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Commodore 64 (ASSY 250407)

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Commodore 64
Additional pictures. Note that loading the page could take a while...
(pictures copyright by M.A.Grundke)

serial number: WG A408473 The 'WG' in the serial number indicates that it was produced in Western Germany (you probably guessed that because it's also written in the next line below...)

Board overview ASSY 250407 (often called 'version A'-board): The CIAs (U1, U2) are left beneath the user-port. Below them, the RAM bank, consisting of 8 pcs. Micron Semiconductor MT4264-20 (locations U9-U12 and U21-U24), together with some TTL and two RAM multiplexers at U13 and U25. Right from the CIAs are the ROMs (U3, U4, U5), the CPU (U7) and the PLA (U17). Below the PLA, you can see SID (socketed at U8), with the color RAM (National Semiconductor MM2114N-3, location U6) to its left. The right side of the board holds the RF-modulator and, right from that, the cartridge interface (top). Below them you can spot VIC-II, i.e. its shielding (see below for a VIC-II picture). And, probably most important: this board revision was the first to make use of 8 pin video, which all later machines have. Its predecessors, ASSY 326106 and ASSY 326298-01 and their revisions came with 5 pin video

board silkscreening, part 1 (lower side of the board): FAB NO.250406 REV.A ASSY NO.250407

board silkscreening, part 2: (C)1983 COMMODORE INTL COMMODORE 64 ASSY-NO. 250407

board silkscreening, part 3: ARTWORK NO. 251137 REV. B (partly covered by the voltage regulator's large heatsink)

The MOS 6510 is fully compatible with its ancestor, the MOS 6502, and is also produced in NMOS-process. In later C64s it was replaced by the HMOS-2 produced MOS 8500, which maintains full compatibility, of course. Both were the most common members of the popular MOS 6500-family, which was conceived by large parts of the Motorola 6800 development team in 1975. Their first CPU, the MOS 6501, was mothballed almost immediately after it was created. Why? Because it was abolutely pin-compatible with Motorola's bestseller MC6800, so Motorola threatened MOS Technology with a lawsuit. The result was creation of the MOS 6502 - almost identical to its forerunner, except for having a new pinout. Generally, the 650x family members are 8 bit CPUs, but with variable address-buses and different features - the rare MOS 6504 (used in the CHESSmate), for example, only has 13 address lines for 8K addressable memory. Compared to the MOS 6502, the MOS 6510 adds 256 bytes on-chip SRAM and an 8 bit bi-directional I/O port, used mainly for the cassette interface

The machine's RAM bank, consisting of 8 pcs. Micron Semiconductor MT4264-20 (64Kx1). Later machines (like C64C/ C64G) came with 2 pcs. 64Kx4 instead, making some TTL ICs obsolete and thus reducing overall production cost. Memory expansion was unnecessary (a RAM disk like in GEOS is probably the only sensible use for more than 64 KBytes), but possible with additional bankswitching logic

The C64s ROMs: C901225-01 character generator ROM, C901227-03 CBM-Kernal Rev.3 and C901226-01 BASIC. Interestingly, this ROM set has been used in many different C64-related machines over the time, such as Commodore 128 series and the Commodore 65/ 64 DX prototype (to maintain C64 compatibility in both cases), the Commodore MAX machine (in the form of an external Mini BASIC cartridge), the portable SX-64 (without tape routines and different startup colors) and the CBM 4064 aka 'Educator PET' (which was a C64 in a PET-case - the color version used an unmodified C64 ROM set, the version with monochrome monitor had different startup colors). Of course in all machines without C64 compatibility mode (i.e. C128 and C65), the startup messages was also different

VIC-II (MOS 6569R3 at U19, among other video logic and within its own shielding) is an enhanced version of the VC-20's VIC; its main improvement is the capability of creating hardware sprites, which were also called 'movable image blocks' (MIBs), MOBs ('movable object blocks') or 'movable display objects'. Another improvement was 40 column text - the VC-20 could only display 22 columns, which wasn't enough for the professional markets these days. By the way, there have also been prototypes of an improved 40 column-capable VIC, which were called 'VIC-40'. However, that new VIC-I was never officially released - there was just no need, because the VIC-30 (i.e. C64) was launched shortly after its development

The MOS 6581 ('SID'), is the C64-series' soundchip. Some of its impressive features are three independant voices, four waveforms and an envelope generator. Many say it was clearly the best soundchip of its time, and if you ever heard sound masterpieces by Chris Hülsbeck or Rob Hubbard, you'll know they're right! The ill-fated Commodore 65/ 64DX-prototypes, by the way, even had two SIDs for stereo-output. Before it, however, you can see the machines color RAM, a National Semiconductor MM2114N-3 (1Kx4 SRAM), which is used for storing screen color data

The two 'complex interface adaptors' (CIAs), MOS 6526 at locations U1 and U2. As the designation implies, they were specifically designed for the 6500-family bus; they are successors to the PIAs and VIAs used in older Commodore computers. They offer 16 programmable I/O lines (i.e. two 8 bit ports) for peripheral control and general I/O operations, as well as timer functions. In the C64 design, the CIAs always run at 1 MHz, even if a 2 MHz capable version (MOS 6526A) is used. Generally, the CIA at U1 is mainly responsible for joystick- and keyboard-I/O, whereas U2 controls serial I/O and userport. Before them, the keyboard-connector with some pins connected directly to the U1 CIA

The 'Programmable Logic Array' (MOS 906114-01) located at U17. It's a gate array used for I/O address decoding and device selection, which is necessary because the MOS 6510 only has 16 I/O lines - not enough to address all devices directly. The PLA acts like a redirector so that ROM, RAM or device memory areas can be selected for I/O operations. However, since the PLA is not a standard part but especially configured for the C64 architecture, you need another C64 PLA to replace it in case of defect (which happened quite often, btw)

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Commodore 64 (ASSY 250407)

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