Add printf facility for kernel to allow debug messages on phys. system

kernel/drive.asm

@@ -46,9 +46,21 @@ dinit:		; copy previously set DPT to our data area
 
 		; int 13 stub
 		; place reserved for stack switches
-int13:		;int 3
+int13:		; debug output
+		push bx
+		push dx
+		push cx
+		push ax
+		call printf
+		db "int13 CALL AX=",2," CX=",2," DX=",2," BX=",2,0x0A,0x0D,0
+		; do the call
 		int 0x13
-		;int 3
+		jc .err
+		ret
+.err:		push ax
+		call printf
+		db "int13 ERR AX=",2,0x0A,0x0D,0
+		stc
 		ret
 
 		; restore DPT to default values if possible
@@ -156,7 +168,9 @@ read:		push ax
 		mov [dskseek+2], dx
 		; do the actual read
 
-		; set the operation code and back it up to the stack
+		; low level read and write
+		; call again to retry, no input registers
+		; read or write is configured in cx
 _read:		mov ch, 2
 		db 0x3D ; cmp ax, imm16: causes next instr to be skipped
 _write:		mov ch, 3