From a46114f6bd5ec6e0fd9067c09ea78d4baf4d51b8 Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Wed, 27 Mar 2019 23:07:04 +0000
Subject: [PATCH] kernel: strip down to dumb binary, expect bootloader to load
 into sane location and setup stack

---
 Makefile        |  8 ++++----
 kernel/main.asm | 50 +++++--------------------------------------------
 2 files changed, 9 insertions(+), 49 deletions(-)

diff --git a/Makefile b/Makefile
index 13d32e4..7cec100 100644
--- a/Makefile
+++ b/Makefile
@@ -1,15 +1,15 @@
 default: kernel.com
 
-kernel.com: kernel/*.asm
+boot.bin: kernel/*.asm
 	nasm -s -o $@ -I kernel kernel/main.asm
 
 vbr.bin: boot/vbr.asm
 	nasm -s -o $@ -I lib $<
 
-fdimage.img: vbr.bin kernel.com
+fdimage.img: vbr.bin boot.bin
 	dd if=/dev/zero bs=512 count=720 of=$@
-	mformat -i $@ -f 360 -B vbr.bin -v "BOOT2B" ::
-	mcopy -i fdimage.img kernel.com ::boot.bin
+	mformat -i $@ -f 360 -B vbr.bin ::
+	mcopy -i fdimage.img boot.bin ::boot.bin
 
 clean:
 	rm -f *.com *.bin *.img
diff --git a/kernel/main.asm b/kernel/main.asm
index 55bb8fb..30640e6 100644
--- a/kernel/main.asm
+++ b/kernel/main.asm
@@ -3,51 +3,9 @@ cpu 8086
 org 0x0000
 
 _startup:
-  xor ax, ax ; trash ax
   mov ax, cs
-  cmp ax, 0x0000 ; MBR or NBP, CS=0000 IP=7C00
-  je .mbr
-  cmp ax, 0x07C0 ;  MBR on weird BIOS, CS=07C0 IP=0000
-  je .mbr
-.dos:
-  ; ROP because call only works on immediate or memory
-  mov ax, cs
-  push ax ; return CS
-  mov ax, .dosret
-  add ax, 0x0100
-  push ax ; return IP
-  mov ax, cs
-  add ax, 0x0010
-  push ax ; main CS
-  mov ax, main
-  push ax ; main IP
-  retf
-.dosret:
-  int 0x20
-  jmp .halt
-.mbr:
-  push si ; index of partition table entry
-  push ds ; segment for ^
-  push dx ; dl might contain drive number
-  push bx ; possible offset for pxeenv+ struct
-  push es ; segment for ^
-
-  mov ax, 0x0050
-  mov es, ax
-  mov di, 0x0000
-
-  xor ax, ax
-  mov ds, ax
-  mov si, 0x7C00
-
-  mov cx, _reloc_end
-  rep movsb
-
-  call 0x0050:main
-.halt:
-  hlt
-  jmp .halt
-
+  mov ss, ax
+  mov sp, 0x0000
 main:
   mov ax, cs
   mov ds, ax
@@ -58,7 +16,9 @@ main:
 
   int 0x2E
 
-  retf
+.halt:
+  hlt
+  jmp .halt
 
 %include "heap.asm"
 %include "intr.asm"