Once the ASIC is unlocked, we get access to a new [[Gate Array]] register called RMR2. It is accessible in the same way as other Gate Array registers.
Locking the ASIC again doesn't disable any of its functionality, it just prevents you changing it. [https://www.cpcwiki.eu/forum/programming/asm-source-code/msg249856/#msg249856 Source]
<br>
ld hl,sequence
ld e,17
.seq
ld a,(hl)
dec e
jr nz,seq
ei
ret
<br>
= Optimized Z80 Assembler version versions =
The unlocking sequence can be reconstituted from simple bit operations instead of being stored in memory.
This allowed == [[Madram]] to create this optimized unlock routine:version == It still uses some magic numbers.
<pre>
UnlockAsicdi
ld bc,#BCFF
out (c),c
out (c),0
ld hl,%1001000011101010
.loop:
out (c),c
ld a,h:rlca:ld h,l:ld l,a
srl c:res 3,cxor c:and #88or :xor c
ld c,a
cp #4D
jr nz,.loop ld a,#CD ; a = #CD for unlock, another value for lockout (c),a : out (c),aei
ret
</pre>
== [[Urusergi]] version == No magic numbers here. <pre>di ; v3.1 -> 30 bytes!ld bc,#BCFFout (c),cout (c),0 ; db #ED,#71ld a,c loop:out (c),ald h,a ; h = 7654 3210add hl,hl ; h = 6543 210*rra ; a = 7765 4321add hl,hl ; h = 5432 10**xor h:and #F7:xor h ; a = 7765 1321ld l,a ; l = 7765 1321ld a,h ; a = 5432 10**rla ; a = 4321 0***and #88:xor l ; a = (7 xor 4)765 (1 xor 0)321cp cjr nz,loop eiret</pre> == [[Longshot]] version == Interleave keys here28 bytes - 349 nops <pre>Asic_Unlock ld de,#533e ; interleave key 32 bits ld hl,#849 ; interleave key (#841 to lock)ld a,#ffld bc,#bcef ; crtc input io address and 1st nibblesout (c),a ; RQ00 <>0out (c),0 ; defb #ed,#71 ; Send 00 (out (c),0Asic_Unlock_Loop:out (c),a ; Send 2 nibblesxor h ; Put bit 12 of the key in the bit 4 of A for high nibbleand c ; erase bit 4 to put key bit insteadxor hadc hl,hl ; Extract left incoming bit of high nibble of interleave keyret z ; exit when key is empty (extra value sent to io address)ex de,hl ; Switch the interleave key (add hl,hl is good for you) rra ; Bit 7 high nibble=Cf and rotate the complete byteadd hl,hl ; Extract left incoming bit of low nibble of interleave key jr Asic_Unlock_Loop ; Send at last 18 values to crtc input io address</pre> == Algorithm == <pre>def unlock_asic(): b, c = 0xBC, 0xFF out(b, c) out(b, 0) a = c while True: out(b, a) # a = (7 xor 4)765 (1 xor 0)321 a = ((a >> 1) & 0x77) | ((a ^ (a << 3)) & 0x80) | (((a << 2) ^ (a << 3)) & 0x08) if a == c: break def out(port, value): print(f"Port: {hex(port)}xx Out: {hex(value)}") unlock_asic()</pre> <br> =Visual representation==
As one may see, the nybbles in the sequence are based on two 4bit shift registers.
=[[File:AsicUnlockSequence.png]] Visual by [[Hwikaa]] <br> =Patent==
For one reason or another, Amstrad has patented the verification mechanism ([[Media:Patent GB2243701A.pdf|GB2243701A]]). The patent seems to focus on ''verifying'' (rather than on ''sending'') the sequence, so its legal use is a bit unclear.
On the [[Original Arnold V Specs]] - Issue 1.5 - 10th April 1990, it is precised at §2.11 "Locking of enhanced features":
<code>
it should be noted that unauthorised use of this mechanism may infringe Amstrad's patent.
</code>
According to [https://patents.google.com/patent/GB2243701A/en Google patents for GB2243701A] the patent was withdrawn on 1994-12-21. This means that this particular patent cannot be enforced.
<br>
[[Category:Programming]]
[[Category:CPC Plus]]