Tuesday, 2 January 2018

A Journey Into Capcom's CPS2 Silicon - Part 2

Welcome to the second post in the Capcom CPS-2 reverse engineering series, if you missed the previous post you can find it here:

Inside the custom chips of CPS2

Capcom's Play System 2, also known as CPS2, was a new arcade platform introduced in 1993 and a firm call on bootlegging. Featuring similar but improved specs to its predecessor CPS1, the system introduced a new security architecture that gave Capcom for the first time a piracy-free platform. A fact that remained true for its main commercial lifespan and that even prevented projects like Mame from gaining proper emulation of the system for years.

Chip Makers

Capcom's extensive use of customs in CPS2 spreads over a total 11 QFP type chips, as part of this project each of the chips were decapped and identified as follows:

A board (Base board)

DL-0311: Ricoh A5C series, standard cells. (Also found in CPS1) Datasheet
DL-0921: Ricoh A5C series, standard cells. (Also found in CPS1) Datasheet
DL-1123: Hitachi HG62F series model 22, gate array. Datasheet
DL-1425: AT&T Digital Signal Processor WEDSP16A-M14. (Also found in CPS1.5) Datasheet
DL-1625: VLSI Technology (VTI) VGT300 series model 022, gate array. Datasheet
DL-2227: Hitachi HG62E series model 08, gate array. Datasheet

CPS2 A Board 93646A-3 Custom chips highlighted

B board (Top board)

DL-1525: Motorola H4C series model 057, gate array in combination with a 68000 cpu megacell (CPM68K REV7-89). Datasheet
DL-1727: Fujitsu CG24 series model 692, gate array. *
DL-1827: Fujitsu CG24 series model 692, gate array. *
DL-1927: Fujitsu CG24 series model 512, gate array. *
DL-2027: Fujitsu CG24 series model 512, gate array. *

* No datasheet available for the Fujitsu CG24 series, please share any.

CPS2 B Board 93646B-6 Custom chips highlighted

Gate Array technology

Used in most CPS2 custom chips, a gate array circuit is a prefabricated silicon chip circuit with no defined functionality, in which transistors, standard NAND or NOR logic gates, are placed following a regular pattern and manufactured on a wafer, this half baked wafer is known as master slice.

Common advantages of Gate Arrays designs over Full-Customs according to TU Delft:

Minimization of the fabrication time: Because the chips are prefabricated (the transistors are already on the master image), the silicon foundry only processes the masks related to metal wires. As compared to full custom layout, the number of masks processed by the silicon foundry is often reduced by more than 60%.

Minimization of the design time: The time involved in designing a cell layout is reduced dramatically (as compared to full-custom) because the transistors are pre placed on the image. Typically, it takes only a few minutes to layout a flipflop or a combinatorial gate, and the designer does not need to know much about the process design rules.

Minimization of the chip cost: The layout design starts with a prefabricated master image. This is a semi-manufactured article that can be produced in large quantities. Consequently, it can be cheap.

Gate Array die size and development time compassion versus other chip design technologies

The Fujitsu gate array chips featured in CPS2's B board belong to the CG24 series and use a 0.8 micron CMOS process. Fujitsu uses a block-level placement and routing scheme commonly known as "fishbone".

Markings inside CPS2 Gate Array chip DL-2027

Unwired section of NAND sea-of-gates inside a Fujitsu CG24 chip

Logic inverter (NOT) implemented in Fujitsu's NAND sea-of-gates

Fujitsu's gate array technology is discussed in more detail in 1978 USPTO patent 4,412,237: https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US4412237.pdf

Capcom's deep pockets

Interestingly enough, several of the B board's chips used by Capcom show a very low utilization of resources being the worst offender chip DL-2027. In IC density terms its contents could be classified as mostly empty space. 

Given the expensive nature of the end to end design and fabrication of these devices one must think that perhaps Capcom's market successes enabled the company not to spare in resources.

Highlighted in yellow: total die area utilization inside DL-2027 

The Mysterious CPU

Contrary to popular belief, Capcom's CPS-2 cpu does not reside on the A bottom board of the system, instead the cpu is found on the B board and inside the big 208 pin QFP chip labeled as DL-1525. MAME's own documentation on CPS-2 does not help this belief either as it also states the system cpu is DL-1625, an A board chip.

Capcom DL-1525 dated 1993 week 51 source id JSX02RJ524AU03

DL-1525 hosts inside a massive die measuring around 7x7mm in size featuring a majestic Motorola 68000 megacell core surrounded by a vast 3-layer gate array. This monster IC is based on the Motorola H4C gate array series and uses a gate length of 0.7 microns (700 nanometers). To date it is the smallest feature sized chip I have worked on since I began reverse engineering ICs.

DL-1525 is a Motorola H4C057 class gate array in combination with a 68k cpu core (top right)

Small section of DL-1525 captured at 50x magnification. Three routing metal layers are visible.

Cross-section view of a Motorola H4C gate array describing its composition

DL-1525 Ancestry

A newsletter from Dataquest from May 1988 traces back the origins of Motorola's blending of 68000 cores with gate arrays to the world of laser printers. An extract of such IC industry newsletter reads as follows: 
Motorola is designing gate-array-based interface chips for use in laser printers. The chips will contain a core of the 68000 microprocessor and the dedicated laser printer functions. The LPC-1 will have 5,000 gates and will be fabricated with a 2-micron CMOS technology, while the ALPC-1 will have 16,000 gates and will be the first commercial application of Motorola's HDC series of 1-micron CMOS channelless architecture gate arrays. The LPC-1 is currently available in sample quantities; samples of the ALPC-1 will be available in December, with volume production scheduled for February 1989.

In fact, additional research shows chips with similar source identification marks to Capcom's DL-1525 have been in use in commercial laser printers such as models A258/A259/A260 made by Ricoh. The following parts catalog mentions at least two relevant ICs listed as follows:

 JSC05RR519AU15   208QFP // RICOH IPU BOARD A259 5146 / A260 5146
 JSC05SV519AY17   240QFP // Ricoh main control board A258 5090

Another close brother to DL-1525 is Motorola's own MC68302 "Integrated Multiprotocol Processor" chip. This IC employs a similar gate array and embedding of a 68k cpu core inside. More details about it can be found in the following document and product manual.

MC68302 internals description found in "Image Processing For Future High Energy Physics Detectors"

Other chips from Motorola are known to exist with even closer source id numbers to Capcom's DL-1525, their purpose or end product usage are unknown: 

 JSX02RJ514AU17   208QFP // H4C057-68K 
 JSX02RJ524AU03   208QFP // Capcom CPS2 DL-1525
 JSX05PR511AW26  144QFP
 JSX05PR511AW27  No info
 JSX38PG511AJ03   No info

DL-1525 in the wild

Another interesting finding regarding DL-1525 was the availability of chip stock in Alibaba.com marketplace, during March of 2017 and to test the listing veracity I was able to successfully purchase brand new stock of JSX02RJ524AU03 from a Chinese reseller. At the time of writing of this blog post such stock seems to be still listed on sale online. This chip doesn't seem to be the only Capcom device being sold in the wild, other chip codes are available to purchase online. 

I guess this is of no commercial relevance to Capcom anymore, but overall it doesn't show great asset control practices.

Two NOS units of Capcom's DL-1525 chip sourced from China, chips dated 1998 week 24

This is all for now, I hope you have enjoyed Part 2 of the CPS2 reverse engineering series. On the next post we will explore how and where Capcom hided its CPS2 security implementation. Stay tuned.

Part 3