poc

.gitignore

@@ -0,0 +1,2 @@
+*.o
+*.bin

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 $^

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