[romprefix] If we hook INT 19, prompt before attempting boot

On non-BBS systems we hook INT 19, since there is no other way we can
guarantee gaining control of the flow of execution.  If we end up
doing this, prompt the user before attempting boot, since forcibly
capturing INT 19 is rather antisocial.

src/arch/i386/prefix/romprefix.S

 	cld
 	pushw	%cs
 	popw	%ds
-	pushw	$0x40
-	popw	%fs
 
 	/* Shuffle some registers around.  We need %di available for
 	 * the print_xxx functions, and in a register that's
 	movw	$init_message_prompt, %si
 	xorw	%di, %di
 	call	print_message
-	/* Empty the keyboard buffer before waiting for input */
-empty_keyboard_buffer:
-	movb	$0x01, %ah
-	int	$0x16
-	jz	1f
-	xorw	%ax, %ax
-	int	$0x16
-	jmp	empty_keyboard_buffer
-1:	/* Wait for up to 3s for a key press */
-	movw	$(18 * 3), %cx	/* Approx 3s worth of timer ticks */
-wait_for_key:
-	decw	%cx
-	jz	no_key_pressed
-	/* Wait for timer tick to be updated */
-	movl	%fs:(0x6c), %eax
-1:	pushf
-	sti
-	hlt
-	popf
-	cmpl	%fs:(0x6c), %eax
-	je	1b
-	/* Check to see if a key was pressed */
-	movb	$0x01, %ah
-	int	$0x16
-	jz	wait_for_key
-	/* Check to see if key was Ctrl-B */
-	cmpb	$0x02, %al
-	je	1f
-	/* Key was not Ctrl-B: remove from buffer and stop waiting */
-	xorw	%ax, %ax
-	int	$0x16
-	jmp	no_key_pressed
-1:	/* Key was Ctrl-B: leave in keyboard buffer and invoke gPXE.
-	 * The keypress will be picked up by the initial shell
-	 * prompt, and we will drop into a shell.
+	/* Wait for Ctrl-B */
+	movw	$0xff02, %bx
+	call	wait_for_key
+	jnz	1f
+	/* Ctrl-B was pressed: invoke gPXE.  The keypress will be
+	 * picked up by the initial shell prompt, and we will drop
+	 * into a shell.
 	 */
 	pushw	%cs
 	call	exec
-no_key_pressed:
-
+1:
 	/* Print blank lines to terminate messages */
 	movw	$init_message_end, %si
 	xorw	%di, %di
 /* INT19 entry point
  *
  * Called via the hooked INT 19 if we detected a non-PnP BIOS.  We
- * attempt to return via the original INT 19 vector (if we were able to
- * store it).
+ * attempt to return via the original INT 19 vector (if we were able
+ * to store it).
  */
 int19_entry:
 	pushw	%cs
+	popw	%ds
+	/* Prompt user to press B to boot */
+	movw	$int19_message_prompt, %si
+	xorw	%di, %di
+	call	print_message
+	movw	$prodstr, %si
+	call	print_message
+	movw	$int19_message_dots, %si
+	call	print_message
+	movw	$0xdf42, %bx
+	call	wait_for_key
+	jnz	1f
+	/* Leave keypress in buffer and start gPXE.  The keypress will
+	 * cause the usual initial Ctrl-B prompt to be skipped.
+	 */
+	pushw	%cs
 	call	exec
+1:	/* Print blank lines to terminate messages */
+	movw	$int19_message_end, %si
+	xorw	%di, %di
+	call	print_message
+	/* Try to call original INT 19 vector */
 	movl	%cs:orig_int19, %eax
 	testl	%eax, %eax
-	je	1f
-	/* Chain to original INT 19 vector */
+	je	2f
 	ljmp	*%cs:orig_int19
-1:	/* No chained vector: issue INT 18 as a last resort */
+2:	/* No chained vector: issue INT 18 as a last resort */
 	int	$0x18
 	.size	int19_entry, . - int19_entry
 orig_int19:
 	.long	0
 	.size	orig_int19, . - orig_int19
 
+int19_message_prompt:
+	.asciz	"Press B to boot from "
+	.size	int19_message_prompt, . - int19_message_prompt
+int19_message_dots:
+	.asciz	"..."
+	.size	int19_message_dots, . - int19_message_dots
+int19_message_end:
+	.asciz	"\n\n\n"
+	.size	int19_message_end, . - int19_message_end
+	
 /* Execute as a boot device
  *
  */
 	popl	%esi
 	lret
 	.size undiloader, . - undiloader
+
+/* Wait for key press specified by %bl (masked by %bh)
+ *
+ * Used by init and INT19 code when prompting user.  If the specified
+ * key is pressed, it is left in the keyboard buffer.
+ *
+ * Returns with ZF set iff specified key is pressed.
+ */
+wait_for_key:
+	/* Preserve registers */
+	pushw	%cx
+	pushw	%ax
+1:	/* Empty the keyboard buffer before waiting for input */
+	movb	$0x01, %ah
+	int	$0x16
+	jz	2f
+	xorw	%ax, %ax
+	int	$0x16
+	jmp	1b
+2:	/* Wait for up to 5s for a key press */
+	movw	$(18 * 5), %cx	/* Approx 5s worth of timer ticks */
+3:	decw	%cx
+	js	99f		/* Exit with ZF clear */
+	/* Wait for timer tick to be updated */
+	call	wait_for_tick
+	/* Check to see if a key was pressed */
+	movb	$0x01, %ah
+	int	$0x16
+	jz	3b
+	/* Check to see if key was the specified key */
+	andb	%bh, %al
+	cmpb	%al, %bl
+	je	99f		/* Exit with ZF set */
+	/* Not the specified key: remove from buffer and stop waiting */
+	pushfw
+	xorw	%ax, %ax
+	int	$0x16
+	popfw			/* Exit with ZF clear */
+99:	/* Restore registers and return */
+	popw	%ax
+	popw	%cx
+	ret
+	.size wait_for_key, . - wait_for_key
+
+/* Wait for timer tick
+ *
+ * Used by wait_for_key
+ */
+wait_for_tick:
+	pushl	%eax
+	pushw	%fs
+	movw	$0x40, %ax
+	movw	%ax, %fs
+	movl	%fs:(0x6c), %eax
+1:	pushf
+	sti
+	hlt
+	popf
+	cmpl	%fs:(0x6c), %eax
+	je	1b
+	popw	%fs
+	popl	%eax
+	ret
+	.size wait_for_tick, . - wait_for_tick