PRO-CT0 is an early gate array chip used as:

A sprite graphics multiplexer on AES carts and MVS boards, sometimes named ALPHA-8921, later replaced by NEO-ZMC2 and then NEO-CMC.
Security device in Super Sidekicks and Fatal Fury 2 AES and MVS cartridges, also named SNK-9201.

The device probably wasn't intended to be used as as security measure at first.

ALPHA-8921 was the original chip name, from when Alpha Denshi used it on their Alpha68k system.

Pinouts

Sprite graphics mux

Security device

the 68k bus when /PORTOEL goes low ($200000-$2FFFFF odd byte read).

Internal logic

Kyuusaku's Verilog definition: File:Ct0.zip

furrtek's version:

SNK-9201 chip found in a Super Sidekicks MVS cartridge. Picture courtesy of [MVS-Scans].

Used on PROG-G2 boards. It is found in Fatal Fury 2 and Super Sidekicks only. It was SNK's first attempt at protecting NeoGeo games.

The chip is tied to the 68k address and data bus, listens to word writes and replies on odd addresses in the $200000-$2FFFFF address range.

The game program expects the chip to reply with specific data depending on the values written to it and will freeze if they are wrong.

Address bus mapping:

A19	A18	A17	A16	A15	A14	A13	A12	A11	A10	A9	A8	A7	A6	A5	A4	A3	A2	/PORTWEL	A1
C31	C29	C27	C25	C30	C28	C26	C24	C23	C21	C19	C17	C22	C20	C18	C16	H	EVEN	12M	LOAD

Data bus mapping:

D15	D14	D13	D12	D11	D10	D9	D8	D7	D6	D5	D4	D3	D2	D1	D0
C15	C13	C11	C9	C14	C12	C10	C8	C7	C5	C3	C1	C6	C4	C2	C0

For specific details see Fatal Fury 2 and Super Sidekicks.

MAME's HLE emulation (fatfury2_protection): [[1]]

Reads:

Writes:

To $211112, with data = $1111: dataout = $FF000000
To $233332, with data = $3333: dataout = $0000FFFF;
To $244442, with data = $4444: dataout = $00FF0000;
To $255552, with data = $5555, read back from $255551, $2FFFF1, $200001, $2FF001: dataout = $FF00FF00
To $256782, with data = $1234, read back from $36000 or $36004: dataout = $F05A3601
To $242812, with data = $1824, read back from $36008 or $3600C: dataout = $81422418
To $255551, $2FFFF1, $2FF001, $236001, $236005, $236009, $23600D, with data = $20: shift dataout left one byte

D15	D14	D13	D12	D11	D10	D9	D8	D7	D6	D5	D4	D3	D2	D1	D0
C15	C13	C11	C9	C14	C12	C10	C8	C7	C5	C3	C1	C6	C4	C2	C0

D15	D14	D13	D12	D11	D10	D9	D8	D7	D6	D5	D4	D3	D2	D1	D0
C15	C13	C11	C9	C14	C12	C10	C8	C7	C5	C3	C1	C6	C4	C2	C0

D15	D14	D13	D12	D11	D10	D9	D8	D7	D6	D5	D4	D3	D2	D1	D0
C15	C13	C11	C9	C14	C12	C10	C8	C7	C5	C3	C1	C6	C4	C2	C0