhackers.net/docs/vga_without_bios/
 GUI:
 https://github.com/vurtun/nuklear
 http://webserver2.tecgraf.puc-rio.br/iup/
+
+linker:
+https://stackoverflow.com/questions/26143123/how-do-i-ensure-the-entry-function-at-a-fixed-address-of-bare-metal-arm
diff --git a/src/Makefile b/src/Makefile
index 876376d..62909a1 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1,6 +1,6 @@
 CC := gcc
 INCLUDES = -I. -Ilibc -Ihardware -Idrivers -Idrivers/hdi -Idrivers/hdi/ps2 -Idrivers/video -Ikernel -Igui
-CFLAGS := -std=c99 -m32 -ffreestanding -O0 -g -Wall -Werror $(INCLUDES)
+CFLAGS := -std=c99 -m32 -ffreestanding -O0 -g -Werror $(INCLUDES)
 LD := ld
 NASMFLAGS := -f elf32
 NASM := nasm
@@ -33,8 +33,9 @@ kernel.bin: kernel.elf
 kernel.sym: kernel.elf
 	$(OBJCOPY) --only-keep-debug kernel.elf kernel.sym
 
-kernel.elf: kernel/kernel.o kernel/kernel_asm.o kernel/console.o kernel/vgatext.o kernel/serial.o kernel/memorymanagement.o kernel/tasks.o hardware/port.o hardware/port_asm.o hardware/interrupts.o hardware/interrupts_asm.o hardware/pci.o drivers/driver.o drivers/hdi/ps2/keyboard.o drivers/hdi/ps2/mouse.o drivers/video/vga.o drivers/video/vga_font.o gui/widget.o gui/composite_widget.o gui/window.o gui/desktop.o gui/text_widget.o libc/string.o libc/stdlib.o libc/stdio.o libc/setjmp.o
+kernel.elf: kernel/entry.o kernel/kernel.o kernel/kernel_asm.o kernel/console.o kernel/vgatext.o kernel/serial.o kernel/memorymanagement.o kernel/tasks.o hardware/port.o hardware/port_asm.o hardware/interrupts.o hardware/interrupts_asm.o hardware/pci.o drivers/driver.o drivers/hdi/ps2/keyboard.o drivers/hdi/ps2/mouse.o drivers/video/vga.o drivers/video/vga_font.o gui/widget.o gui/composite_widget.o gui/window.o gui/desktop.o gui/text_widget.o libc/string.o libc/stdlib.o libc/stdio.o libc/setjmp.o
 	$(LD) -o kernel.elf -N -n -Ttext 0x8800 --oformat elf32-i386 \
+		kernel/entry.o  \
 		kernel/kernel.o kernel/kernel_asm.o \
 		kernel/console.o kernel/vgatext.o kernel/serial.o \
 		kernel/memorymanagement.o kernel/tasks.o \
@@ -51,6 +52,9 @@ kernel.elf: kernel/kernel.o kernel/kernel_asm.o kernel/console.o kernel/vgatext.
 magic.bin: boot/magic.asm
 	$(NASM) boot/magic.asm -DMAGIC='"$(MAGIC)"' -f bin -o magic.bin
 	
+kernel/entry.o: kernel/entry.c
+	$(CC) $(CFLAGS) -c -o kernel/entry.o kernel/entry.c
+
 kernel/kernel.o: kernel/kernel.c
 	$(CC) $(CFLAGS) -c -o kernel/kernel.o kernel/kernel.c
 
diff --git a/src/README b/src/README
index b529282..9269936 100644
--- a/src/README
+++ b/src/README
@@ -22,6 +22,7 @@ kernel
 
 Kernel main and utility routines like early consoles.
 
+* entry.c - the kernel entry from the bootloader
 * kernel.c - kernel helper functions
 * console.c - the kernel console, can use VGA or serial port for now
 * vga.c - VGA basic output routines for early kernel output
diff --git a/src/drivers/video/vga.c b/src/drivers/video/vga.c
index 146194e..0cd623d 100644
--- a/src/drivers/video/vga.c
+++ b/src/drivers/video/vga.c
@@ -210,7 +210,7 @@ static uint8_t *get_frame_buffer_segment( vga_t *vga )
 {
 	port8_write( &vga->graphics_controller_index_port, 0x06 );
 	uint8_t segment_no = ( port8_read( &vga->graphics_controller_data_port ) >> 2 ) & 0x03;
-	uint8_t *segment;
+	uint8_t *segment = 0x0;
 
 	switch( segment_no  ) {
 		case 0: // A0000h - BFFFFh, 128k
diff --git a/src/kernel/entry.c b/src/kernel/entry.c
new file mode 100644
index 0000000..d0e75a7
--- /dev/null
+++ b/src/kernel/entry.c
@@ -0,0 +1,16 @@
+#include "kernel.h"
+
+// must be first entry in kernel.bin (0x8800) as stage 2 of
+// the boot loader expects the entry point to b here!
+//
+// compilers (at least gcc) mess with the order of functions when
+// compiling with optimization levels, so kernel_main ends up somewhere
+// in the first compilation unit. I didn't find an elegand way (but for
+// an ld script with custom section and some compiler specific attributes
+// how to solve this issue). This solution here is sort of a nice
+// workaround working no matter what the compilers try to do (of course
+// till we get optimization across compilation units that is)
+void kernel_entry( void ) 
+{
+	kernel_main( );
+}
-- 
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