diff --git a/src/@rdos.asm b/src/@rdos.asm
index 2dfac29..cd96907 100644
--- a/src/@rdos.asm
+++ b/src/@rdos.asm
@@ -2,15 +2,13 @@
 cseg	equ 0x0
 	jmp init
 
-%include "bpb.inc"
-%include "farptr.inc"
+;;; Character I/O
 
-	; dflags bitfields
-DRVLOG	equ 0x01 ; bit 0 - drive logged in (see drvnum)
-DRVCTL	equ 0x02 ; bit 1 - controller configured
-DRVEXT	equ 0x04 ; bit 2 - EBIOS supported
-DIRTY	equ 0x08 ; bit 3 - dskbuf dirty
-DRVCHS	equ 0x10 ; bit 4 - CHS geometry known
+	; direct console I/O
+	; IN	dl	character to output or 0xFF if input
+	; OUT	al	character to read
+conio	cmp dl, 0xFF
+	je conin
 
 	; raw console output
 	; IN	dl	character
@@ -20,16 +18,6 @@ conout	xchg ax, dx
 	int 0x10
 	ret
 
-	; check if character has characters ready
-	; OUT	al	0xFF if data, 0 if none
-const	mov ah, 1
-	int 0x16
-	jz .emp
-	mov ax, 0xFF
-	ret
-.emp	mov al, 0
-	ret
-
 	; read character from console
 	; OUT	al	character or zero
 conin	call const
@@ -39,6 +27,16 @@ conin	call const
 	int 0x16
 .ret	ret
 
+	; check if character has characters ready
+	; OUT	al	0xFF if data, 0 if none
+const	mov ah, 1
+	int 0x16
+	jz .emp
+	mov ax, 0xFF
+	ret
+.emp	mov al, 0
+	ret
+
 	; dummy: write to auxillary dev
 auxout	ret
 
@@ -71,13 +69,6 @@ readc	call const
 	; IN	dl	character
 putc	jmp conout
 
-	; direct console I/O
-	; IN	dl	character to output or 0xFF if input
-	; OUT	al	character to read
-dcon	cmp dl, 0xFF
-	je conin
-	jmp conout
-
 	; output character string
 	; IN	ds:dx	far ptr to string, '$'-terminated
 puts	push si
@@ -147,6 +138,18 @@ gets	mov si, dx
 	call putc
 	ret
 
+;;; Disk i/o
+
+%include "bpb.inc"
+%include "farptr.inc"
+
+	; dflags bitfields
+DRVLOG	equ 0x01 ; bit 0 - drive logged in (see drvnum)
+DRVCTL	equ 0x02 ; bit 1 - controller configured
+DRVEXT	equ 0x04 ; bit 2 - EBIOS supported
+DIRTY	equ 0x08 ; bit 3 - dskbuf dirty
+DRVCHS	equ 0x10 ; bit 4 - CHS geometry known
+
 	align 16
 	; IBM Interrupt Sharing Protocol structure
 i13isp	jmp short int13i
@@ -436,29 +439,6 @@ diskio	mov cl, 1 ; read len
 	stc
 	ret
 
-	; Get interrupt vector
-	; IN	al	interrupt number
-	; OUT	es:bx	vector
-getint	mov bl, al
-	mov bh, 0
-	; multiply with 4
-	add bx, bx
-	add bx, bx
-	les bx, [cs:bx]
-	ret
-
-	; Set interrupt vector
-	; IN	al	interrupt number
-	;	ds:dx	vector
-setint	mov bl, al
-	mov bh, 0
-	; multiply with 4
-	add bx, bx
-	add bx, bx
-	mov [cs:bx], dx
-	mov [cs:bx+2], ds
-	ret
-
 init	mov ax, cs
 	; boot failure if loaded wrong
 	cmp ax, cseg
@@ -481,11 +461,130 @@ init	mov ax, cs
 	mov word [4*0x13], int13i
 	mov [4*0x13+2], cs
 
-	mov ax, 0x0e37
-	int 0x10
+	; install int21h handler
+	mov word [4*0x21], int21h
+	mov [4*0x21+2], cs
+
+	mov dl, 0x36
+	mov ah, 2
+	int 0x21
+
 halt	hlt
 	jmp halt
 
+;;; Export functions as syscalls
+; SP, BP, SI, DI and ES must be preserved by kernel code
+; DL, DX or DS:DX is input argument from prog
+; AL, BX or ES:BX is return value to prog
+
+srb	equ 0x8000
+srw	equ 0x4000
+
+; DOS style syscall interface
+int21h	push bx
+	xor bx, bx
+	mov bl, ah
+	jmp sysc
+
+; CP/M-86 style syscall interface
+intE0h	push bx
+	xor bx, bx
+	mov bl, cl
+
+	; bx = syscall number
+	; stack must be -> BX IP CS FL
+	; for return to prog
+sysc	add bx, bx
+	mov bx, [cs:bx+stab]
+	test bh, srw >> 8 ; fork off bx returns
+	jnz .l02
+	test bh, srb >> 8 ; fork off al returns
+	jnz .l01
+	; syscall with no return value
+	push dx
+	push cx
+	push ax
+	push ds
+	call bx
+	pop ds
+	pop ax
+	pop cx
+	pop dx
+	pop bx
+	iret
+	; syscalls returning byte in al
+.l01	push dx
+	push cx
+	push ds
+	and bh, ~(srb >> 8)
+	call bx
+	pop ds
+	pop cx
+	pop dx
+	pop bx
+	iret
+	; syscalls returning bx or es:bx
+.l02	add sp, 2 ; discard bx on the stack
+	push dx
+	push cx
+	push ax
+	push ds
+	and bh, ~(srw >> 8)
+	call bx
+	pop ds
+	pop ax
+	pop cx
+	pop dx
+	ret
+
+section .rodata
+
+	; syscall table
+stab	dw 0 ; 0 reboot
+	dw getc + srb ; 1 console input
+	dw putc ; 2 console output
+	dw auxin ; 3 reader in
+	dw auxout + srb ; 4 puncher out
+	dw lstout ; 5 lister out
+	dw 0 ; 6 direct console i/o
+	dw 0 ; 7 read i/o byte
+	dw 0 ; 8 get i/o byte
+	dw 0 ; 9 string output
+	dw 0 ; 10 string input
+	dw 0 ; 11 console status
+	dw 0 ; 12 get version number
+	dw 0 ; 13 reset disks
+	dw 0 ; 14 set drive
+	dw 0 ; 15 open file
+	dw 0 ; 16 close file
+	dw 0 ; 17 find first
+	dw 0 ; 18 find next
+	dw 0 ; 19 delete file
+	dw 0 ; 20 sequential read
+	dw 0 ; 21 sequential write
+	dw 0 ; 22 create file
+	dw 0 ; 23 rename file
+	dw 0 ; 24 get login vector
+	dw 0 ; 25 get drive
+	dw 0 ; 26 set dma addr
+	dw 0 ; 27 alloc vector
+	dw 0 ; 28 set drive r/o
+	dw 0 ; 29 get r/o vector
+	dw 0 ; 30 file attr
+	dw 0 ; 31 get addr of disk parameter block
+	dw 0 ; 32 get user number
+	dw 0 ; 33 random read
+	dw 0 ; 34 random write
+	dw 0 ; 35 calculate file size
+	dw 0 ; 36 calculate cur record number
+	dw 0 ; 37 reset drive
+	dw 0 ; 38
+	dw 0 ; 39
+	dw 0 ; 40 random write with block init
+stab_e	nop
+
+section .data
+
 dap	db 0x10, 0
 	dw 1
 	dw dskbuf, cseg