[zbin] Allow decompressor to generate debug output via BIOS console

The 0xe9 debug port exists only on virtual machines.  Provide an
option to print debug output on the BIOS console, to allow for
debugging on real hardware.

Note that this option can be used only if the decompressor is called
in flat real mode; the easiest way to achieve this is to build with
DEBUG=libprefix.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/unlzma.S

 #endif /* CODE16 */
 
 /****************************************************************************
- * Debugging (via 0xe9 debug port)
+ * Debugging
  ****************************************************************************
+ *
+ * This code will usually run in 16-bit protected mode, in which case
+ * only the 0xe9 debug port (present on some virtual machines) can be
+ * used.
+ *
+ * To debug on real hardware, build with DEBUG=libprefix.  This will
+ * cause this code to be called in flat real mode, and so DEBUG_INT10
+ * may be used.
  */
 
-#define DEBUG 0
+/* Enable debugging via 0xe9 debug port */
+#define DEBUG_E9 0
 
-#if DEBUG
-	.macro	print_character, char
+/* Enable debugging via BIOS INT 10 (works only when in flat real mode) */
+#define DEBUG_INT10 0
+
+#if ( DEBUG_E9 || DEBUG_INT10 )
+	.macro	print_character, reg
+	pushfl
 	pushw	%ax
-	movb 	$\char, %al
+	pushw	%bx
+	pushw	%bp
+	movb 	\reg, %al
+	movw	$0x0007, %bx
+	movb	$0x0e, %ah
+#if DEBUG_E9
 	outb	%al, $0xe9
+#endif
+#if DEBUG_INT10
+	cmpb	$('\n'), %al
+	jne	L\@
+	int	$0x10
+	movb	$('\r'), %al
+L\@:	int	$0x10
+#endif
+	popw	%bp
+	popw	%bx
 	popw	%ax
+	popfl
 	.endm
 
 	.macro	print_hex_nibble
+	pushfl
+	pushw	%ax
 	cmpb	$10, %al
 	sbb	$0x69, %al
 	das
-	outb	%al, $0xe9
+	print_character %al
+	popw	%ax
+	popfl
 	.endm
 
 	.macro	print_hex_byte, reg
  * Returns:
  *   %ds:%esi : compressed input data pointer (possibly updated)
  *   %eax : current range
- * Corrupts:
- *   %eax
  *****************************************************************************
  */
 rc_normalise:
 	/* Store output byte */
 	ADDR32 stosb
 	print_hex_byte %al
-	print_character ' '
+	print_character $(' ')
 	/* Update LZMA state */
 	subb	$3, %dl
 	jns	1f
  *****************************************************************************
  */
 match:	/* Update repeated match list */
-	print_character '['
+	print_character $('[')
 	movl	$3, %ecx
 	jmp	1f
 match_rep:
-	print_character '['
-	print_character 'R'
+	print_character $('[')
+	print_character $('R')
 	print_hex_byte %cl
-	print_character '='
+	print_character $('=')
 	movzbl	%cl, %ecx
 	movl	reps(%ebp,%ecx,4), %eax
 	jcxz	2f
 	/* Get stored match length */
 	movzwl	len(%ebp), %ecx
 	print_hex_dword	%eax
-	print_character '+'
+	print_character $('+')
 	print_hex_word %cx
-	print_character ']'
-	print_character ' '
+	print_character $(']')
+	print_character $(' ')
 	/* Abort with CF set if match distance is out of range */
 	movl	out_start(%ebp), %esi
 	negl	%esi
 	/* Initialise remaining parameters */
 	movl	%esi, in_start(%ebp)
 	movl	%edi, out_start(%ebp)
-	print_character '\n'
+	print_character $('\n')
 	ADDR32 lodsb	/* discard initial byte */
 	print_hex_byte %al
 	ADDR32 lodsl
 	bswapl	%eax
 	print_hex_dword %eax
-	print_character '\n'
+	print_character $('\n')
 	movl	%eax, rc_code(%ebp)
 	decl	rc_range(%ebp)
 	movl	$STATE_LIT_LIT, %edx
 	call	lzma_decode
 	jnc	1b
 	call	rc_normalise
-	print_character '\n'
+	print_character $('\n')
 	/* Undo BCJ filter */
 	pushl	%esi
 	movl	out_start(%ebp), %esi