Implement file descriptor table and CON as file descriptor

kernel/con.asm

@@ -0,0 +1,22 @@
+con_ftab:	dw con_read
+		dw con_write
+
+con_read:	stc
+		ret
+
+con_write:	push si
+		push bx
+		push cx
+		test cx, cx
+		jz .end
+		mov si, dx
+		mov ah, 0x0E
+		xor bx, bx
+.loop:		lodsb
+		int 0x10
+		loop .loop
+		pop cx
+.end:		pop bx
+		pop si
+		ret
+

kernel/fd.asm

@@ -0,0 +1,32 @@
+%define	fd_num	32
+
+		section .bss
+
+fd_table:	resb (0x10 * fd_num)
+
+		section .text
+
+fd_to_ptr:	push cx
+		mov cl, 4
+		sal bx, cl
+		add bx, fd_table
+		pop cx
+		ret
+
+fd_read:	cmp bx, fd_num
+		jnc fd_inv_hnd
+		call fd_to_ptr
+		; read ptr to ftab
+		mov bx, [bx]
+		; jmp to addr in ftab[0]
+		jmp [bx]
+
+fd_write:	cmp bx, fd_num
+		jnc fd_inv_hnd
+		call fd_to_ptr
+		mov bx, [bx]
+		jmp [bx+2]
+
+fd_inv_hnd:	mov ax, 6
+		stc
+		ret