Implement some syscall handling

boot/kernel.asm

@@ -1,7 +1,7 @@
 	cpu 8086
 	org 0x7C00
-stack:	equ 0x7C00
 
+stack:	equ 0x7C00
 framsz: equ 0x300
 
 	jmp init
@@ -68,23 +68,43 @@ getax:	push ds
 	pop si
 	ret
 
-sftab:	dw 0, 0, 0, 0 ; 00-03
+	; No such subfunction - set carry and exit
-	dw 0, 0, 0, 0 ; 04-07
+sferr:	stc
-	dw 0, 0, 0, 0 ; 08-0b
+	ret
-	dw 0, 0, 0, 0 ; 0c-0f
-	dw 0, 0, 0, 0 ; 10-13
-	dw 0, 0, 0, 0 ; 14-17
-	dw 0, 0, 0, 0 ; 18-1b
-	dw 0, 0, 0, 0 ; 1c-1f
-	dw 0, 0, 0, 0 ; 20-23
-	dw 0, dos25, 0, 0 ; 24-27
-	dw 0, 0, 0, 0 ; 28-2b
-	dw 0, 0, 0, 0 ; 2c-2f
 
-	; DOSINT 25h: Set interrupt vector
+	; Subfunction table
+	align 2
+sftab:	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, setint, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+	dw sferr, sferr, sferr, sferr
+
+	; DOS 25h: Set interrupt vector
 	; IN	al	interrupt number
 	;	ds:dx	entry point
-dos25:	
+setint: push ax
+	push bx
+	xor ah, ah
+	add al, al
+	add al, al
+	mov bx, ax
+	mov [cs:bx], dx
+	mov ax, ds
+	mov [cs:bx+2], ds
+	pop bx
+	pop ax
 	ret
 
 ; ===== end of resident, begin of transient startup code
@@ -96,16 +116,13 @@ init:	xor ax, ax
 	mov ss, ax
 	xor sp, sp
 
-	mov ax, 0x2521
+	mov al, 0x21
 	mov dx, int21h
-	pushf
+	call setint
-	push cs
+
-	call int21h
 	mov ax, 0x2520
 	mov dx, int20h
-	pushf
+	call setint
-	push cs
-	call int21h
 
 main:	push cs
 	pop ds
@@ -120,5 +137,4 @@ loop:	lodsb
 end:	hlt
 	jmp end
 
-	align 2048
 msg:	db "rdos kernel stub", 0x0A, 0x0D, 0