ATARI 800XL
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(pictures copyright by M.A.Grundke)
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| ATARI® ATARI 800XL
ATARI,INC.
MADE IN TAIWAN SERIAL NO.
72R3CG AT84199527
Interesting to see that even joysticks patents are mentioned there... |
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ATARI 800XL board overview:
not 'highly integrated', but after all, there's plenty of room in the machine... Beginning from the left, before power switch and DC-in at the machine's back, is the RAM bank (8 pcs. 64Kx1, U9-U16). Right from there some TTL ICs, the RF-modulator and the cartridge slot (with monitor connector behind it). Below RAM and modulator are the custom chips, from left to right: GTIA (U17), ANTIC (U7), CPU (U8, with some 74xx above it)
and PIA (U23). Above PIA is POKEY (U22) with the fixed-mounted keyboard connector (covering the two joystick ports here) to its right. Above POKEY, finally, the ROMs (U4, U5), and still above the SIO-connector. Note that the board is normally completely covered by a PCB shield (on the upper and lower side, like most ATARIs) |
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board silkscreening, part 1 (lower board side):
800XL
C061851 REV C
MADE IN HONGKONG
Below it, there's 0484 manually imprinted (most probably board production date) |
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board silkscreening, part 2 (upper board side):
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| The 800XL's CPU, 6502C (C014806-12). The 6502C is an ATARI-customized 6502, designed to cooperate with a second DMA-busmaster (ANTIC in the XL/XE-series).
ATARI 400 and 800 both had a standard 6502(B), which caused further cost for additional DMA-circuitry (well, at least in the XL-design). So essentially, the additional circuitry is required to coordinate the two busmasters (e.g. halt the CPU via its 'HALT-line' (pin 35), when ANTIC accesses data- or address-buses) |
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| C061618 selects parts of the XL/XE's memory-subsystem (cartridge ROM, OS/BASIC ROM, RAM bank) or its coprocessors (GTIA, POKEY or PIA) for memory operations (and thus is the memory management unit, of course). That's required because all these devices must be accessible within the first 64 KBytes of address space (which is all that a 6502 can address directly) |
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| The 800XL's RAM bank, consisting of eight pieces OKI M3764-15RS RAMs, 8 KBytes each. Since the 6502 can only address 64 KBytes, parts of the RAM are only accessible when one of the ROMs is disabled (if you disabled both, you had lots of memory - but to do what, without OS?). So those address selection operations are done by the MMU. By the way, the later 130XE (128K) came with a dedicated IC especially for bankswitching (C025953) |
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| ATARI XL OS Rev.B (C061598B-29, 128Kx8) and ATARI BASIC Rev.B (C060302A-29, 64Kx8). The older ATARI 400, 800 and 1200XL didn't have BASIC in ROM, which was a great disadvantage, of course, since you had to purchase a BASIC cartridge seperately. The OS REV.B, furthermore, contains parallel bus handlers for controlling the machines PBI.
By the way, the CA061854 imprinted below the ROMs is the ATARI part-no. for an '800XL NTSC U.S. Mother Board Populated' - seems to be a silkscreening they 'forgot' to remove when producing this PAL-board |
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| ANTIC (here a C014887-01) is a second microprocessor in the XL/XE-design, dedicated to generating its TV-output. It is DMA-capable and manages and processes the so-called display list, which contains the instructions for GTIA (see below). As mentioned earlier, ANTIC acts as a second busmaster besides the CPU (a very interesting design, btw), which requires some additional logic on the board |
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| The XL/XE GTIA is the actual 'television interface' and generates the output according to the inputted commands. It can be controller either by the CPU or the ANTIC, and is also responsible for other things such as collision detection, color-luminance control and player-missile graphics |
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| The machine's 30 pin cartridge connector. Software on cartridge was common in the early 80s, but then more and more pushed aside by cassette tapes and diskettes - ironic, since software companies complained about too much illegal copies soon after... However, for the last of the XL-series, the XE SYSTEM, cartridges were intended to be the only media |
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| POKEY (C012294B-01) is a multi-function chip that generates the 800XL's sound (with four 8 bit/ two 16 bit audio channels and indepent volume- and noise-control for each), includes a keyboard scanner (six key scan-lines, two sense-lines), controls the serial I/O (SIO)-port, provides three timers, a random-number generator, 8 potentiometer inputs, and an IRQ line. Whow! |
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| The P6520A 'Peripheral Interface Adaptor' (PIA, C014795) is the interface to the machines's controller ports as well as the PBI. It is an ancestor to the 6522 'VIA' and 6526 'CIA' which were often used in Commodore-computers (the 6520 was used in some PETs as well). Later machines used a 68B21 instead |
See the CBM 3016 page for MOS 6502-picture, ancestor of the 6502C!