GMA87 - Revision history

Enigma at 00:55, 21 September 2022

2022-09-21T00:55:17Z

2022-09-21T00:54:43Z

2022-09-21T00:54:21Z

2022-09-21T00:54:11Z

2022-09-21T00:53:05Z

2022-09-21T00:52:15Z

2022-09-21T00:51:37Z

2022-09-21T00:50:39Z

2020-08-19T18:56:58Z

2019-10-27T18:16:48Z

Created page with "Descriptions needs to be done. ==== Pages that refer to this protection ==== {{Special:Whatlinkshere/GMA87}}"

New page

Descriptions needs to be done.

==== Pages that refer to this protection ====

{{Special:Whatlinkshere/GMA87}}

@@ Line 14: / Line 14: @@
 if a sync length is longer than the second length a 1 is shifted in a byte<br>
 if a sync length is shorter than the second length a 0 is shifted in a byte.
 GMA uses a single byte as EOR decode code that is determined on the basis of sync length on track 38.
+===== Example from Exolon =====
 We know that the GMA byte decode code is 4F -> 01001111

@@ Line 11: / Line 11: @@
 The second sync length is used as reference length.
-The following 8 sync lengths are compared to the second sync length,
+The following 8 sync lengths are compared to the second sync length<br>
+if a sync length is longer than the second length a 1 is shifted in a byte<br>
 if a sync length is shorter than the second length a 0 is shifted in a byte.

← Older revision		Revision as of 00:54, 21 September 2022
Line 14:		Line 14:

	if a sync length is longer than the second length a 1 is shifted in a byte		if a sync length is longer than the second length a 1 is shifted in a byte
−
	if a sync length is shorter than the second length a 0 is shifted in a byte.		if a sync length is shorter than the second length a 0 is shifted in a byte.

@@ Line 6: / Line 6: @@
 ===== Logic =====
 Starting from next sync 10 sync lengths are determined by counting on X register.
 The second sync length is used as reference length.

@@ Line 1: / Line 1: @@
 ==== Description ====
-On Track 38 look for code of raw bytes after a sync: 69 XX XX XX A9
+On Track 38 look for signature of raw bytes after a sync: 69 XX XX XX A9
 Should occur only once per track.

@@ Line 26: / Line 26: @@
 First the signature bytes:
-sync 42
+ sync 42
     ; Following raw bytes:  69 57 57 a9 a9 55 55 55 55 55 55 55 55 55 55 aa ff

@@ Line 40: / Line 40: @@
     sync 136  <- This is longer -> 1
     sync 136  <- This is longer -> 1
-    result of 0 bits: 01001111 -> 4F
+    result of 8 bits after shift: 01001111 -> 4F
 [[File:GMA87.png|thumb|none]]

← Older revision		Revision as of 00:50, 21 September 2022
Line 1:		Line 1:
−	~~Descriptions needs~~ to ~~be done~~.	+	==== Description ====
		+	On Track 38 look for code of raw bytes after a sync: 69 XX XX XX A9
		+
		+	Should occur only once per track.
		+
		+	===== Logic =====
		+	Starting from next sync 10 sync lengths are determined by counting on X register.
		+
		+	The second sync length is used as reference length.
		+
		+	The following 8 sync lengths are compared to the second sync length,
		+
		+	if a sync length is longer than the second length a 1 is shifted in a byte
		+
		+	if a sync length is shorter than the second length a 0 is shifted in a byte.
		+
		+
		+	GMA uses a single byte as EOR decode code that is determined on the basis of sync length on track 38.
		+
		+	Example from Exolon:
		+
		+	We know that the GMA byte decode code is 4F -> 01001111
		+
		+	On Track 38 we see in G64Conv text format of the track:
		+
		+	First the signature bytes:
		+
		+	sync 42
		+	; Following raw bytes: 69 57 57 a9 a9 55 55 55 55 55 55 55 55 55 55 aa ff
		+
		+	Then the following syncs appear (raw bytes after syncs omitted):
		+	sync 39
		+	sync 87 <- This is the reference sync length
		+	sync 40 <- This is shorter -> 0
		+	sync 136 <- This is longer -> 1
		+	sync 40 <- This is shorter -> 0
		+	sync 40 <- This is shorter -> 0
		+	sync 136 <- This is longer -> 1
		+	sync 136 <- This is longer -> 1
		+	sync 136 <- This is longer -> 1
		+	sync 136 <- This is longer -> 1
		+	result of 0 bits: 01001111 -> 4F

	[[File:GMA87.png\|thumb\|none]]		[[File:GMA87.png\|thumb\|none]]

@@ Line 1: / Line 1: @@
 Descriptions needs to be done.
 ==== Pages that refer to this protection ====
 {{Special:Whatlinkshere/GMA87}}