From 07db0aabb3ad8dbb0a3cee2d3837dd93792ae825 Mon Sep 17 00:00:00 2001
From: John Tsiombikas <nuclear@member.fsf.org>
Date: Tue, 10 Apr 2018 04:18:43 +0300
Subject: [PATCH] second stage loader loading semi-works

---
 Makefile         |    6 +++-
 gdbnotes         |    2 +-
 src/boot/boot.s  |   85 +++++++++++++++++++++++++++++++++++++++++++++++-------
 src/boot/boot2.s |   47 +++++++++++++++++++++++++-----
 4 files changed, 119 insertions(+), 21 deletions(-)

diff --git a/Makefile b/Makefile
index d034876..9117c3f 100644
--- a/Makefile
+++ b/Makefile
@@ -20,6 +20,10 @@ ifneq ($(shell uname -m), i386)
 	ldarch = -m elf_i386
 endif
 
+floppy.img: $(bin)
+	dd if=/dev/zero of=$@ bs=512 count=2880
+	dd if=$< of=$@ conv=notrunc
+
 $(bin): $(elf)
 	objcopy -O binary $< $@
 
@@ -41,4 +45,4 @@ cleandep:
 
 .PHONY: run
 run: $(bin)
-	qemu-system-i386 -fda $(bin)
+	qemu-system-i386 -fda floppy.img
diff --git a/gdbnotes b/gdbnotes
index e03261f..fb31bbf 100644
--- a/gdbnotes
+++ b/gdbnotes
@@ -7,4 +7,4 @@ b *0x7c00
 
 examine register with: i r eax
 
-$ objdump -D test
+$ objdump -D test -m i8086
diff --git a/src/boot/boot.s b/src/boot/boot.s
index 499c386..ac4a424 100644
--- a/src/boot/boot.s
+++ b/src/boot/boot.s
@@ -3,10 +3,10 @@
 
 	cli
 	cld
-	# move stack to the top of 512k
-	mov $0x7000, %ax
+	# move stack to just below the code
+	xor %ax, %ax
 	mov %ax, %ss
-	xor %sp, %sp
+	mov $0x7c00, %sp
 	# use the code segment for data access
 	mov %cs, %ax
 	mov %ax, %ds
@@ -25,7 +25,7 @@
 	shr $9, %eax
 	and $0x1ff, %ebx
 	jz 0f
-	inc %eax
+	inc %ax
 0:	pushw %ax
 	pushw $1
 	# set es to the start of the destination buffer to allow reading in
@@ -34,7 +34,7 @@
 	shr $4, %bx
 	mov %bx, %es
 	xor %bx, %bx
-	call read_sect
+	call read_sectors
 	jmp boot2_addr
 
 	cli
@@ -45,31 +45,39 @@
 	.set ARG_NSECT, 6
 	.set ARG_SIDX, 4
 
-# read_sect(first, num)
-read_sect:
+# read_sectors(first, num)
+read_sectors:
 	push %bp
 	mov %sp, %bp
 
 	mov ARG_SIDX(%bp), %ax
-	mov ARG_NSECT(%bp), %cx
+	xor %cx, %cx
 
 	jmp 1f
 0:	push %ax
 	call read_sector
-	add $2, %sp
+	pop %ax
+	inc %ax
+	inc %cx
 1:	cmp ARG_NSECT(%bp), %cx
 	jnz 0b
 
 	pop %bp
 	ret
 
+	.set VAR_ATTEMPTS, -2
+
 # read_sector(sidx)
 read_sector:
 	push %bp
 	mov %sp, %bp
+	sub $2, %sp
 	push %cx
 	push %dx
 
+	movw $3, VAR_ATTEMPTS(%bp)
+
+.Lread_try:
 	# calculate the track (sidx / sectors_per_track)
 	mov 4(%bp), %ax
 	xor %dx, %dx
@@ -92,7 +100,22 @@ read_sector:
 	mov $0x0201, %ax
 	movb drive_number, %dl
 	int $0x13
+	jnc .Lread_ok
+
+	# abort after 3 attempts
+	decw VAR_ATTEMPTS(%bp)
+	jz .Lread_fail
+
+	# error detected, reset controller and retry
+	xor %ah, %ah
+	int $0x13
+	jmp .Lread_try
 
+.Lread_fail:
+	mov 4(%bp), %ax
+	jmp abort_read
+
+.Lread_ok:
 	# increment es:bx accordingly (advance es if bx overflows)
 	add $512, %bx
 	jno 0f
@@ -100,11 +123,26 @@ read_sector:
 	add $4096, %ax
 	mov %ax, %es
 
-	pop %dx
+0:	pop %dx
 	pop %cx
+	add $2, %sp
 	pop %bp
 	ret
 
+str_read_error: .asciz "Failed to read sector: "
+
+abort_read:
+	push %ax
+	mov $str_read_error, %si
+	call print_str
+
+	xor %eax, %eax
+	pop %ax
+	call print_num
+
+	cli
+	hlt
+
 clearscr:
 	push %es
 	pushw $0xb800
@@ -116,6 +154,28 @@ clearscr:
 	pop %es
 	ret
 
+cursor_x: .byte 0
+
+# expects string pointer in ds:si
+print_str:
+	push %es
+	pushw $0xb800
+	pop %es
+	xor %di, %di
+	movb $0, cursor_x
+
+0:	mov (%si), %al
+	mov %al, %es:(%di)
+	movb $7, %es:1(%di)
+	inc %si
+	add $2, %di
+	incb cursor_x
+	cmp $0, %al
+	jnz 0b
+
+	pop %es
+	ret
+
 # expects number in eax
 print_num:
 	# save es
@@ -137,7 +197,10 @@ print_num:
 	# print the backwards string
 	pushw $0xb800
 	pop %es
-	xor %di, %di
+	movb cursor_x, %al
+	xor %ah, %ah
+	shl $1, %ax
+	mov %ax, %di
 
 0:	dec %si
 	mov (%si), %al
diff --git a/src/boot/boot2.s b/src/boot/boot2.s
index 47121ee..7b8c7bb 100644
--- a/src/boot/boot2.s
+++ b/src/boot/boot2.s
@@ -5,20 +5,44 @@
 	mov $0x13, %ax
 	int $0x10
 
-	pushw $63
-	pushw $63
-	pushw $0
-	pushw $0xff
-	call set_palette
-	add $8, %sp
+	# copy palette
+	mov $logo_pal, %si
+	xor %cl, %cl
 
+0:	mov $0x3c8, %dx
+	movb %cl, %al
+	outb %al, %dx
+	inc %dx
+	# red
+	movb (%si), %al
+	inc %si
+	shr $2, %al
+	outb %al, %dx
+	# green
+	movb (%si), %al
+	inc %si
+	shr $2, %al
+	outb %al, %dx
+	# blue
+	movb (%si), %al
+	inc %si
+	shr $2, %al
+	outb %al, %dx
+	inc %cl
+	jno 0b
+
+	# copy pixels
 	pushw $0xa000
 	pop %es
 	xor %di, %di
+	mov $logo_pix, %eax
+	shr $4, %eax
+	mov %ax, %ds
+	xor %si, %si
 	mov $16000, %ecx
-	mov $0xffffffff, %eax
-	rep stosl
+	rep movsl
 
+	cli
 	hlt
 
 set_palette:
@@ -34,3 +58,10 @@ set_palette:
 	movw 8(%bp), %ax
 	outb %al, %dx
 	ret
+
+logo_pal:
+	.incbin "logo.pal"
+
+	.align 16
+logo_pix:
+	.incbin "logo.raw"
-- 
1.7.10.4