From 11db15bb0f0f3d23aa9b2966c2f65a0379246813 Mon Sep 17 00:00:00 2001
From: John Tsiombikas <nuclear@mutantstargoat.com>
Date: Fri, 27 Apr 2018 16:56:33 +0300
Subject: [PATCH] generalizing the 16bit interrupt hack to an arbitray int86()
 call

---
 src/boot/boot2.s |   42 ++++++++++++++++++++++++++++++++----------
 src/int86.h      |   15 +++++++++++++++
 src/kmain.c      |   12 +++++++++++-
 3 files changed, 58 insertions(+), 11 deletions(-)
 create mode 100644 src/int86.h

diff --git a/src/boot/boot2.s b/src/boot/boot2.s
index 37f1026..e4088f4 100644
--- a/src/boot/boot2.s
+++ b/src/boot/boot2.s
@@ -554,10 +554,9 @@ kbc_wait_write:
 numbuf: .space 16
 
 
-# this is not boot loader code. It's called later on by the main
-# kernel code in 32bit protected mode. It's placed here because
-# it needs to be in base memory as it returns and runs in real mode.
-
+# this is not boot loader code. It's called later on by the main kernel
+# code in 32bit protected mode. It's placed here because it needs to be
+# located in base memory as it returns and runs in real mode.
 	.code32
 	.align 4
 	# place to save the protected mode IDTR pseudo-descriptor
@@ -571,9 +570,14 @@ idtaddr:.long 0
 rmidt:	.short 0x3ff
 	.long 0
 
-	# drop back to unreal mode to set video mode
-	.global set_mode13h
-set_mode13h:
+saved_esp: .long 0
+saved_ebp: .long 0
+
+	# drop back to unreal mode to call 16bit interrupt
+	.global int86
+int86:
+	push %ebp
+	mov %esp, %ebp
 	pushal
 	cli
 	# save protected mode IDTR and replace it with the real mode vectors
@@ -597,9 +601,26 @@ set_mode13h:
 	mov %ax, %ss
 	nop
 
-	# switch video mode by calling the video bios
-	mov $0x13, %ax
-	int $0x10
+	# modify the int instruction
+	mov $int_op, %ebx
+	movb 4(%ebp), %al
+	movb %al, 1(%ebx)
+
+	# load registers from the int86regs struct
+	mov %esp, saved_esp
+	mov %ebp, saved_ebp
+	mov 8(%ebp), %esp
+	popal
+	mov saved_esp, %esp
+
+	# call 16bit interrupt
+int_op:	int $0
+
+	mov saved_ebp, %ebp
+	mov 8(%ebp), %esp
+	add $32, %esp
+	pushal
+	mov saved_esp, %esp
 
 	# re-enable protection
 	mov %cr0, %eax
@@ -620,6 +641,7 @@ set_mode13h:
 	lidt (saved_idtr)
 	sti
 	popal
+	pop %ebp
 	ret
 
 
diff --git a/src/int86.h b/src/int86.h
new file mode 100644
index 0000000..fdae525
--- /dev/null
+++ b/src/int86.h
@@ -0,0 +1,15 @@
+#ifndef INT86_H_
+#define INT86_H_
+
+#include <inttypes.h>
+
+struct int86regs {
+	uint32_t edi, esi, ebp, esp;
+	uint32_t ebx, edx, ecx, eax;
+	uint16_t flags;
+	uint16_t es, ds, fs, gs;
+} __attribute__ ((packed));
+
+void int86(int inum, struct int86regs *regs);
+
+#endif	/* INT86_H_ */
diff --git a/src/kmain.c b/src/kmain.c
index af7bc65..fab4e6f 100644
--- a/src/kmain.c
+++ b/src/kmain.c
@@ -16,14 +16,24 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 #include <stdio.h>
+#include <string.h>
 #include <ctype.h>
 #include "segm.h"
 #include "intr.h"
 #include "keyb.h"
 #include "timer.h"
 #include "contty.h"
+#include "int86.h"
+
+static void set_mode13h(void)
+{
+	struct int86regs regs;
+
+	memset(&regs, 0, sizeof regs);
+	regs.eax = 0x13;
+	int86(0x10, &regs);
+}
 
-void set_mode13h(void);
 void logohack(void);
 
 void pcboot_main(void)
-- 
1.7.10.4