GMA87 - Revision history

Enigma at 22:16, 28 August 2026

2026-08-28T22:16:44Z

2026-08-28T19:42:41Z

2026-08-28T19:39:17Z

2026-08-24T22:05:20Z

2026-08-24T21:12:19Z

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

@@ Line 35: / Line 35: @@
 GMA87 variants is only what happens to that measured byte afterward:
-# '''C64-side bulk decrypt'''
+# '''C64-side bulk decrypt''' (20 of 21 titles). The measured byte is sent to the C64 over a raster-line-synced <code>$DD00</code> handshake and used as a single repeating-byte XOR key over almost the entire game, decrypted in place in one pass before jumping straight into the result. This is the scheme's "standard" form, and the one this article's disassembly is drawn from directly (Cholo, the disk the algorithm was originally worked out against).
- (20 of 21 titles). The measured byte is sent
+# '''Drive-side per-sector decrypt''' ('''Eagles''' only, within this survey). The measured byte is instead baked as a literal <code>EOR #imm</code> operand into a second, freshly-uploaded drive-resident program, and the 1541's own 6502 decrypts every subsequent sector before it ever reaches the C64, streaming the result out over the same bit-banged serial link one byte at a time. Structurally identical, address-for-address, to the drive-side variant already documented for [[GMA86]]'s Firelord — this is an established technique in the same author's toolkit, not a one-off invention for Eagles.
 On any genuine failure — marker not found within the retry budget, or a

@@ Line 35: / Line 35: @@
 GMA87 variants is only what happens to that measured byte afterward:
-# '''C64-side bulk decrypt''' (20 of 21 titles). The measured byte is sent
+# '''C64-side bulk decrypt'''
    to the C64 over a raster-line-synced <code>$DD00</code> handshake and used
    as a single repeating-byte XOR key over almost the entire game, decrypted
@@ Line 42: / Line 43: @@
    from directly (Cholo, the disk the algorithm was originally worked out
    against).
-# '''Drive-side per-sector decrypt''' ('''Eagles''' only, within this
+# '''Drive-side per-sector decrypt''' ('''Eagles''' only, within this survey).
-  survey). The measured byte is instead baked as a literal <code>EOR #imm</code>
+ The measured byte is instead baked as a literal <code>EOR #imm</code>
    operand into a second, freshly-uploaded drive-resident program, and the
 's own 6502 decrypts every subsequent sector before it ever reaches
@@ Line 364: / Line 365: @@
 or applies the key itself, which is why this variant can't be cracked by
 watching the C64 decrypt a static block the way every other GMA87 title can.
 == Relationship to GMA85, GMA86, and GMA88 ==
@@ Line 455: / Line 406: @@
 * [[GMA86]]
 * [[GMA88]]

@@ Line 41: / Line 41: @@
 [[File:GMA87.png|thumb|none]]
-[[Eagles GMA87]] detailed description
+* [[Cholo GMA87]] detailed description
 ==== Pages that refer to this protection ====
 {{Special:Whatlinkshere/GMA87}}

@@ Line 40: / Line 40: @@
 [[File:GMA87.png|thumb|none]]
 ==== 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.