From 3fe0dccba6a36462dff8e870b0441bf4a09c409e Mon Sep 17 00:00:00 2001
From: dzwdz <kg67199@gmail.com>
Date: Mon, 16 Aug 2021 19:15:38 +0200
Subject: [PATCH] poc

---
 .gitignore |  2 ++
 Makefile   | 12 ++++++++++++
 boot.s     | 39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 Makefile
 create mode 100644 boot.s

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..8deeab7
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*.o
+*.bin
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..6a5b710
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,12 @@
+boot.bin: boot.o
+	ld -o $@ --oformat binary -Ttext 0x7c00 $<
+
+boot.o: boot.s
+	as -o $@ $<
+
+.PHONY: boot disasm
+boot: boot.bin
+	qemu-system-i386 -drive file=$^,index=0,if=floppy,format=raw
+
+disasm: boot.bin
+	objdump -D -b binary -mi8086 $^
diff --git a/boot.s b/boot.s
new file mode 100644
index 0000000..df6f49c
--- /dev/null
+++ b/boot.s
@@ -0,0 +1,39 @@
+.code16
+
+.global _start # the linker needs to find it
+_start:
+	# enter mode 13h
+	mov $0x13, %ax
+	int $0x10
+
+	mov $0xA000, %ax # ES = vga memory
+	mov %ax, %es
+
+	mov $0x7c0, %ax # DS = the bootsector
+	mov %ax, %ds
+
+	# %eax - position in sector
+	#        it has to take up all of eax because of as's lack of real support
+	#        for 16bit
+	# %ebx - position in vga mem
+	mov $0, %eax
+	mov $0, %ebx
+loop:
+	movb %ds:(%eax), %cl
+	movb %cl, %es:(%ebx)
+
+	inc %bx
+	mov $16, %cl # if we've crossed a 16 pixel boundary, skip to the next line
+	and %bx, %cx
+	jz not_eol
+	add $304, %bx
+not_eol:
+	inc %ax      # only iterate 512 times
+	cmp $512, %ax
+	jl loop
+
+	incb %es:319
+	hlt
+
+.fill 510-(.-_start), 1, 0x90 # fill with 0x90 (NOPs)
+.word 0xaa55                  # boot sector magic