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I'm remembering a difference in the design of Commodore computers like the Vic and 64, versus the Atari 8-bits, and game consoles from the likes of Atari and Nintendo: they all had cartridge slots, but Commodore put them at the back instead of on top. From a technical perspective this was a minor difference, but it may have had consequences: one of the answers to Why did Commodore 64 cartridge games disappear? says

I used multiple Commodore 64s in the past, and I do not remember any of them ever having a cartridge slot. If any of them did, I never noticed them nor used them, and nobody I knew ever even mentioned anything about cartridges on their C64s.

Out of sight, out of mind?

Now I'm wondering why Commodore made that design decision. Aesthetics is always one possibility, but there is another that occurs to me: maybe it's just that it was a few cents cheaper to build that way. The company was certainly very focused on cost reduction.

Was it in fact cheaper to put the cartridge slot at the back?

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    A larger case would have been required for a larger main board, and a socket for connecting the vertical cartridge with the horizontal motherboard. The C64 slot was just there in the back: you horizontally inserted the cartridge into the horizontal motherboard. Much more efficient.
    – RonJohn
    Jul 21 at 16:08
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    Yeah I think it's the vertical slot that made it expensive. If the slot's parallel with the main board, it can be cheaper. For example, the ZX Spectrum had an expansion port (could have been used for cartridges), and that was very inexpensively just printed onto the circuit board. The cost of the mating connector is then pushed to the cartridge.
    – Omar and Lorraine
    Jul 21 at 16:10
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    It's not just "horizontal vs. vertical": Comparing the relatively simple C64 cartridge slot to the Atari 400/800 cartridge slot that was apparently built using WWII tank technology involving a big chunk of solid aluminium and had a pretty elaborate lid mechanism maybe isn't quite fair and might result not in "a few cents" difference in cost, but rather $$$. There must be a reason the XLs had a much more simple design (but still was vertical).
    – tofro
    Jul 21 at 17:19
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    @tofro Top loading is (almost) mandatory for game consoles which the Atari are descendants of. The heavy shielding of the 400/800 was due the very strict 1977 EM rules, which got relaxed by the time the XL was designed.
    – Raffzahn
    Jul 21 at 21:11
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    It is interesting that only top and back loading is discussed here. Why front loading is so uncommon? I think that it could be even more user-friendly – see, for example, many VHS video recorders.
    – jiwopene
    Jul 22 at 11:00

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Maybe it's just that it was a few cents cheaper to build that way. The company was certainly very focused on cost reduction.

Yes, it was. Not much, but there are several small savings:

  • The board needs no additional support as forces for insert and removal go along the board's orientation.

  • Simpler case structure as there is no need for another hole on top with opening being just a cutout at a fringe.

  • Any top opening would need some kind of lid which is at least one more part, if not two and an additional spring.

In addition it leaves more freedom in manufacturing, as it doesn't mater much if misaligned a bit.

But there is also a Case of Usability and History:

In case of Atari it's also a matter of heritage: The VCS was already a top loader. For a game console it's mandatory that the cartridge can be inserted and removed while sitting in some TV furniture where it's usually less possible to access the back. Same reason why next to all others from NES to MSX had them as top loader.

In contrast, the VIC20's port was from the start meant less as a module than as an expansion port. Of course, a single one could be always plugged in, but Commodore expected users to get a 1020 expansion unit. With a top slot the VIC20 would have needed another expansion bus connector. Making that only one and on the back served both use cases and saved the money of a second, more expensive one.

The C64 in turn was a quick-shot design based in many ways, especially mechanical, on the VIC20, thus using the same design for its cartridge/expansion board.

Notable here is that the later Atari XL series added an expansion bus at the back, so offering both: Expansion on the back (notably for the 1090 interface box) and modules on top.

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    I'm not a gamer: I've never touched an Atari and the last Commodore I had anything to do with was the PET 2001. However from an engineering POV and to get this into the record, it's worth remarking that photos show that both the Atari and Commodore used simple PCBs with edge connector "fingers" for the cartridge with the corresponding (relatively expensive) receptacle inside the main unit. Hence in terms of manufacturing cost, the cartridge PCBs etc. should have been comparable, and if there was a significant retail price difference it was either because of the shell or royalty policies etc.
    – Mark Morgan Lloyd
    Jul 22 at 11:48
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    The VIC-20 cartridge connector was quite wide, and IIRC the connectors were on .15625" centers rather than 0.1". I suspect the size was chosen to allow a cartridge with eight 2Kx8 SRAM chips and chip-select logic to fit in the same size housing as all the other cartridges, and having the edge connector be as long as the board was better than having a short edge connector in the middle of a longer board. A minimalist VIC-20 cart would have needed a fair bit more board surface area than a minimalist Atari 2600 cart.
    – supercat
    Jul 22 at 18:25
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    @supercat you're putting way too much afterthought into this. I'd say the connector was chosen simply for already being part of Commodore part stock. Each KIM-4 needed two of them. (also all edge connectors of commodore up to this point were 0.15 6spaced)
    – Raffzahn
    Jul 22 at 18:55
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    @Raffzahn: Fair 'nuff. My main point was that the cartridge used a rather long edge connector, which necessitated a fairly large board.
    – supercat
    Jul 22 at 19:16
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    The NES wasn't a top-loader in its original form--it was a front-loader. They made a top-loader later, though.
    – Hearth
    Jul 22 at 21:57
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If a machine has a vertical cartridge slot on the top, it's liable to have junk fall into it unless it either has a door, or unless something is left in it. The Odyssey2 video game system simply had its cartridge slot open on the top, but it would only be used with a cartridge in place. Most computers, however, would be used most of the time without a cartridge installed.

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