From 098b0dd7bf6600ed6b904cd531a55a6415996900 Mon Sep 17 00:00:00 2001
From: John Tsiombikas <nuclear@member.fsf.org>
Date: Sun, 8 Apr 2018 08:11:47 +0300
Subject: [PATCH] hacking the 1st stage loader

---
 gdbnotes         |   10 ++++
 pcboot.ld        |   14 +++++-
 src/boot/boot.s  |  147 ++++++++++++++++++++++++++++++++++++++++++++----------
 src/boot/boot2.s |   36 +++++++++++++
 4 files changed, 178 insertions(+), 29 deletions(-)
 create mode 100644 gdbnotes
 create mode 100644 src/boot/boot2.s

diff --git a/gdbnotes b/gdbnotes
new file mode 100644
index 0000000..e03261f
--- /dev/null
+++ b/gdbnotes
@@ -0,0 +1,10 @@
+$ qemu-system-i386 -fda test.bin -s -S
+$ gdb
+target remote localhost:1234
+set architecture i8086
+display/i $pc
+b *0x7c00
+
+examine register with: i r eax
+
+$ objdump -D test
diff --git a/pcboot.ld b/pcboot.ld
index f629124..28fb7e6 100644
--- a/pcboot.ld
+++ b/pcboot.ld
@@ -1,10 +1,20 @@
 OUTPUT_ARCH(i386)
 
 SECTIONS {
-	/* BIOS will load us at 0x7c000h */
-	. = 0x7c000;
+	/* BIOS loads the boot code at 0000:7c00 */
+	. = 0x7c00;
 
 	.boot : { * (.boot); }
+
+	/* second stage boot loader */
+	.boot2 : { * (.boot2); }
+	_boot2_size = SIZEOF(.boot2);
+
+	/* main program will be loaded at 1MB by the second stage
+	 * boot loader
+	 */
+	. = 1M;
+
 	.startup : { * (.startup); }
 	.text : { * (.text); }
 	.rodata : { * (.rodata); }
diff --git a/src/boot/boot.s b/src/boot/boot.s
index e3fd206..ffd1408 100644
--- a/src/boot/boot.s
+++ b/src/boot/boot.s
@@ -1,37 +1,130 @@
 	.code16
 	.section .boot,"a"
 
-	mov $0x13, %ax
-	int $0x10
-
-	mov $1, %al
-	mov $0x3c8, %dx
-	outb %al, %dx
-	mov $0x3c9, %dx
-	mov $63, %al
-	outb %al, %dx
-	xor %al, %al
-	outb %al, %dx
-	outb %al, %dx
-
-	mov $200, %ebx
-	mov $0x00000101, %eax
-	pushl $0xa000
-	popl %es
-	xor %di, %di
-fill:
-	mov %ebx, %ecx
-	and $1, %ecx
-	jnz 0f
-	rol $16, %eax
-0:	mov $80, %ecx
-	rep stosl
-	dec %ebx
-	jnz fill
+	cli
+	cld
+	# move stack to the top of 512k
+	mov $0x7000, %ax
+	mov %ax, %ss
+	xor %sp, %sp
+	# use the code segment for data access
+	mov %cs, %ax
+	mov %ax, %ds
+	mov %ax, %es
+
+	call clearscr
+
+	mov $_boot2_size, %eax
+	call print_num
+
+	# load the second stage boot loader and jump to it
+	mov $_boot2_size, %eax
+	mov %eax, %ebx
+	shr $9, %eax
+	and $0x1ff, %ebx
+	jz 0f
+	inc %eax
+0:	pushw %ax
+	pushw $1
+	# set es to the start of the destination buffer to allow reading in
+	# full 64k chunks
+	mov $boot2_addr, %bx
+	shr $4, %bx
+	mov %bx, %es
+	xor %bx, %bx
+	call readsect
+	jmp boot2_addr
 
 	cli
 	hlt
 
+	.set SECT_PER_TRACK, 18
+
+	.set ARG_NSECT, 6
+	.set ARG_SIDX, 4
+	.set VAR_NTRACKS, -2
+
+# readsect(first, num)
+readsect:
+	push %bp
+	mov %sp, %bp
+	sub $2, %sp
+
+	# calculate how many tracks to read
+	mov ARG_NSECT(%bp), %ax
+	xor %dx, %dx
+	mov $SECT_PER_TRACK, %cx
+	div %cx
+	cmp $0, %dx
+	jz 0f
+	inc %ax
+0:	mov %ax, VAR_NTRACKS(%bp)
+
+	xor %cx, %cx
+0:	cmp VAR_NTRACKS(%bp), %cx
+	jz 0f
+	push %cx
+	call read_track
+	pop %cx
+	jmp 0b
+0:
+	# TODO cont.
+	pop %bp
+	ret
+
+read_track:
+	ret
+
+clearscr:
+	push %es
+	pushw $0xb800
+	pop %es
+	xor %eax, %eax
+	xor %di, %di
+	movl $500, %ecx
+	rep stosl
+	pop %es
+	ret
+
+print_num:
+	# save es
+	push %es
+
+	xor %cx, %cx
+	movw $numbuf, %si
+	mov $10, %ebx
+
+0:	xor %edx, %edx
+	div %ebx
+	add $48, %dl
+	mov %dl, (%si)
+	inc %si
+	inc %cx
+	cmp $0, %eax
+	jnz 0b
+
+	# print the backwards string
+	pushw $0xb800
+	pop %es
+	xor %di, %di
+
+0:	dec %si
+	mov (%si), %al
+	movb %al, %es:(%di)
+	inc %di
+	mov $7, %al
+	movb %al, %es:(%di)
+	inc %di
+	dec %cx
+	jnz 0b
+
+	# restore es
+	pop %es
+	ret
+
+numbuf: .space 10
 	.org 510
 	.byte 0x55
 	.byte 0xaa
+
+boot2_addr:
diff --git a/src/boot/boot2.s b/src/boot/boot2.s
new file mode 100644
index 0000000..47121ee
--- /dev/null
+++ b/src/boot/boot2.s
@@ -0,0 +1,36 @@
+# this is the second-stage boot loader
+	.code16
+	.section .boot2,"a"
+
+	mov $0x13, %ax
+	int $0x10
+
+	pushw $63
+	pushw $63
+	pushw $0
+	pushw $0xff
+	call set_palette
+	add $8, %sp
+
+	pushw $0xa000
+	pop %es
+	xor %di, %di
+	mov $16000, %ecx
+	mov $0xffffffff, %eax
+	rep stosl
+
+	hlt
+
+set_palette:
+	mov %sp, %bp
+	mov $0x3c8, %dx
+	movw 2(%bp), %ax
+	outb %al, %dx
+	inc %dx
+	movw 4(%bp), %ax
+	outb %al, %dx
+	movw 6(%bp), %ax
+	outb %al, %dx
+	movw 8(%bp), %ax
+	outb %al, %dx
+	ret
-- 
1.7.10.4