From 8131db9b2925b2a94e45f8e911bd52eed44b6ed0 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Thu, 18 Feb 2021 19:00:54 +0000
Subject: [PATCH] Re-add support for booting from qemu harddisk

---
 Makefile     |   9 ++++
 boot/mbr.asm | 141 ++++++++-------------------------------------------
 2 files changed, 31 insertions(+), 119 deletions(-)

diff --git a/Makefile b/Makefile
index 3012944..2b90815 100644
--- a/Makefile
+++ b/Makefile
@@ -64,6 +64,12 @@ fd%.img: $(DISTFILES) $(SYS)
 	mcopy      -i $@ $(DISTFILES) ::
 	$(SYS)        $@ @rdos.com
 
+hdd.img: mbr.bs @rdos.com fd1440.img
+	dd if=mbr.bs bs=512 count=1 of=$@
+	dd if=@rdos.com bs=512 seek=1 of=$@
+	dd if=fd1440.img bs=512 seek=63 of=$@
+	dd if=/dev/zero bs=512 seek=$$(( 63 * 23 )) count=63 of=$@
+
 clean:
 	rm -f *.com *.bs *.0 *.lst *.img *.bin *.rom
 	rm -f host/*.elf
@@ -73,3 +79,6 @@ qemu-floppy3: fd1440.img $(ROMS)
 
 qemu-floppy5: fd360.img $(ROMS)
 	$(QEMU) $(QEMU_ARGS) -boot a -fda fd360.img
+
+qemu-hdd: hdd.img $(ROMS)
+	$(QEMU) $(QEMU_ARGS) -boot c -hda hdd.img
diff --git a/boot/mbr.asm b/boot/mbr.asm
index 873380a..38e7f37 100644
--- a/boot/mbr.asm
+++ b/boot/mbr.asm
@@ -1,139 +1,42 @@
-	; blah dont judge me, i miss 8080
-	%macro retnz 0
-	jz short ($+2+1)
-	ret
-	%endmacro
-	%macro callc 1
-	jnc short ($+2+3)
-	call %1
-	%endmacro
-
-	%include "inc/bpb.asm"
-
-	%define bpb	bs+0x0B
-
 	cpu 8086
-	org 0x0600
-bs:	equ 0x7C00
+
+%define seg 0x060
+
+	org 0x7C00 - (seg << 4)
 
 init:	cli
-	xor ax, ax
+	mov ax, seg
 	mov ss, ax
-	xor sp, sp
-
-	; save driver number
-	push dx
-
-	; save PnP ptr
-	push es
-	push di
-
-	; Relocate away from bootsector loading area
 	mov ds, ax
 	mov es, ax
-	mov si, bs
-	mov di, $$
-	mov cx, 0x100
-	rep movsw
+	xor sp, sp
 
-	jmp 0:main
+.retry:	mov bx, 7
+	mov ax, 0x0e2e
+	int 0x10
 
-	%include "inc/booterr.asm"
+	mov ax, 0x0210
+	mov cx, 2
+	mov dh, 0
+	mov bx, 0x100
 
-	; find active partition or bail out
-	; result in si
-findp:	mov si, part1
-	mov cx, 4
-.loop:	test BYTE [si], 0x80
-	retnz
-	add si, 0x10
-	loop .loop
-	call errcll
-
-main:	; find active partition
-	call findp
-
-	; transfer starting address into DAP
-	push si
-	add si, 0x08
-	mov di, dap.num
-	movsw
-	movsw
-	pop si
-
-	; load sector
-	push si
-	mov si, dap
-	mov ah, 0x42
-	stc
 	int 0x13
-	callc errcll
-	pop si
+	jc .retry
 
-	; FAT bpb has "bytes per sector" at this place
-	; we support only 512 byte sectors anyways
-	cmp word [bpb+BPBSS], 512
-	jne chain
-
-	; Media descriptor byte is never smaller than 0xF0
-	cmp byte [bpb+BPBMD], 0xF0
-	jc chain
-
-	; ask BIOS what our disk geometry is
-	mov ah, 0x08
-	stc
-	int 0x13
-	jc chain
-
-	; ES might be messed up from int 13h, fix it
-	push cs
-	pop es
-
-	; update sectors per track
-	xor ax, ax
-	mov al, cl
-	mov [bpb+BPBSPT], ax
-
-	; update number of sides
-	xor ax, ax
-	mov al, dh
-	mov [bpb+BPBNOS], ax
-
-	; update partition offset
-	push si
-	add si, 0x08
-	mov di, bpb+BPBHS
-	movsw
-	movsw
-	pop si
-
-chain:	; restore PnP structure
-	pop di
-	pop es
-	pop dx
-	xor cx, cx
-	mov bp, si
-	; chain into partition boot sector
-	jmp 0:bs
-
-dap:
-.size:	db 0x10
-	db 0
-.count:	dw 1
-.buf:	dw bs
-	dw 0
-.num:	dq 0
+	jmp seg:0x100
 
 	times (0x1BE - ($-$$)) db 0
 
+	; assumes 16 heads, 63 sectors per track
+	; image of 1.44 MB floppy starting on 2nd track
 part1:	db 0x80
+	db 1, 1, 0
+	db 1
 	db 0xFF, 0xFF, 0xFF
-	db 0x01
-	db 0xFF, 0xFF, 0xFF
-	dd 1
-	dd (1440 * 2)
+	dd 63
+	dd 0xb40
 
 	times (0x1FE - ($-$$)) db 0
 
 	; Boot signature
-	dw 0xAA55
+sig:	dw 0xAA55