Add prototype for serial bootloader

Makefile

@@ -4,7 +4,7 @@ FD_SECTORS = 9
 
 QEMU_ARCH = $(shell uname -m)
 QEMU = qemu-system-$(QEMU_ARCH)
-QEMU_ARGS =
+QEMU_ARGS = -serial stdio
 
 ifdef KVM
 QEMU_ARGS += --enable-kvm
@@ -39,3 +39,6 @@ qemu-rom: kernel.rom
 
 qemu-floppy: fdimage.img
 	$(QEMU) $(QEMU_ARGS) -boot c -hda fdimage.img
+
+qemu-serial: boot/serial.bin
+	$(QEMU) $(QEMU_ARGS) -hda boot/serial.bin

boot/serial.asm

@@ -0,0 +1,90 @@
+org 0x7c00
+
+xor ax, ax
+mov ds, ax
+mov es, ax
+mov ss, ax
+mov sp, 0x7c00
+jmp 0:start
+
+getc:
+  mov ah, 0x02
+  mov dx, 0x0000
+  int 0x14
+  test ah, 0x80
+  jnz getc
+  ret
+
+putc:
+  mov ah, 0x01
+  mov dx, 0x0000
+  int 0x14
+  ret
+
+init_port:
+  mov ah, 0x00
+  mov al, 0b11100011
+  mov dx, 0x0000
+  int 0x14
+  ret
+
+dump:
+  mov si, 0x500
+  mov cx, di
+  sub cx, si
+.loop:
+  lodsb
+  call print8
+  loop .loop
+  mov al, 0x0A
+  call putc
+  mov al, 0x0D
+  call putc
+  ret
+
+line_reset:
+  cmp di, 0x500
+  je .reset
+  call line_process
+.reset:
+  mov di, 0x500
+  xor bp, bp
+  jmp mainloop
+
+line_process:
+  call dump
+  ret
+
+start:
+  call init_port
+  mov di, 0x500
+  jmp line_reset
+
+mainloop:
+  call getc
+  cmp al, ':'
+  je line_reset
+  cmp al, 0x20
+  jb line_reset
+  sub al, 0x30
+  cmp al, 9
+  jbe .noadjust
+  sub al, 7
+.noadjust:
+  test bp, bp
+  jnz .secondnib
+  mov cl, 4
+  shl al, cl
+  mov [di], al
+  not bp
+  jmp mainloop
+.secondnib:
+  or [di], al
+  inc di
+  not bp
+  jmp mainloop
+
+%include "print.asm"
+
+times 510-($-$$) db 0x00
+db 0x55,0xaa