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Commodore 64
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(pictures copyright by M.A.Grundke)

Commodore 64 UK serial number (27 KBytes)

serial number:
U.K. B 2157824
MADE IN ENGLAND

ASSY 250425 board overview (70 KBytes)

ASSY 250425 board overview (often called 'version B' board):
this PCB is a cost-reduced variant of the older 'version A boards' (like ASSY 250407). The cost-reduction is accomplished by replacing some TTL-chips with a lone MOS 8701 clock generator (found under VIC-II's shielding). Now to the chips' locations in detail: the CIAs (U1,U2) are still below the user-port. The ROM bank (U3,U4,U5) and the CPU (U7) haven't changed locations as well. But next to the CPU, below the serial port, there's now SID (U18), and at its former location further below, the PLA (U17). The color RAM (U6) has also moved away from left to the PLA, down to the board's bottom. Other small changes are VIC-II's shielding (which is soldered to the board in this machine) and alignment (in this revision, for example, VIC-II is aligned horizontally, heading to the RAM-bank). Worth a note is also the 'cooling tongue' in VIC-II's shielding, which, together with some cooling paste, makes it an excellent heat spreader. Also worth mentioning is that the board is normally covered by an additional 'cardboard shielding' with an aluminium layer (you can see part of that below)

Commodore 64 UK board silkscreening part 1 (21 KBytes)

board silkscreening, part 1 (lower side of the board):
P/N 251470-01 REV.B
ASSY NO. 250425

Commodore 64 UK board silkscreening part 2 (28 KBytes)

board silkscreening, part 2:
(C)1984
COMMODORE INTL COMMODORE 64
ASSY-NO. 250425

Commodore 64 UK CPU (39 KBytes)

The MOS 6510 is a slightly modified MOS 6502, coming with an additional 8 bit I/O-port and 256 bytes SRAM integrated, but otherwise fully opcode compatible. In the C64, the additional features are used for address decoding and controlling the cassette port - using a standard 6502 would require additional logic in these architectures. Like its ancestor, the MOS 6510 has an 8 bit data- and 16 bit address-bus, allowing it to access a 64K address space. Although this is certainly good for an 8 bit CPU, it's not enough for the C64 - because of ROM and I/O areas, the memory map is significantly larger, and that's where address decoding is required (se PLA below). But there was a member of the 6500-family which could, due to an 20 bit address bus and integrated memory management, access up to 1 MB RAM: the MOS 6509 used in the CBM-II series. Also interesting, btw, is this CPU's production date, which is week 20 in 1985. By that time, production of the 6510's successor MOS 8500 had already begun (using Commodore's newer HMOS-2 production process)

Commodore 64 UK RAMs (29 KBytes)

The machine's RAM bank, 8 pcs. Motorola MCM6665BP20 (64Kx1 DRAM) for a total of 64 KBytes. In later machines, these RAMs were replaced by 64Kx4 DRAM, so that only 2 pcs. and less TTL were required, reducing overall production cost and power consumption. As mentioned earlier, the 6510 CPU's addressing range is 64 KBytes, which also includes other memory areas. Practically that means that e.g. ROMs always overlay parts of the RAM, so that address/ device selection is required to access all RAM areas (provided by PLA and CPU, essentially)

color RAM (63 KBytes)

The machine's 2114-type color SRAM (middle, in this case a Toshiba TMM314APL-3), is, as the name implies, used to store screen color data. Note that in comparison to the older board-revisions, the color RAM has changed its location. It once was next to the SID (which has also swapped location with the PLA), and now is located at the lower side of the board. Behind it, however, a SGS 4066BE quad analog switch, and a SN74LS373N octal transparent latch before it (part of the machine's TTL)

Commodore 64 UK ROMs (62 KBytes)

The C64's ROMs:
C901225-01 Char-ROM, C901227-03 Kernal-ROM Rev.3 and C901226-01 BASIC-ROM
The BASIC V2 used in all C64 was always a point of criticism, because other 8 bit architectures (like ATARI's XL/XE-series) had a faster BASIC with richer command set. As a result, there were several 3rd party 'BASIC extensions' for the C64, until Commodore released BASIC 3.5 (in the 264-series) to the public. It was the direct predecessor to the C128-series' BASIC 7.0, and featured lots of new commands, such as useful disk- and graphic-functions (even window drawing routines were implemented, which had already been part of the Commodore 64GS' BASIC V2 ROMs before). The latest BASIC V2 offspring was BASIC V10, which was used in the C64DX/ C65 prototypes and was never completely finished (well, at least to the current state of knowledge)

Commodore 64 UK shielded VIC & clock generator MOS 8701 (38 KBytes)

This machine's VIC-II is covered by shielding that's directly soldered to the board, and I don't want to risk damage by removing it, so no VIC-II photo this time. But because it's a PAL-B machine with NMOS-chipset it should be a MOS 6569 (see the ASSY 250407 C64 for a MOS 6569 picture). Behind it, however, the MOS 8701 clock generator. The MOS 8701 reduces production cost because it replaces some TTL ICs used on older boards (e.g. ASSY 250407). This is called 'improved system clock circuit design' in the C64 service manual. Also interesting is that the shielding actually serves as a heat spreader (with a sheet tongue and lots of cooling paste touching VIC's surface), so VIC-II seems to get quite warm upon operation

Commodore 64 UK SID (30 KBytes)

The MOS 6581 'SID' was probably the most powerful and flexible soundchip of its time. This, and VIC-II's capabilities like hardware sprites, made the C64 such a successful gaming platform. It's capabilities like 3 tone generators (with 4 possible waveforms each), 3 envelope generators and programmable filter make it a nicely versatile synthesizer and sound effects generator. There were essentially three different SID variants: the older MOS 6581 (like this one, requiring 12V supply), the newer MOS 6582 (now getting by with only 9V) and the latest MOS 8580 (which essentially is the HMOS-2 variant of MOS 6582). Also interesting on this ASSY 250425 PCB is that SID and PLA have swapped locations - the latter's now left from VIC-II, whereas SID now is located next to CPU and serial port

Commodore 64 UK CIAs (51 KBytes)

The C64's two MOS 6526 'CIA' ('complex interface adaptor'), responsible for most of the machine's I/O functions. In detail, they handle keyboard- and joystick I/O (U1 CIA), and CBM serial bus and userport respectively (U2 CIA). They offer two programmable 8 bit I/O ports (i.e. 16 I/O lines) - the 'bigger brothers', MOS 6523/ MOS 6525 ('tri port interface', used e.g. in the CBM-II series), even feature three ports (i.e. 24 I/O lines). Interestingly, these 8 bit CIAs are very similar to the 16 bit ones used in the AMIGA-series (CBM 8520) - in fact, they can be considered their direct predecessors. Before them, btw, the machine's keyboard-connector

Commodore 64 UK PLA (39 KBytes)

MOS 906114-01 is the machine's 'programmable logic array' (PLA) and used for address decoding and device selection. As mentioned above, this is necessary because of the CPU's limited address range - not all memory areas can be accessed directly, so PLA is used to select the appropriate area for the current operation (such as selecting a ROM area for read access, or a RAM area for read/ write access). Note the production date of this chip (week 29, 1992), seven years after this particular C64 was built. So the machine's original PLA must have been 'blown away' in the meantime. Note also the changes in board design compared to the older revisions: SID and PLA have swapped places in this ASSY 250425. As you can see on the picture, the color RAM that would normally reside there (before the PLA/ former SID-location), has also moved - now there's a TTL-circuit (here a MB74LS258 by Fujitsu)

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