Programming:Unlocking ASIC

To unlock the ASIC, a 17-byte "unlock" sequence must be sent to the CRTC's selection port (&BC00) : RQ00, 0, 255, 119, 179, 81, 168, 212, 98, 57, 156, 70, 43, 21, 138, STATE, <ACQ>

• RQ00 must be different from the value 0.
• STATE=205 for UNLOCK otherwise another value for LOCK.
• ACQ represents sending any value if STATE=205 (not needed otherwise).

Note that the ASIC is already unlocked after the STATE phase, before ACQ.

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.

Z80 Assembler version

;; This example shows how to unlock the ASIC
;;
;; This example is designed for CPC+ only and will
;; not work on CPC or KC Compact.
;;
;; This example will compile with the MAXAM assembler
;; or the built-in assembler of WinAPE32.

org &8000

;;--------------------------------------------------
;; Unlock CPC+ additional features

di
ld b,&bc
ld hl,sequence
ld e,17
.seq 
ld a,(hl)
out (c),a
inc hl
dec e
jr nz,seq
ei
ret

;;----------------------------------------------------------
;; this is the sequence to unlock the ASIC extra features
.sequence
defb &ff,&00,&ff,&77,&b3,&51,&a8,&d4,&62,&39,&9c,&46,&2b,&15,&8a,&cd,&ee

Note: As one may see, the nybbles in the sequence are based on two 4bit shift registers. For one reason or another, Amstrad has patented the verification mechanism (GB2243701A). The patent seems to focus on verifying (rather than on sending) the sequence, so its legal use is a bit unclear.

BASIC version

1000 RESTORE 1010:FOR I=0 TO 16:READ A:OUT &BC00,A:NEXT I:RETURN
1010 DATA 255, 0, 255, 119, 179, 81, 168, 212, 98, 57, 156, 70, 43, 21, 138, 205, 238