diff --git a/Makefile b/Makefile
index 1057db0..4e6b0d8 100644
--- a/Makefile
+++ b/Makefile
@@ -1,13 +1,18 @@
-ordos.img: BOOT.BIN
+ordos.img: BOOT.BIN IO.SYS ORDOS.SYS COMMAND.COM
 	rm -f ordos.img
 	mkdosfs -C $@ -M 0xff 320
 	rw -I 0x7c00 -i BOOT.BIN -o $@ -c 512
+	mcopy -i $@ IO.SYS ::
+	mcopy -i $@ ORDOS.SYS ::
+	mcopy -i $@ COMMAND.COM ::
 
-BOOT.BIN ORDOS.SYS COMMAND.COM:
+BOOT.BIN IO.SYS ORDOS.SYS COMMAND.COM:
 	dosbox -exit build.bat
 
 BOOT.BIN: boot.asm
 
+IO.SYS: io.asm
+
 ORDOS.SYS: msdos.asm stddos.asm
 
 COMMAND.COM: command.asm
diff --git a/README b/README
index 4070b88..d466baf 100644
--- a/README
+++ b/README
@@ -15,6 +15,7 @@ command.asm  ms-dos  v1.25/source/COMMAND.ASM
 debug.com    ms-dos  v1.25/bin/DEBUG.COM
 edlin.com    ms-dos  v1.25/bin/EDLIN.COM
 exe2bin.exe  ms-dos  v1.25/bin/EXE2BIN.EXE
+io.asm       ordos
 link.exe     ms-dos  v1.25/bin/LINK.EXE
 masm.exe     ms-dos  v2.0/bin/MASM.EXE
 msdos.asm    ms-dos  v1.25/source/MSDOS.ASM
diff --git a/build.bat b/build.bat
index 6e12eb0..b12c88e 100644
--- a/build.bat
+++ b/build.bat
@@ -9,3 +9,7 @@ exe2bin stddos.exe ordos.sys
 masm boot;
 link boot;
 exe2bin boot.exe boot.bin
+
+masm io;
+link io;
+exe2bin io.exe io.sys
diff --git a/clean.bat b/clean.bat
index dacf244..f2a8415 100644
--- a/clean.bat
+++ b/clean.bat
@@ -9,3 +9,7 @@ del ordos.sys
 del boot.obj
 del boot.exe
 del boot.bin
+
+del io.obj
+del io.exe
+del io.sys
diff --git a/io.asm b/io.asm
new file mode 100644
index 0000000..1b56611
--- /dev/null
+++ b/io.asm
@@ -0,0 +1,157 @@
+; SPDX-License-Identifier: MIT
+; Copyright (c) 2021 Juhani 'nortti' Krekelä.
+
+iosegment equ 60h
+dossegment equ iosegment + 1*1024/16 ; DOS starts 1KiB after IO system
+
+dos segment at dossegment
+org 0
+dosinit proc far
+dosinit endp
+dos ends
+
+code segment
+code ends
+
+constants segment
+constants ends
+
+data segment
+data ends
+
+iogroup group code, constants, data
+
+code segment
+assume cs:iogroup, ds:iogroup, es:iogroup
+org 0
+
+; Jump table
+jmp init
+jmp status
+jmp getch
+jmp putch
+jmp unimplemented ; Output to printer
+jmp unimplemented ; Serial read
+jmp unimplemented ; Serial write
+jmp diskread
+jmp diskwrite
+jmp diskchange
+jmp setdate
+jmp settime
+jmp gettime
+jmp flush
+jmp mapdev
+
+init:
+	cld
+
+	cli
+	mov ax, cs
+	mov ds, ax
+	mov es, ax
+	; Put setup stack just below 32K
+	xor ax, ax
+	mov ss, ax
+	mov sp, 8000h
+	sti
+
+	; Figure out memory size
+	int 12h
+	; AX = memory size in kilobytes
+	; We want it in paragraphs
+	; There are 64 paragraphs in a kilobyte
+	mov cx, 64
+	shl al, cl
+	; Memory size is passed in dx
+	mov dx, ax
+
+	; Disk table is passed in si
+	mov si, offset iogroup:disks_table
+
+	call dosinit
+
+	mov al, 'c'
+	jmp error
+
+; OUT:
+;  al = character if any
+;  zf = there were no characters
+status proc far
+	; TODO: Implement
+	xor ax, ax
+	test ax, ax
+	ret
+status endp
+
+getch:
+	mov al, 'g'
+	jmp error
+
+; IN:
+;  ax = character
+putch proc far
+	push ax
+	mov ah, 0eh
+	int 10h
+	pop ax
+	ret
+putch endp
+
+diskread:
+	mov al, 'r'
+	jmp error
+diskwrite:
+	mov al, 'w'
+	jmp error
+diskchange:
+	mov al, 'c'
+	jmp error
+setdate:
+	mov al, 'd'
+	jmp error
+settime:
+	mov al, 't'
+	jmp error
+gettime:
+	mov al, 'T'
+	jmp error
+flush:
+	mov al, 'f'
+	jmp error
+
+mapdev:
+	mov al, 'm'
+	jmp error
+
+unimplemented:
+	mov al, '@'
+
+error:
+	mov ah, 0eh
+	int 10h
+
+hang:
+	hlt
+	jmp hang
+
+code ends
+
+constants segment
+
+disks_table:
+	db 1 ; 1 drive
+
+	db 0 ; Physical drive 0
+	dw offset iogroup:parameters_320k
+
+parameters_320k:
+	dw 512 ; Sector size in bytes
+	db 2 ; Sectors per cluster
+	dw 1 ; Number of reserved sectors
+	dw 2 ; Number of FATs
+	dw 112 ; Number of directory entries
+	dw 320*2 ; Number of sectors
+
+constants ends
+
+end